Outside Front, Back, and Spines

HST CoversMel Higashi Outside Front, Back, and Spines

= 3-Inch Binders= 2.5-Inch Binders= 1.5-Inch Binders= 1-Inch Binders= 0.5-Inch Binders

Hubble Space Telescope

S e r v i c i n g M i s s i o n 3 A

M e d i a R e f e r e n c e G u i d e

HST S e r v i c i n g M i s s i o n 3 A

M e d i a R e f e r e n c e G u i d e

Hubble Space Telescope Servicing Mission 3A Media Reference Guide

Prepared for:

By:

Missiles & Space

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2

ne of the great pioneers of modern astronomy, theAmerican astronomer Edwin Powell Hubble(1889–1953), started out by getting a law degree andserving in World War I. However, after practicing law

for one year, he decided to “chuck law for astronomy andI knew that, even if I were second rate or third rate, it wasastronomy that mattered.”

He completed a Ph.D. thesis on the PhotographicInvestigation of Faint Nebulae at the University ofChicago and then continued his work at Mount WilsonObservatory, studying the faint patches of luminous“fog” or nebulae in the night sky.

Using the largest telescope of its day, a 2.5-m reflector,he studied Andromeda and a number of other nebulaeand proved that they were other star systems (galaxies)similar to our own Milky Way.

He devised the classification scheme for galaxiesthat is still in use today, and obtained extensive evidence that the laws of physics outside the

Galaxy are the same as on Earth – in his own words:“verifying the principle of the uniformity of nature.”

In 1929, Hubble analyzed the speeds of recession of a numberof galaxies and showed that the speed at which a galaxymoves away from us is proportional to its distance (Hubble’sLaw). This discovery of the expanding universe marked thebirth of the “Big Bang Theory” and is one of the greatesttriumphs of 20th-century astronomy.

In fact, Hubble’s remarkable discovery could have been predicted some 10 years earlier by none other than Albert Einstein. In 1917, Einstein applied his newly developedGeneral Theory of Relativity to the problem of the universe asa whole. Einstein was very disturbed to discover that his theorypredicted that the universe could not be static, but had toeither expand or contract. Einstein found this prediction sounbelievable that he went back and modified his originaltheory in order to avoid this problem. Upon learning ofHubble’s discoveries, Einstein later referred to this as “the biggest blunder of my life.”

—ESA Bulletin 58

Edwin Hubble (1889–1953) at the 48-in.

Schmidt telescope on Palomar Mountain

O

Photo courtesy of the Carnegie Institution of Washington

Who Was Edwin P. Hubble?


1 INTRODUCTION 1-11.1 Hubble Space Telescope Configuration 1-31.1.1 Optical Telescope Assembly 1-41.1.2 The Science Instruments 1-51.1.3 Support Systems Module 1-71.1.4 Solar Arrays 1-71.1.5 Computers 1-71.2 The Hubble Space Telescope Program 1-71.3 The Value of Servicing 1-9

2 HUBBLE SPACE TELESCOPE SERVICING MISSION 3A 2-12.1 Reasons for Orbital Servicing 2-12.2 Orbital Replacement Units 2-22.3 Shuttle Support Equipment 2-32.3.1 Remote Manipulator System 2-42.3.2 Space Support Equipment 2-42.3.3 Orbital Replacement Unit Carrier 2-52.4 Astronaut Roles and Training 2-72.5 Extravehicular Crew Aids and Tools 2-92.6 Astronauts of the Servicing Mission 3A 2-92.7 Servicing Mission Activities 2-122.7.1 Rendezvous With the Hubble Space Telescope 2-122.7.2 Extravehicular Servicing Activities – Day by Day 2-132.8 Future Servicing Plans 2-26

3 HUBBLE SPACE TELESCOPE SCIENCE AND DISCOVERIES 3-13.1 Planets 3-13.2 Formation and Evolution of Stars and Planets 3-63.3 Galaxies and Cosmology 3-103.4 Summary 3-15

4 SCIENCE INSTRUMENTS 4-14.1 Space Telescope Imaging Spectrograph 4-14.1.1 Physical Description 4-14.1.2 Spectra Operational Modes 4-74.1.3 STIS Specifications 4-84.1.4 Observations 4-84.2 Wide Field and Planetary Camera 2 4-94.2.1 Physical Description 4-104.2.2 WFPC2 Specifications 4-134.2.3 Observations 4-134.3 Astrometry (Fine Guidance Sensors) 4-144.3.1 Fine Guidance Sensor Specifications 4-144.3.2 Operational Modes for Astrometry 4-144.3.3 Fine Guidance Sensor Filter Wheel 4-154.3.4 Astrometric Observations 4-15

Section Page

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CONTENTS

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5 HUBBLE SPACE TELESCOPE SYSTEMS 5-15.1 Support Systems Module 5-25.1.1 Structures and Mechanisms Subsystem 5-25.1.2 Instrumentation and Communications Subsystem 5-75.1.3 Data Management Subsystem 5-85.1.4 Pointing Control Subsystem 5-105.1.5 Electrical Power Subsystem 5-145.1.6 Thermal Control 5-165.1.7 Safing (Contingency) System 5-165.2 Optical Telescope Assembly 5-185.2.1 Primary Mirror Assembly and Spherical Aberration 5-195.2.2 Secondary Mirror Assembly 5-235.2.3 Focal Plane Structure Assembly 5-245.2.4 OTA Equipment Section 5-245.3 Fine Guidance Sensor 5-255.3.1 Fine Guidance Sensor Composition and Function 5-255.3.2 Articulated Mirror System 5-275.4 Solar Array and Jitter Problems 5-275.4.1 Configuration 5-275.4.2 Solar Array Subsystems 5-285.4.3 Solar Array Configuration for Servicing Mission 3A 5-295.5 Science Instrument Control and Data Handling Unit 5-295.5.1 Components 5-295.5.2 Operation 5-305.6 Space Support Equipment 5-315.6.1 Flight Support System 5-325.6.2 Orbital Replacement Unit Carrier 5-335.6.3 Crew Aids 5-35

6 HST OPERATIONS 6-16.1 Space Telescope Science Institute 6-16.1.1 Scientific Goals 6-16.1.2 Institute Software 6-16.1.3 Selecting Observation Proposals 6-26.1.4 Scheduling Selected Observations 6-26.1.5 Data Analysis and Storage 6-26.2 Space Telescope Operations Control Center 6-36.3 Operational Characteristics 6-36.3.1 Orbital Characteristics 6-46.3.2 Celestial Viewing 6-46.3.3 Solar System Object Viewing 6-56.3.4 Natural Radiation 6-56.3.5 Maneuver Characteristics 6-66.3.6 Communication Characteristics 6-66.4 Acquisition and Observation 6-7

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7 VALUE ADDED: The Benefits of Servicing Hubble 7-17.1 Cost-Effective Modular Design 7-17.1.1 Processor Improvements 7-17.1.2 Data Archiving Rate 7-17.1.3 Detector Technology 7-47.1.4 Cryogenic Cooler 7-47.1.5 Solar Arrays 7-57.1.6 Simultaneous Science 7-57.2 Accelerated Innovations 7-57.2.1 Detecting Breast Cancer Before Black Holes 7-57.2.2 Image Processing: Diagnosing Cancer Earlier 7-6

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ivK9322-TOC

Figure Page

1-1 The Hubble Space Telescope (HST) – shown in a clean room at Lockheed 1-2Martin Missiles & Space in Sunnyvale, California, before shipment to Kennedy Space Center – is equipped with science instruments and engineeringsubsystems designed as orbital replacement units.

1-2 Schedule of extravehicular activities 1-31-3 HST overall configuration 1-41-4 HST exploded view 1-51-5 Hubble Space Telescope specifications 1-61-6 Organization summary for HST program operational phase 1-81-7 HST data collecting network 1-92-1 Hubble Space Telescope Servicing Mission 3A Orbital Replacement Units 2-32-2 Servicing Mission 3A Payload Bay configuration 2-42-3 Flight Support System configuration 2-52-4 Orbital Replacement Unit Carrier 2-62-5 Neutral Buoyancy Laboratory at NASA Johnson Space Center 2-82-6 The STS-103 mission has seven crewmembers. They are (from left) 2-10

Mission Specialist C. Michael Foale, Mission Specialist Claude Nicollier, Pilot Scott J. Kelly, Commander Curtis L. Brown, Jr., Mission Specialist Jean-François Clervoy, Mission Specialist John M. Grunsfeld, and Mission Specialist and Payload Commander Steven L. Smith.

2-7 Detailed schedule of extravehicular activities and SA and FSS positions 2-14during SM3A

2-8 Change-out of Rate Sensor Unit 2-162-9 Voltage/Temperature Improvement Kit installation 2-172-10 Fine Guidance Sensor change-out 2-192-11 S-Band Single Access Transmitter change-out 2-222-12 Installation of Solid State recorder 2-232-13 New outer blanket layer installation 2-242-14 Redeploying the Space Telescope 2-263-1 On April 27, 1999, Hubble took pictures of a Martian storm more than 1000 3-2

miles (1600 km) across. Left: an image of the polar storm as seen in blue light (410 nm). Upper right: a polar view of the north polar region, showing the location of the storm relative to the classical bright and dark features in thisarea. Lower right: an enhanced view of the storm processed to bring out additional detail in its spiral cloud structures.

3-2 The HST WFPC2 captured these images between April 27 and May 6, 1999, 3-3when Mars was 54 million miles (87 million kilometers) from Earth. From this distance the telescope could see Martian features as small as 12 miles(19 kilometers) wide.

3-3 This is the first image of Saturn’s ultraviolet aurora taken by the STIS in 3-4A bright knot appears in the Supernova 1987A Ring.

3-4 Saturn viewed in the infrared shows atmospheric clouds and hazes. 3-53-5 The crisp resolution of the Telescope reveals various stages of the life cycle 3-6

of stars in this single view of the giant galactic nebula NGC 3603.

ILLUSTRATIONS

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Figure Page

3-6 In this October 1998 image of the Ring Nebula (M57), Hubble looks down 3-8a barrel of gas cast off by a dying star thousands of years ago.

3-7 Hubble sees supersonic exhaust from nebula M2-9, a striking example of 3-9a “butterfly” or bipolar planetary nebula.

3-8 A bright knot appears in the Supernova 1987A Ring. 3-103-9 In an observation called the Hubble Deep Field South (HDF-S), the Telescope 3-12

peered down an 11-billion-light-year-long corridor loaded with thousands of never-before seen galaxies.

3-10 This HST image provides a detailed look at a “fireworks show” in the center 3-13of a collision between two galaxies.

3-11 Hubble offers an unprecedented close-up view of a turbulent firestorm of 3-14starbirth along a nearly edge-on dust disk girdling Centaurus A.

4-1 Space Telescope Imaging Spectrograph 4-14-2 STIS components and detectors 4-34-3 STIS spectroscopic modes 4-44-4 Multi-Anode Microchannel Plate Array (MAMA) detector 4-54-5 Simplified MAMA system 4-64-6 STIS filter set 4-84-7 STIS specifications 4-84-8 Wide Field and Planetary Camera (WFPC) overall configuration 4-114-9 WFPC optics design 4-124-10 WFPC2 imaging 4-134-11 WFPC2 specifications 4-134-12 Fine Guidance Sensor (FGS) 4-144-13 FGS specifications 4-145-1 Hubble Space Telescope – exploded view 5-15-2 Hubble Space Telescope axes 5-25-3 Design features of Support Systems Module 5-35-4 Structural components of Support Systems Module 5-35-5 Aperture door and light shield 5-45-6 Support Systems Module forward shell 5-45-7 Support Systems Module Equipment Section bays and contents 5-55-8 Support Systems Module aft shroud and bulkhead 5-65-9 High Gain Antenna 5-75-10 Data Management Subsystem functional block diagram 5-85-11 Advanced computer 5-95-12 Data Management Unit configuration 5-105-13 Location of Pointing Control Subsystem equipment 5-125-14 Reaction Wheel Assembly 5-135-15 Electrical Power Subsystem functional block diagram 5-155-16 Placement of thermal protection on Support Systems Module 5-175-17 Light path for the main Telescope 5-195-18 Instrument/sensor field of view 5-205-19 Optical Telescope Assembly components 5-215-20 Primary mirror assembly 5-21

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Figure Page

5-21 Primary mirror construction 5-225-22 Main ring and reaction plate 5-225-23 Secondary mirror assembly 5-235-24 Focal plane structure 5-245-25 Optical Telescope Assembly Equipment Section 5-255-26 Cutaway view of Fine Guidance Sensor 5-265-27 Optical path of Fine Guidance Sensor 5-265-28 Solar Array wing detail 5-285-29 Fitting for Solar Array manual deployment 5-285-30 Science Instrument Control and Data Handling unit 5-295-31 Command flow for Science Instrument Control and Data Handling unit 5-315-32 Flow of science data in the Hubble Space Telescope 5-325-33 Flight Support System configuration 5-335-34 Flight Support System Berthing and Positioning System ring pivoted up 5-33

with Telescope berthed5-35 Orbital Replacement Unit Carrier 5-345-36 Portable Foot Restraint 5-356-1 “Continuous-zone” celestial viewing 6-46-2 HST single-axis maneuvers 6-56-3 Sun-avoidance maneuver 6-66-4 TDRS-HST contact zones 6-67-1 Advanced scientific instruments installed (or to be installed) on HST 7-27-2 Systems maintained and upgraded during each servicing mission 7-37-3 Processor improvements on HST 7-37-4 Data archiving rate improvements 7-37-5 Increase in onboard pixels 7-47-6 Increase in HST infrared capability 7-47-7 Productivity gains on HST with new solar arrays 7-57-8 Simultaneous use of HST science instruments 7-57-9 Projected medical savings 7-6

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umankind has sought to expand its knowledgeof the universe by studying the stars.Throughout history, great scientists such as

Nicholaus Copernicus, Galileo Galilei, JohannesKepler, Issac Newton, Edwin Hubble, and Albert Einstein have each contributed significantlyto our understanding of the universe. The launch of theHubble Space Telescope in 1990 signified another greatstep toward unraveling the mysteries of space.Spectacular discoveries such as massive black holes atthe center of galaxies, the common existence of precursorplanetary systems like our own, and the quantity anddistribution of cold dark matter are just a few examplesof the Telescope’s findings. Now, with NASA’sServicing Mission 3A, we are equipped to carry thequest for knowledge into the 21st century.

H

Another Step in Our Journey to the Stars


4

n a single stunning image of the giant galactic nebulaNGC 3603, the crisp resolution of NASA’s HubbleSpace Telescope captures various stages in the life cycle

of stars.

The back cover shows the evolved supergiant star calledSher 25 (upper left center). The star has a unique circum-stellar ring of glowing gas that is a galactic twin to thefamous ring around the supernova 1987A.

Young, hot Wolf-Rayet stars and early O-type stars dominatea starburst cluster near the center of the image. A torrentof ionizing radiation and fast stellar winds from thesemassive stars has blown a large cavity around the cluster.

The giant gaseous pillars to the right and below thecluster are the most spectacular evidence for the interac-tion of ionizing radiation with cold molecular-hydrogencloud material.

Bok globules, the dark clouds in the upper right, are probably in an earlier stage of star formation.

Two compact, tadpole-shaped emission nebulae appearnear the lower left of the cluster. Hubble found similarstructures in Orion that have been interpreted as gas anddust evaporation from possible protoplanetary disks.

The life cycle of stars begins with the Bok globules andgiant gaseous pillars, followed by circumstellar disks, andprogresses to evolved massive stars in the young starburstcluster. The blue supergiant with its ring and bipolaroutflow marks the end of the life cycle.

The inside covers show 3-D computer models of some ofthe tasks to be performed in orbit by the STS-103 crewduring Servicing Mission 3A. The computer models enabledengineers to study task feasibility and to confirm thatastronauts could safely reach and service components andlocations on the spacecraft. They provided dimensionallyaccurate, visually correct images to help the extravehicularactivity servicing team prepare to install new componentsand upgrade functional systems on the Telescope.

IAbout the Covers


1-1K9322-01M

azing through the first crude telescope inthe 17th century, Galileo discovered thecraters of the Moon, the satellites of

Jupiter, and the rings of Saturn. These observa-tions led the way to today’s quest for in-depthknowledge and understanding of the cosmos.And for nearly a decade NASA’s Hubble SpaceTelescope (HST) has continued this historic quest.

Since its launch in April 1990, Hubble hasprovided scientific data and images ofunprecedented resolution from which manynew and exciting discoveries have been made.Even when reduced to raw numbers, theaccomplishments of the 12.5-ton orbitingobservatory are impressive:

• Hubble has taken about 259,000 exposures. • Hubble has observed nearly 13,000

astronomical targets. • Astronomers using Hubble data have

published over 2,400 scientific papers. • Circling Earth every 90 minutes, Hubble has

traveled about 1.425 billion miles, which isnearly the distance from Earth to Uranus.

This unique observatory operates around theclock above the Earth’s atmosphere gatheringinformation for teams of scientists who studyvirtually all the constituents of the universe. TheTelescope is an invaluable tool for examiningplanets, stars, star-forming regions of the MilkyWay, distant galaxies and quasars, and thetenuous hydrogen gas lying between the galaxies.

The HST can produce images of the outerplanets in our solar system that approach theclarity of those from Voyager flybys.Astronomers have resolved previously unsus-pected details of star-forming regions of theOrion Nebula in the Milky Way and havedetected expanding gas shells blown off byexploding stars.

Using the Telescope’s high-resolution andlight-gathering power, scientists have cali-brated the distances to remote galaxies. Theyhave detected and measured the rotation ofcool disks of matter trapped in the gravita-tional field at the cores of galaxies that portendthe presence of massive black holes.

Spectroscopic observations at ultraviolet wave-lengths inaccessible from the ground have givenastronomers their first opportunity to study theabundance and spatial distribution of intergalactichydrogen in relatively nearby regions of theuniverse – and forced scientists to rethink some oftheir earlier theories about galactic evolution.(Section 4 of this guide contains additional infor-mation on the Telescope’s scientific discoveries.)

The Telescope’s purpose is to spend 20 yearsprobing the farthest and faintest reaches of thecosmos. Crucial to fulfilling this objective is aseries of on-orbit manned servicing missions.The First Servicing Mission (SM1) took place inDecember 1993 and the Second ServicingMission (SM2) was flown in February 1997.

During these missions, astronauts performplanned repairs and maintenance activities torestore and upgrade the observatory’s capabil-ities. To facilitate this process, the Telescope’sdesigners configured science instruments andseveral vital engineering subsystems as OrbitalReplacement Units (ORU) – modular packageswith standardized fittings accessible to astro-nauts in pressurized suits (see Fig. 1-1).

Hubble’s Third Servicing Mission has been sepa-rated into two parts: Servicing Mission 3A(SM3A) will fly in Fall of 1999 and ServicingMission 3B (SM3B) is planned for 2001.

The principal objective of SM3A is to replace allsix gyroscopes that compose the three Rate

GINTRODUCTION

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Fig. 1-1 The Hubble Space Telescope (HST) – shown in a clean room at Lockheed MartinMissiles & Space in Sunnyvale, California, before shipment to Kennedy SpaceCenter – is equipped with science instruments and engineering subsystemsdesigned as orbital replacement units.

K70110-101

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Sensor Units (RSU). In addition, space-walkingastronauts will install a new AdvancedComputer that will dramatically increase thecomputing power, speed, and storage capabilityof HST. They will change out one of the FineGuidance Sensors (FGS) and replace a taperecorder with a new Solid State Recorder (SSR).The Extravehicular Activity (EVA) crew also willinstall a new S-band Single-Access Transmitter(SSAT), and Voltage/Temperature ImprovementKits (VIK) for the Telescope’s nickel-hydrogenbatteries. Finally, they will begin repair of themultilayer insulation on Hubble’s outer surface.

During SM3B astronauts will install a new scienceinstrument, the Advanced Camera for Surveys(ACS), and an Aft Shroud Cooling System (ASCS)for the other axial science instruments. They willattach a new cryogenic cooler to the Near-InfraredCamera and Multi-Object Spectrometer(NICMOS). They also will replace the HST flexibleSolar Arrays with new high-performance rigidarrays designed and built by the Goddard Space

Flight Center (GSFC), the European Space Agency(ESA) and Lockheed Martin Missiles & Space.Additionally, they will complete repair of themultilayer surface insulation begun on SM3A.

Figure 1-2 shows the SM3A activities as sched-uled at press time. Section 2 provides moredetails of SM3A.

Since 1979 the HST team has overcome enormoustechnical obstacles to successfully develop andlaunch the orbiting observatory. The ThirdServicing Mission continues this tradition. Its twochallenging flights promise to upgrade theHubble Space Telescope with the latest tech-nology hardware for spacecraft systems and toincorporate advanced instruments that will signif-icantly expand Hubble’s scientific capabilities.

1.1 Hubble Space Telescope Configuration

Figures 1-3 and 1-4 show the overall Telescopeconfiguration. Figure 1-5 lists specifications for

Fig. 1-2 Schedule of extravehicular activities

K90322-102A

6-hour EVA period

EV

A D

ay 1

EV

A D

ay 2

EV

A D

ay 3

EV

A D

ay 4

VIKClose0:30

Close0:30

RSU 1, 2 & 3Setup°1:00

ComputerSet:15

Sun protect attitude


MF

R

MF

R

AS

D

Val

ves

FGS2

Priority Task Times (stand-alone)

1. RSUs (RSU-1, -2, -3)2. VIK3. Advanced Computer4. FGS (FGS-2)5. SSAT (SSAT-2)6. SSR (ESTR-3)7. Bay 5-10 MLI Repair8. NICMOS valves open

1:55 ea1:101:503:001:301:152:25 total 0:20

3:07 OT#1. FS/LS MLI RepairOT#2. Handrail Covers Bays A & JOT#3. ASLR for +V2 Doors

Required Setup

Optional Tasks & Priorities

Task Times1:00 (1st day)0:15 (nth day)0:30 (nth day)1:00 (last day)

Setup

Closeup

Includes BAPS Post installationAft Shroud Door latch contingencycrew change positions in Manipulator Foot RestraintHigh Gain Antenna deployment

° =ASD =MFR =HGA =

SSA XmtrSet:15 SSR

MLI RepairBays 5, 6, 7, 8, 9, 10

Close0:30

OT#1 MLIFS/LS 5, 6 & 7

Set:15 H

GA Closeout

1:00OT #2&/or #3

OT#1 MLIFS/LS 1 & 2

OT#1 MLIFS/LS 3 & 4

OCE

MF

R

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the Telescope. The major elements are:

• Optical Telescope Assembly (OTA) – twomirrors and associated structures thatcollect light from celestial objects

• Science instruments – devices used toanalyze the images produced by the OTA

• Support Systems Module (SSM) – spacecraftstructure that encloses the OTA and scienceinstruments

• Solar Arrays (SA).

1.1.1 Optical Telescope Assembly

The OTA consists of two mirrors, supporttrusses, and the focal plane structure. Theoptical system is a Ritchey-Chretien design, in

which two special aspheric mirrors formfocused images over the largest possible fieldof view. Incoming light travels down a tubularbaffle that absorbs stray light. The concaveprimary mirror – 94.5 in. (2.4 m) in diameter –collects the light and converges it toward theconvex secondary mirror, which is only 12.2 in.(0.3 m) in diameter. The secondary mirrordirects the still-converging light back towardthe primary mirror and through a 24-in. hole inits center into the Focal Plane Structure, wherethe science instruments are located.

Shortly after launch in 1990, a spherical aber-ration of the primary mirror was detected. Aninvestigation revealed the specific errors thatcaused the fault. This knowledge allowed

PRIMARY MIRROR

FORWARD SHELL

HIGH GAIN ANTENNA (2)

SECONDARYMIRROR (2)

APERTUREDOOR

LIGHT SHIELD

AFT SHROUD

COSTAR AND SCIENCEINSTRUMENTS

AXIAL (4)

RADIAL (1)

SOLAR ARRAY (2)FINE GUIDANCESENSORS (3)

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Fig. 1-3 HST overall configuration

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optical experts to design and fabricate smallpairs of corrective mirrors that successfullyrefocused the light reflected from the primarymirror before it entered the axial scienceinstruments.

The Corrective Optics Space TelescopeAxial Replacement (COSTAR) wasinstalled on HST in December 1993. Inaddition, WFPC2, a radial instrument withcorrective optics incorporated, wasinstalled on the same mission. The subse-quent increase in optical performancehelped to restore the Telescope’s capabilityto original expectations.

All new instruments installed after 1993correct for spherical aberration internallywithin their own optical systems.

1.1.2 The Science Instruments

Hubble can accommodate eight science instru-ments. Four are aligned with the Telescope’smain optical axis and are mounted immedi-ately behind the primary mirror. The currentsuite of axial science instruments consists of:

• Space Telescope Imaging Spectrograph (STIS)•Faint Object Camera (FOC) {decommissioned}•Near Infrared Camera and Multi-Object

Spectrometer (NICMOS) {dormant}• Corrective Optics Space Telescope Axial

Replacement (COSTAR) {not currently inuse}.

In addition to the four axial instruments, fourother instruments are mounted radially(perpendicular to the main optical axis). Theseradial science instruments are:

Fig. 1-4 HST exploded view

SECONDARY MIRROR BAFFLE


SUPPORT SYSTEMS MODULEFORWARD SHELL

APERTUREDOOR

LIGHT SHIELD

CENTRAL BAFFLE

AXIAL SCIENCEINSTRUMENTMODULE (3)AND COSTAR

LOW GAINANTENNA (2)

SUPPORT SYSTEMSMODULE AFT SHROUD

K70110-104

MAGNETIC TORQUER (4)

OPTICAL TELESCOPE ASSEMBLYSECONDARY MIRROR ASSEMBLY

OPTICAL TELESCOPE ASSEMBLYPRIMARY MIRROR AND MAIN RING

FINE GUIDANCE OPTICALCONTROL SENSOR (3)

OPTICAL TELESCOPEASSEMBLY FOCAL PLANESTRUCTURE

FIXED HEAD STAR TRACKER (3)AND RATE GYRO ASSEMBLY

RADIAL SCIENCEINSTRUMENT MODULE (1)

OPTICAL TELESCOPE ASSEMBLYEQUIPMENT SECTION

SUPPORT SYSTEMS MODULEEQUIPMENT SECTION

OPTICAL TELESCOPE ASSEMBLYMETERING TRUSS

MAIN BAFFLESOLAR ARRAY (2)

MAGNETOM-ETER (2)

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Hubble Space Telescope (HST)

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WeightLength

DiameterOptical systemFocal lengthPrimary mirrorSecondary mirrorField of viewPointing accuracyMagnitude rangeWavelength rangeAngular resolutionOrbitOrbit timeMission

24,500 lb (11,110 kg)43.5 ft (15.9 m)10 ft (3.1 m) Light Shield and Forward Shell14 ft (4.2 m) Equipment Section and Aft ShroudRitchey-Chretien design Cassegrain telescope189 ft (56.7 m) folded to 21 ft (6.3 m)94.5 in. (2.4 m) in diameter12.2 in. (0.3 m) in diameterSee instruments/sensors0.007 arcsec for 24 hours5 mv to 29 mv (visual magnitude)1100 to 11,000 Å0.1 arcsec at 6328 Å320 nmi (593 km), inclined 28.5 degrees from equator97 minutes per orbit20 years

Fig. 1-5 Hubble Space Telescope specifications

•Wide Field and Planetary Camera 2 (WFPC2)•Three Fine Guidance Sensors (FGS).

Space Telescope Imaging Spectrograph. STISseparates incoming light into its componentwavelengths, revealing information about theatomic composition of the light source. It candetect a broader range of wavelengths than ispossible from Earth because there is no atmo-sphere to absorb certain wavelengths. Scientistscan determine the chemical composition,temperature, pressure, and turbulence of thetarget producing the light – all from spectral data.

STIS also provides a two-dimensional capa-bility to HST spectroscopy. The two dimen-sions can be used for “long-slit” spectroscopy,where spectra of many different points acrossan object are obtained simultaneously, or in amode that obtains more wavelength coveragein a single exposure. In addition, STIS canproduce visible or ultraviolet (UV) imagesand can provide objective prism spectra in theintermediate UV region of the spectrum. STISalso has a coronagraph capability and a hightime-resolution capability in the UV. A team atGSFC manages the STIS instrument and itsobservations.

Wide Field and Planetary Camera 2. WFPC2 isan electronic camera that records images attwo magnifications. A team at the JetPropulsion Laboratory (JPL) in Pasadena,California, built the first WFPC and developedthe WFPC2. The team incorporated an opticalcorrection by refiguring relay mirrors in theoptical train of the cameras. Each relay mirroris polished to a prescription that compensatesfor the incorrect figure on HST’s primarymirror. Small actuators fine-tune the posi-tioning of these mirrors on orbit.

Fine Guidance Sensors. The three FGSs havetwo functions: (1) provide data to the space-craft’s pointing system to keep HST pointedaccurately at a target when one or more ofthe science instruments is being used to takedata and (2) act as a science instrument.When functioning as a science instrument,two of the sensors lock onto guide stars andthe third measures the brightness and rela-tive positions of stars in its field of view.These measurements, referred to as astrom-etry, are helping to advance knowledge ofthe distances and motions of stars and maybe useful in detecting planetary-sizedcompanions of other stars.

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1.1.3 Support Systems Module

The SSM encloses the OTA and the scienceinstruments like the dome of an Earth-basedobservatory. It also contains all of the struc-tures, mechanisms, communications devices,electronics, and electrical power subsystemsneeded to operate the Telescope.

This module supports the light shield and anaperture door that, when opened, admits light.The shield connects to the forward shell onwhich the SAs and High Gain Antennas (HGA)are mounted. Electrical energy from the 40-ft(12-m) SAs charges the spacecraft batteries topower all HST systems. Four antennas, twohigh-gain and two low-gain, send and receiveinformation between the Telescope and theSpace Telescope Operations Control Center(STOCC). All commanding occurs through theLow Gain Antennas (LGA).

Behind the OTA is the Equipment Section, a ringof bays that house the batteries and most of theelectronics, including the computer and commu-nications equipment. At the rear of the Telescope,the aft shroud contains the science instruments.

1.1.4 Solar Arrays

The SAs provide power to the spacecraft. Theyare mounted like wings on opposite sides ofthe Telescope, on the forward shell of the SSM.Each array stands on a mast that supports aretractable wing of solar panels 40 ft (12.2 m)long and 8.2 ft (2.5 m) wide.

The SAs are rotated so each wing’s solar cells facethe Sun. The cells absorb the Sun’s light energyand convert it into electrical energy to power theTelescope and charge the spacecraft’s batteries,which are part of the Electrical Power Subsystem(EPS). Batteries are used when the Telescopemoves into Earth’s shadow during each orbit.

Shortly after launch in 1990, it was determinedthat as the Telescope orbited in and out ofdirect sunlight, the resulting thermal gradientscaused oscillation of the SAs, inducing jitter inthe Telescope’s line of site. This in turn causedsome loss of fine lock of the FGSs duringscience observations. New SAs with thermalshields over the array masts installed duringSM1 minimized the effect.

1.1.5 Computers

Hubble’s Data Management Subsystem (DMS)contains two computers: the DF-224 flightcomputer and the Science Instrument Controland Data Handling (SI C&DH) unit. The DF-224performs onboard computations and handlesdata and command transmissions between theTelescope systems and the ground system. TheSI C&DH unit controls commands received bythe science instruments, formats science data,and sends data to the communications systemfor transmission to Earth.

During SM1 astronauts installed a coprocessorto augment the DF-224 capacity. The new 386-co-processor increased flight computer redun-dancy and significantly enhanced on-orbitcomputational capability.

During SM3A astronauts will replace the DF-224and its co-processor with an AdvancedComputer. It will assume all of the DF-224functions while running 20 times faster andproviding six times as much onboardmemory.

1.2 The Hubble Space Telescope Program

Hubble Space Telescope represents the fulfill-ment of a 50-year dream and 23 years of dedi-cated scientific effort and political vision toadvance humankind’s knowledge of theuniverse.

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The HST program comprises an internationalcommunity of engineers, scientists, contractors, andinstitutions. It is managed by GSFC for the Office ofSpace Science (OSS) at NASA Headquarters.

The program falls under the Search for Originsand Planetary Systems scientific theme. WithinGSFC, the program is in the Flight ProjectsDirectorate, under the supervision of the asso-ciate director of Flight Projects for HST.

The HST program is organized as two flightprojects: (1) the HST Operations and GroundSystems Project and (2) the HST Flight Systemsand Servicing Project.

Responsibilities for scientific oversight on HST aredivided among the members of the Project ScienceOffice (PSO). The PSO is designed to interact effec-tively and efficiently with the HST flight projectand the wide range of external organizationsinvolved with the HST. The senior scientist for theHST and supporting staff work in the Office of theAssociate Director of Flight Projects for HST. Thisgroup is concerned with the highest level of scien-tific management for the project.

The roles of NASA centers and contractors foron-orbit servicing of the HST are:

• Goddard Space Flight Center (GSFC) –Overall management of daily on-orbit opera-tions of HST and the development, integra-tion, and test of replacement hardware, spacesupport equipment, and crew aids and tools

• Johnson Space Center (JSC) – Overallservicing mission management, flight crewtraining, and crew aids and tools

• Kennedy Space Center (KSC) – Overallmanagement of launch and post-landingoperations for mission hardware

• Ball Aerospace – Design, development, andprovision of axial science instruments

• JPL – Design, development, and provision ofWFPC1 and WFPC2

• Lockheed Martin – personnel support forGSFC to accomplish (1) development, inte-gration, and test of replacement hardwareand space support equipment; (2) systemintegration with the Space TransportationSystem (STS); (3) launch and post-landingoperations; and (4) daily HST operations.

Major subcontractors for SM3A includeRaytheon Optical Systems, Inc., Allied Signal,Jackson and Tull, Orbital Sciences Corporation,Odetics, Honeywell, ETA, and Hughes STX.

The HST program requires a complex networkof communications among GSFC, theTelescope, Space Telescope Ground System,and the Space Telescope Science Institute.Figure 1-6 summarizes the major organizations

Organization

K70110-108

NASA Headquarters

Goddard Space FlightCenter– Office of the Associate Director of Flight Projects for HST– HST Operations and Ground Systems Project

Space TelescopeOperations ControlCenter

Space TelescopeScience Institute

Goddard Space FlightCenter– HST Flight Systems and Servicing Project

Function

• Provides overall responsibility for the program

• Overall HST Program Management• HST Project Management• Responsible overseeing all HST operations

• Provides minute-to-minute spacecraft control• Schedules, plans, and supports all science operations when required• Monitors telemetry communications data to the HST

• Selects observing programs from numerous proposals• Analyzes astronomical data

• Responsible for implementing HST Servicing Program• Manages development of new HST spacecraft hardware and service instruments• Manages HST Servicing Payload Integration and Test Program• Primary interface with the Space Shuttle Program at JSC

Fig. 1-6 Organization summary for HST programoperational phase

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K70110-109A

Fig. 1-7 HST data collecting network

STARLIGHT

TRACKING ANDDATA RELAYSATELLITE SYSTEM(TDRSS)

PAYLOAD OPERATIONSCONTROL CENTER

TDRSSGROUNDSTATION

SPACETELESCOPESCIENCEINSTITUTE(STScI)

SPACE TELESCOPEOPERATIONSCONTROL CENTER(STOCC)

that oversee the program. Figure 1-7 showscommunication links.

1.3 The Value of Servicing

Hubble’s visionary modular design allowsNASA to equip it with new, state-of-the-art

instruments every few years. These servicingmissions enhance the Telescope’s sciencecapabilities, leading to fascinating newdiscoveries about the universe. Periodicservice calls also permit astronauts to “tuneup” the Telescope and replace limited-lifecomponents.

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he Hubble Space Telescope (HST) is thefirst observatory designed for extensivemaintenance and refurbishment in

orbit. While other U.S. spacecraft have beenretrieved or repaired by astronauts, none wasso thoroughly designed for orbital servicing asHST. Its science instruments and many othercomponents were designed as OrbitalReplacement Units (ORU) – modular in con-struction with standardized fittings and acces-sible to spacewalking astronauts. Features suchas handrails and foot restraints are built intothe Telescope to help astronauts perform ser-vicing tasks in the Shuttle cargo bay as theyorbit Earth at 17,500 mph.

For Servicing Mission 3A (SM3A), the Discoverycargo bay is equipped with several devices tohelp the astronauts. The Flight SupportSystem (FSS) will berth and rotate theTelescope. Large, specially designed equip-ment containers house the ORUs. Astronautscan work and be maneuvered as needed fromthe Shuttle robot arm.

SM3A will benefit from lessons learned onNASA’s previous on-orbit servicing missions,ranging from the 1984 Solar Maximum repairmission to the 1993 HST First ServicingMission (SM1) and the 1997 Second ServicingMission (SM2). NASA has incorporated theselessons in detailed planning and training ses-sions for astronauts Curtis Brown, Jr., ScottKelly, Jean-François Clervoy, Steven Smith,John Grunsfeld, Michael Foale, and ClaudeNicollier. All of NASA’s planning and theastronauts’ skills will be put to the test duringthe SM3A mission in 1999. Four extravehicularactivity (EVA) days are scheduled for theservicing.

2.1 Reasons for Orbital Servicing

The Hubble Telescope is a national asset and aninvaluable international scientific resource thathas revolutionized modern astronomy. Toachieve its full potential, HST will continue toconduct extensive, integrated scientific obser-vations, including follow-up work on its manydiscoveries.

Although the Telescope has numerous redun-dant parts and safemode systems, such a com-plex spacecraft cannot be designed withsufficient backups to handle every contingencylikely to occur during a 20-year mission.Orbital servicing is the key to keeping Hubblein operating condition. NASA’s orbital servic-ing plans address three primary maintenancescenarios:

• Incorporating technological advances intothe science instruments and ORUs

• Normal degradation of components• Random equipment failure or malfunction.

Technological Advances. Throughout theTelescope’s 20-year life, scientists and engineerswill upgrade the science instruments. Forexample, when Hubble was launched in 1990, itwas equipped with the Goddard HighResolution Spectrograph and the Faint ObjectSpectrograph. A second-generation instrument,the Space Telescope Imaging Spectrograph,took over the function of those two instruments– and added considerable new capability –when it was installed during the SecondServicing Mission in February 1997. This leftopen a slot for the Near Infrared Camera andMulti-Object Spectrometer (NICMOS), whichexpanded the Telescope’s vision into the

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infrared region of the spectrum. Additionally,on SM2 a new Solid State Recorder (SSR)replaced an Engineering/Science TapeRecorder (E/STR).

Component Degradation. Servicing plans takeinto account the need for routine replacementof some items, for example, restoring HST sys-tem redundancy and limited-life items such astape recorders and gyroscopes.

Equipment Failure. Given the enormous sci-entific potential of the Telescope – and theinvestment in designing, developing, building,and putting it into orbit – NASA must be ableto correct unforeseen problems that arise fromrandom equipment failures or malfunctions.The Space Shuttle program provides a provensystem for transporting astronauts to theTelescope fully trained for its on-orbit servicing.

Originally, planners considered using theShuttle to return the Telescope to Earthapproximately every five years for mainte-nance. However, the idea was rejected for bothtechnical and economic reasons. ReturningHubble to Earth would entail a significantlyhigher risk of contaminating or damaging del-icate components. Ground servicing wouldrequire an expensive clean room and supportfacilities, including a large engineering staff,and the Telescope would be out of actionfor a year or more – a long time to suspendscientific observations.

Shuttle astronauts can accomplish mostmaintenance and refurbishment within a10-day on-orbit mission – with only a briefinterruption to scientific operations andwithout the additional facilities and staffneeded for ground servicing.

2.2 Orbital Replacement Units

Advantages of ORUs include modularity, stan-dardization, and accessibility.

Modularity. Engineers studied various tech-nical and human factors criteria to simplifyTelescope maintenance. Due to the limitedtime available for repairs and the astronauts’limited visibility, mobility, and dexterity in theEVA environment, designers simplified themaintenance tasks by planning entire compo-nents for replacement.

The modular ORU concept is key to success-fully servicing the Telescope on orbit. ORUsare self-contained boxes installed and removedusing fasteners and connectors. They rangefrom small fuses to phone-booth-sized scienceinstruments weighing more than 700 lb (318 kg).Figure 2-1 shows the ORUs for SM3A.

Standardization. Standardized bolts and con-nectors also simplify on-orbit repairs. Captivebolts with 7/16-in., double-height hex headshold many ORU components in place. Toremove or install the bolts, astronauts needonly a 7/16-in. socket fitted to a power tool ormanual wrench. Standardization limits thenumber of crew aids and tools.

Some ORUs do not contain these fasteners.When the maintenance philosophy changedfrom Earth-return to on-orbit-only servicing,other components were selected as replaceableunits after their design had matured. Thisadded a greater variety of fasteners to the ser-vicing requirements, including non-captive5/16-in.-head bolts and connectors withoutwing tabs. Despite these exceptions, the highlevel of standardization among units reduces

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the number of tools needed for the servicingmission and simplifies astronaut training.

Accessibility. To be serviced in space,Telescope components must be seen andreached by an astronaut in a bulky pressuresuit, or they must be within range of the appro-priate tool. Therefore, most ORUs are mountedin equipment bays around the perimeter of thespacecraft. To access these units, astronautssimply open a large door that covers the appro-priate bay.

Handrails, foot restraint sockets, tether attach-ments, and other crew aids are essential to effi-cient, safe on-orbit servicing. In anticipation ofservicing missions, 31 foot restraint socketsand 225 ft of handrails were designed into the

Telescope. The foot restraint sockets andhandrails greatly increase the mobility and sta-bility of EVA astronauts, giving them safeworksites conveniently located near ORUs.

Crew aids such as portable lights, special tools,installation guiderails, handholds, and port-able foot restraints (PFR) also ease servicing ofthe telescope components. In addition, footrestraints, translation aids and handrails arebuilt into various equipment and instrumentcarriers specific to each servicing mission.

2.3 Shuttle Support Equipment

To assist the astronauts in servicing theTelescope, Discovery will carry into orbit severalthousand pounds of hardware and Space

Fig. 2-1 Hubble Space Telescope Servicing Mission 3A Orbital Replacement Units

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Support Equipment (SSE), including theRemote Manipulator System (RMS), FSS, andORU Carrier (ORUC).

2.3.1 Remote Manipulator System

The Discovery RMS, also known as the roboticarm, will be used extensively during SM3A.The astronaut operating this device frominside the cabin is designated intravehicularactivity (IVA) crew member. The RMS will beused to:

• Capture, berth, and release the Telescope• Transport new components, instruments,

and EVA astronauts between worksites• Provide a temporary work platform for one

or both EVA astronauts.

2.3.2 Space Support Equipment

Ground crews will install two major assem-blies essential for SM3A – the FSS and ORUC –in Discovery’s cargo bay. Figure 2-2 shows acargo bay view of these assemblies.

Fig. 2-2 Servicing Mission 3A Payload Bay configuration

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Flight Support System. The FSS is a mainte-nance platform used to berth the HST in thecargo bay after the Discovery crew has ren-dezvoused with and captured the Telescope(see Fig. 2-3). The platform was adapted fromthe FSS first used during the 1984 SolarMaximum repair mission. It has a U-shapedcradle that spans the rear of the cargo bay. Acircular berthing ring with three latchessecures the Telescope to the cradle. The berthingring can rotate the Telescope almost 360 degrees(176 degrees clockwise or counterclockwisefrom its null position) to give EVA astronautsaccess to every side of the Telescope.

The FSS also pivots to lower or raise theTelescope as required for servicing or reboost-ing. The FSS’s umbilical cable provides powerfrom Discovery to maintain thermal control ofthe Telescope and permits ground engineers totest and monitor Telescope systems during theservicing mission.

2.3.3 Orbital Replacement UnitCarrier

The ORUC is centered in Discovery’s cargo bay.A Spacelab pallet modified with a shelf, it hasprovisions for safe transport of ORUs to andfrom orbit (see Fig. 2-4). In the SM3A configu-ration:

• The Fine Guidance Sensor (FGS) #2 is storedin the Fine Guidance Sensor ScientificInstrument Protective Enclosure (FSIPE).

• The Large ORU Protective Enclosure(LOPE) contains the Advanced Computerand two Y-harnesses (going up), two spareVoltage/Temperature Improvement Kits(VIK), and the DF-224 computer and co-processor (returning from orbit).

• The Contingency ORU Protective Enclosure(COPE) houses three Rate Sensor Units(RSU), the Solid State Recorder (SSR) (goingup), the S-band Single Access Transmitter

Fig 2-3 Flight Support System configurationK9322_203

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(SSAT), the Engineering/Science TapeRecorder (E/STR) (coming back), and addi-tional harnesses. Contingency hardwareincluding the Electronics Control Unit(ECU), the spare Optics Control ElectronicsEnhancement Kit (OCE/EK), and the PowerDistribution Unit (PDU) connector coversare stowed in the COPE.

• Connector converters for the AdvancedComputer are on the COPE lid.

• The Axial Scientific Instrument ProtectiveEnclosure (ASIPE) shelf contains the spareAdvanced Computer, the spare RSU, sixMulti-Layer Insulation (MLI) patches (twolarge and four small), seven Shell/ShieldReplacement Fabrics (SSRFs), and six SSRFrib clamps.

• The New Outer Blanket Layer (NOBL)Protective Enclosure (NPE) contains thenew protective coverings to be installed onthe Telescope equipment bay doors.

• The Auxiliary Transport Module (ATM)houses MLI Recovery Bags, the DataManagement Unit (DMU)/AdvancedComputer contingency cables, a debris bag,and a spare SSAT coaxial cable.

The protective enclosures, their heaters, andthermal insulation control the temperature ofthe new ORUs and provide an environmentequivalent to that inside the Telescope. Struts,springs, and foam between the enclosures andthe pallet protect the ORUs from the loadsgenerated at liftoff and during Earth return.

Fig. 2-4 Orbital Replacement Unit Carrier

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2.4 Astronaut Roles and Training

To prepare for SM3A, the seven-memberDiscovery crew trained extensively at NASA’sJohnson Space Center (JSC) in Houston, Texas,and Goddard Space Flight Center (GSFC) inGreenbelt, Maryland.

Although there has been extensive cross train-ing, each crewmember also has trained forspecific tasks. Training for MissionCommander Brown and Pilot Kelly focusedon rendezvous and proximity operations,such as retrieval and deployment of theTelescope. The two astronauts rehearsed theseoperations using JSC’s Shuttle MissionSimulator, a computer-supported trainingsystem. In addition, they received IVA train-ing – helping the EVA astronauts into suitsand monitoring their activities outside theDiscovery cabin.

The five Mission Specialists also receivedspecific training, starting with classroominstruction on the various ORUs, tools andcrew aids, Space Support Equipment (SSE)such as the RMS (the robotic arm), and theFSS. Principal operator of the robotic arm isMission Specialist Clervoy, who also per-forms IVA duties. The alternate operatorsare mission specialists Claude Nicollier andJohn Grunsfeld.

Clervoy trained specifically for capture andredeployment of the Telescope, rotation andpivoting of the Telescope on the FSS, and relatedcontingencies. These operations were simulatedwith JSC’s Manipulator Development Facility,which includes a mockup of the robotic armand a suspended helium balloon with dimen-sions and grapple fixtures similar to those onthe Telescope. RMS training also took place at

JSC’s Neutral Buoyancy Laboratory (NBL),enabling the RMS operator and alternates towork with individual team members. Forhands-on HST servicing, EVA crewmemberswork in teams of two in the cargo bay.Astronauts Smith, Grunsfeld, Foale, andNicolier logged many days of training for thisimportant role in the NBL, a 40-ft (12-m)-deepwater tank (see Fig. 2-5).

In the NBL, pressure-suited astronauts andtheir equipment are made neutrally buoy-ant, a condition that simulates weightless-ness. Underwater mockups of the Telescope,FSS, ORUs, ORUC, RMS, and the Shuttlecargo bay enabled the astronauts to practiceentire EVA servicing. Such training activi-ties help the astronauts efficiently use thelimited number of days (four) and duration(six hours) of each EVA period during theservicing mission.

Other training aids at JSC helped recreateorbital conditions for the Discovery astronauts.In the weightlessness of space, the tiniestmovement can set instruments weighing sev-eral hundred pounds, such as FGS #2, intomotion.

To simulate the delicate on-orbit conditions,models of the instruments are placed on padsabove a stainless steel floor and floated on athin layer of pressurized gas. This allowscrewmembers to practice carefully nudgingthe instruments into their proper locations.

Astronauts also used virtual reality technolo-gies in their training. This kind of ultrarealisticsimulation enabled the astronauts to “see”themselves next to the Telescope as their part-ners maneuver them into position with therobotic arm.

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Fig. 2-5 Neutral Buoyancy Laboratory at NASA Johnson Space Center

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2.5 Extravehicular Crew Aids andTools

Astronauts servicing HST use three differentkinds of foot restraints to counteract theirweightless environment. When anchored in aManipulator Foot Restraint (MFR), an astronautcan be transported from one worksite to the nextwith the RMS. Using either the STS or HST PFR,an astronaut establishes a stable worksite bymounting the restraint to any of 31 differentreceptacles strategically placed on the Telescopeor 16 receptacles on the ORUC and the FSS.

In addition to foot restraints, EVA astronautshave more than 150 tools and crew aids at theirdisposal. Some of these are standard itemsfrom the Shuttle’s toolbox, while others areunique to this servicing mission. All tools aredesigned to be used in a weightless environmentby astronauts wearing pressurized gloves.

The most commonly used ORU fasteners arethose with 7/16-in., double-height hex heads.These bolts are used with three different kindsof fittings: J-hooks, captive fasteners, and key-hole fasteners. To replace a unit, the astronautsuse a 7/16-in. extension socket on a powered ormanual ratchet wrench. Extensions up to 2 ftlong are available to extend an astronaut’sreach. Multi-setting torque-limiters preventover-tightening of fasteners or latch systems.

For units with bolts or screws that are not cap-tive in the ORU frame, astronauts use tools fit-ted with socket capture fittings – and speciallydesigned capture tools – so that nothing floatsaway in the weightless space environment. Togrip fasteners in hard-to-reach areas, the crewcan use wobble sockets.

Some ORU electrical connectors require specialtools, such as a torque tool to loosen coaxial

connectors. If connectors have no wing tabs,astronauts use another special tool to get a firmhold on the rotating ring of the connector.

Portable handles have been attached to manylarger ORUs to facilitate removal or installa-tion. Other tools and crew aids used during theservicing mission are tool caddies (carryingaids), tethers, transfer bags, and protective cov-ers for the Low Gain Antenna (LGA).

When astronauts work within the Telescope’saft shroud area, they must guard against opticscontamination by using special tools that willnot outgas or shed particulate matter. All toolsare certified to meet this requirement.

2.6 Astronauts of the ServicingMission 3A

NASA carefully selected and trained the SM3ASTS-103 crew (see Fig. 2-6). Their unique set ofexperiences and capabilities makes them emi-nently qualified for this challenging assign-ment. Brief biographies of the STS-103astronauts follow.

Curtis L. Brown, Jr., NASA Astronaut(Lieutenant Colonel, USAF). Curtis Brown ofElizabethtown, North Carolina, is commanderof SM3A. He received a bachelor of sciencedegree in electrical engineering from the AirForce Academy in 1978. His career highlightsinclude: 1992 – pilot of STS-47 Spacelab-J, aneight-day cooperative mission between theUnited States and Japan; 1994 – pilot of STS-66,the Atmospheric Laboratory for Applicationsand Science-3 (ATLAS-3) mission; 1996 – pilotof STS-77, whose crew performed a recordnumber of rendezvous sequences (one with aSPARTAN satellite and three with a deployedSatellite Test Unit) and approximately 21 hoursof formation flying in close proximity of the

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satellites; 1997 – commander of STS-85, a 12-day mission that included deployment andretrieval of the CRISTA-SPAS payload; 1998 –commander of STS-95, a nine-day mission dur-ing which the crew supported a variety ofresearch payloads including the Hubble SpaceTelescope Orbital Systems Test Platform anddeployment of the Spartan solar-observingspacecraft.

Scott J. Kelly, NASA Astronaut (LieutenantCommander, USN). Scott Kelly, Discovery piloton SM3A, is from Orange, New Jersey. Hereceived a bachelor of science degree in electri-cal engineering from the State University ofNew York Maritime College in 1987 and a mas-ter of science degree in aviation systems fromthe University of Tennessee, Knoxville, in 1996.Kelly became a naval aviator in 1989 andserved in the North Atlantic, MediterraneanSea, Red Sea and Persian Gulf. He graduatedfrom the U.S. Naval Test Pilot School in 1994,then worked as a test pilot, logging over 2,000flight hours in more than 30 different aircraft.After completing two years of training andevaluation at Johnson Space Center in 1998, hequalified for flight assignment as a pilot. Beforehis selection for the SM3A crew, Kelly per-formed technical duties in the Astronaut OfficeSpacecraft Systems/Operations Branch.

Jean-François Clervoy, ESA Astronaut. Jean-François Clervoy, the RMS operator on SM3A,is from Toulouse, France. Clervoy received hisbaccalauréat from Collège Militaire de SaintCyr l’ Ecole in 1976 and graduated from EcolePolytechnique, Paris, in 1981. He was selectedas a French astronaut in 1985 and became aflight test engineer in 1987. Clervoy served aschief test director of the Parabolic FlightProgram at the Flight Test Center, Brétigny-sur-Orge, where he was responsible for testing andqualifying the Caravelle aircraft for micrograv-

ity. He also worked at the Hermes Crew Office,Toulouse, supporting the European MannedSpace Programs. Clervoy trained in Star City,Moscow, on the Soyuz and Mir systems in1991. He was selected for the astronaut corps ofthe European Space Agency (ESA) in 1992.After a year of training at the Johnson SpaceCenter, he qualified as a mission specialist forSpace Shuttle flights. Clervoy flew twiceaboard Space Shuttle Atlantis and has loggedover 483 hours in space. He served as a missionspecialist on STS-66 in 1994 and as the payloadcommander on STS-84 in 1997.

Steven L. Smith, NASA Astronaut. StevenSmith is payload commander and EVAcrewmember (designated EV1 on EVA Days 1& 3). He was born in Phoenix, Arizona, butconsiders San Jose, California, to be his home-town. He received both bachelor and master ofscience degrees in electrical engineering and amaster’s degree in business administrationfrom Stanford University. Smith served as amission specialist aboard the Space ShuttleEndeavour on STS-68 in 1994. His responsibili-ties during the 11-day flight included Shuttlesystems and Space Radar Lab 2 (the flight’sprimary payload). Smith flew as an EVA crewmember on STS-82, the HST Second ServicingMission, in 1997. He made two 6-hour spacewalks while installing new science instrumentsand upgraded technology. From November1994 until March 1996, Smith was assigned tothe astronaut support team at Kennedy SpaceCenter. The team was responsible for SpaceShuttle prelaunch vehicle checkout, crewingress and strap-in, and crew egress postlanding.

John M. Grunsfeld, Ph.D., NASA Astronaut.John Grunsfeld is an astronomer and an EVAcrewmember (EV2 on EVA Days 1 & 3) on theSM3A mission. He was born Chicago, Illinois.

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Grunsfeld received a bachelor of science degreein physics from the Massachusetts Institute ofTechnology in 1980 and a master of sciencedegree and a doctor of philosophy degree inphysics from the University of Chicago in 1984and 1988, respectively. Grunsfeld reported to theJohnson Space Center in 1992 for a year of train-ing and became qualified for flight selection as amission specialist. A veteran of two spaceflights, he has logged over 644 hours in space.On his first mission, STS-67 in March 1995,Grunsfeld and the crew conducted observationsaround the clock to study the far ultravioletspectra of faint astronomical objects and thepolarization of ultraviolet light coming from hotstars and distant galaxies. Grunsfeld flew onSTS-81 in 1991 on the fifth mission to dock withRussia’s Space Station Mir and the second toexchange U.S. astronauts.

C. Michael Foale, Ph.D., NASA Astronaut.Michael Foale is an EVA crewmember (EV1 onEVA Days 2 & 4) on SM3. He was born inLouth, England, but considers Cambridge,England, to be his hometown. He attended theUniversity of Cambridge, Queens’ College,receiving a bachelor of arts degree in physics,with first-class honors, in 1978. He completedhis doctorate in Laboratory Astrophysics atCambridge in 1982. NASA selected Foale as anastronaut candidate in 1987 and he completedastronaut training and evaluation in 1988. Aveteran of four space flights, Foale has loggedover 160 days in space including 10-1/2 hoursof EVA. He was a mission specialist on STS-45in 1992 and STS-56 in 1993, the first two ATLASmissions to address the atmosphere and itsinteraction with the Sun. He was a member ofthe first Shuttle crew (STS-63) to rendezvouswith Russia’s Mir Space Station in 1995. Foalespent four months aboard Mir in 1997, con-ducting various science experiments and help-ing the crew resolve and repair numerous

malfunctioning systems. He arrived May 17 onSpace Shuttle Atlantis (STS-84) and returnedOctober 6, also on Atlantis (STS-86).

Claude Nicollier, ESA Astronaut. ClaudeNicollier is an EVA crewmember (EV2 on EVADays 2 & 4) on SM3A. A native of Vevey,Switzerland, he received a bachelor of sciencedegree in physics from the University ofLausanne in 1970 and a master of sciencedegree in astrophysics from the University ofGeneva in 1975. He became a Swiss Air Forcepilot in 1966, an airline pilot in 1974, and a testpilot in 1988. In July 1978 ESA selected Nicollieras a member of the first group of Europeanastronauts. Under agreement between ESA andNASA, he joined the NASA astronaut candi-dates selected in 1980 for training as a missionspecialist. A veteran of three space flights,Nicollier has logged more than 828 hours inspace. He participated in the deployment of theEuropean Retrievable Carrier (EURECA) sci-ence platform on STS-46 in 1992. He was theRMS operator on STS-61 in 1993, the first HSTservicing and repair mission. He also served asa mission specialist on STS-75 aboard Columbiain 1996 – the reflight of the Tethered SatelliteSystem and the third flight of the United StatesMicrogravity Payload.

2.7 Servicing Mission Activities

After Discovery berths the Hubble SpaceTelescope in Fall of 1999, the seven-personcrew will begin an ambitious servicing mis-sion. Four days of EVA tasks are scheduled.Each EVA session is scheduled for six hours.

2.7.1 Rendezvous With the HubbleSpace Telescope

Discovery will rendezvous with Hubble in orbit320 nautical miles (512 km) above the Earth.

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Prior to approach, in concert with the SpaceTelescope Operations Control Center (STOCC)at GSFC, Mission Control will command HSTto stow the High Gain Antennas (HGA) andclose the aperture door. As Discoveryapproaches the Telescope, Commander Brownwill control the thrusters to avoid contaminat-ing HST with propulsion residue. During thisapproach the Shuttle crew will be in close con-tact with Mission Control at JSC.

As the distance between Discovery and HSTdecreases to approximately 200 ft (60 m), theSTOCC ground crew will command HST toperform a final roll maneuver to position itselffor grappling. The Solar Arrays (SA) willremain fully deployed parallel to the opticalaxis of the Telescope.

When Discovery and HST achieve the properposition, Mission Specialist Clervoy will oper-ate the robotic arm to grapple the Telescope.Using a camera mounted at the berthing ring ofthe FSS platform in the cargo bay, he willmaneuver HST to the FSS, where the Telescopewill be berthed and latched.

Once the Telescope is secured, the crew willremotely engage the electrical umbilical andswitch Hubble from internal power to externalpower from Discovery. Pilot Kelly also willmaneuver the Shuttle so that the SAs face theSun to recharge the Telescope’s six onboardnickel-hydrogen (NiH2) batteries.

2.7.2 Extravehicular ServicingActivities – Day by Day

Figure 2-7 shows the schedule for four plannedsix-hour EVA servicing periods. These timespans are planning estimates; the schedule will bemodified as needed as the mission progresses.

During the EVAs, HST will be vertical relativeto Discovery’s cargo bay. Four EVA mission spe-cialists will work in two-person teams on alter-nate days. One team is Steve Smith and JohnGrunsfeld; the other is Mike Foale and ClaudeNicollier.

One astronaut, designated EV1, accomplishesprimarily the free-floating portions of the EVAtasks. He can operate from a PFR or while freefloating. The other astronaut, EV2, works pri-marily from an MFR mounted on Discovery’srobotic arm (RMS), removing and installing theORUs on the Hubble. EV1 assists EV2 inremoval of the ORUs and installation of thereplaced units in the SM3A carriers.

To reduce crew fatigue, EVA crewmembersswap places once during each EVA day; thefree floater goes to the RMS MFR and viceversa. Inside Discovery’s aft flight deck, the off-shift EVA crewmembers and the designatedRMS operator assist the EVA team by readingout procedures and operating the RMS.

At the beginning of EVA Day 1 (the fourth dayof the mission), the first team of EVA astronautssuit up and pass through the Discovery airlockinto the cargo bay. To prevent themselves fromaccidentally floating off, they attach safety teth-ers to a cable running along the cargo bay sills.EV1 accomplishes a variety of specific tasks toprepare for that day’s EVA servicing activities.These include removing the MFR from itsstowage location and installing it on the RMSgrapple fixture, installing the Low GainAntenna Protective Cover (LGA PC), deploy-ing the Translation Aids (TA), and removingthe Berthing and Positioning System (BAPS)Support Post from its stowage location andinstalling it on the FSS with the assistance ofEV2.

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Meanwhile, EV2 brings out of the airlock theCrew Aids and Tools (CATs) that will beattached to the handrails of the RMS. Heinstalls the CATs handrail to the RMS and thecolor television camera (CTVC) on the MFR.The IVA RMS operator then moves EV2 to theBAPS Support Post (BSP) installation worksiteto install the BSP forward end on the BAPS.

EVA Day 1: Change out three RSUs. Removecaps and open the NICMOS “Coolant In” and“Coolant Out” valves. Install VIK on HSTbatteries.

During EVA Day 1, Smith and Grunsfeld arescheduled to replace three new RSUs in the

Telescope. They will also remove caps from theNICMOS “Coolant In” and “Coolant Out”valves. Any ice contained within those valveswill sublimate in the vacuum of space and facil-itate installation of a replacement cooler forNICMOS on SM3B. Additionally, the astronautswill install VIKs on the six batteries aboard HST.

Once in the payload bay, the astronauts beginthe initial setup, which includes installation ofthe LGA protective cover, TA deployments,and installation and deployment of the BSP.

The BSP is required to dampen the vibrationthat the servicing activities will induce into thedeployed SAs. Prior to the BSP installation on

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Priority Task Times (stand-alone)

1. RSUs (RSU-1, -2, -3)2. VIK3. Advanced Computer4. FGS (FGS-2)5. SSAT (SSAT-2)

6. SSR (ESTR-3)7. Bay 5-10 MLI Repair8. NICMOS valves open

1:55 ea1:101:503:001:30

1:152:25 total 0:20

3:07 OT#1. FS/LS MLI RepairOT#2. Handrail Covers Bays A & JOT#3. ASLR for +V2 Doors

Required Setup

Optional Tasks & Priorities

Task Times1:00 (1st day)0:15 (nth day)

0:30 (nth day)1:00 (last day)

Setup

Closeup

6-hour EVA period

EVA Day 1

EVA Day 2

EVA Day 3

EVA Day 4

Includes BAPS Post installationAft Shroud Door latch contingencycrew change positions in Manipulator Foot RestraintHigh Gain Antenna deployment

VIKClose0:30

Close0:30

RSU 1, 2 & 3Setup°1:00

ComputerSet:15



MF

R

MF

R

AS

D

Val

ves

Computer FGS2

r rr@305° r@30° r@-V3; p@90°(±)@0°(±)-30°→0°(±)SA@-30°

prp

(±) 0°-30°SA

FSS

SA

FSS

A

FSS

SA

FSS

(±)SA@20°p@90°; r@+V3

(±) 0°→20°

(±)SA@-20°p@90°; r@+V3p@90°; r@-V2 r

(±)SA@0° (±) 0°→100°

p@90°; r@-V3p@85°; r@-V3p:90°→85°(±) 100°→0°

(+) -20°→0°

(±) 20°→0°

(±) 0°→90°

r:-V3→-V2

(-):0°→15°→-20°

p

Goddard Space Flight Center

° =ASD =MFR =HGA =

SSRF NOBL FGS-2 SA slew FSS pivot

FSS rotationberthing position

HST-V3RSU VIK

Valves Open

OCEK Computer

+V345°

90°90°

0° 0°0°

-45°-V3

+V2-V2

Hubble Space TelescopeFlight Systems and Servicing Project

OT#1 MLIFS/LS 5, 6 & 7

Set:15 H

GA Closeout

1:00OT #2&/or #3

OT#1 MLIFS/LS 1 & 2

OT#1 MLIFS/LS 3 & 4

SSA XmtrSet:15 SSR

MLI RepairBays 5, 6, 7, 8, 9, 10

Close0:30OCE

MF

R

Fig. 2-7 Detailed schedule of extravehicular activities and SA and FSS positions during SM3A

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EVA Day 1, the IVA team commands the HSTto an 85-degree pivot angle. The two centerpush-in-pull out (PIP) pins are installed eachday and removed each night in the event thatthe Shuttle must make an emergency return toEarth. Steve Smith (EV1) removes the BSP fromits stowage position in the cradle of the FSSand hands the forward end to John Grunsfeld(EV2) who installs his end to the BAPS ringwith a PIP pin. Smith then installs the aft endof the BSP to the FSS cradle with a PIP pin.Finally the BSP is commanded to its 90-degreelimit and the two center PIP pins are installed.

After the BSP is installed and other initial setuptasks are completed, the crew starts the specifictasks for the RSU change-outs. First Smith,who is free floating, retrieves the STS PFR andArticulating Socket and temporarily stowsthem at the aft ORUC. Smith and Grunsfeld (inthe MFR), move to the COPE to retrieve thereplacement RSU-2. To accomplish theremoval, they open three COPE T-handle lidlatches and the COPE lid, then release thetransport module T-handle latch and open it.They remove the replacement RSU-2 and stowit in the ORU transfer bag. Next they close thetransport module and the COPE lid andengage two T-handle latches.

The astronauts then translate to the aft shroud.They inspect the area for excessive particulatesor debris and when satisfied that the area isclear, Smith and Grunsfeld open the HST –V3aft shroud doors by retracting two Fixed HeadStar Tracker (FHST) seals and disengaging fourdoor latches.

Smith then installs the PFR and articulatingsocket in the HST aft shroud and ingresses.Smith and Grunsfeld remove the old RSU-2 bydemating two wing tab connectors and disen-gaging three 7/16-in. hex-head captive spring-

loaded fasteners. Smith and Grunsfeld removethe RSU-2 replacement from the ORU transferbag. They remove the two connector caps andinstall the replacement RSU-2 in HST, engagingthree fasteners and mating the two connectors(see Fig. 2-8). Finally, they replace the old RSU-2in the ORU transfer bag.

Smith steps out of the PFR, reconfigures it forthe RSU-3 position, and gets back into the PFR.Meanwhile, Grunsfeld maneuvers to the COPEand opens the lid, stowing the old RSU-2 andretrieving the RSU-3 replacement. He stows itin the ORU transfer bag, then temporarily clos-es the COPE lid (two latches) and maneuversback to the aft shroud.

Smith and Grunsfeld remove the old RSU-3 bydemating two wing tab connectors and disen-gaging three 7/16-in. hex captive spring-loaded fasteners. They then remove thereplacement RSU-3 from the ORU transfer bagand install it, first removing the two connectorcaps, then seating it in HST. They engage threefasteners, mate the two connectors and placethe old RSU-3 in the ORU transfer bag.

Smith exits the PFR and reinstalls it in the aftshroud door closing position. Grunsfeld per-forms a video close-out on the two newlyinstalled RSUs. He then maneuvers to the COPEand stows RSU-3. He retrieves the replacementRSU-1 and stows it in the ORU transfer bag. Hetemporarily closes the COPE lid (one latch)and maneuvers back to the aft shroud.

Grunsfeld, on the MFR at the end of the RMS,removes the old RSU-1 by demating two wingtab connectors and disengaging three 7/16-in.hex captive spring-loaded fasteners. Theyinstall the replacement RSU-1 by removing thetwo connector caps, removing the replacementfrom the ORU transfer bag, seating it in HST

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and engaging three fasteners, and mating twoconnectors. After the old RSU-1 is stowed inthe ORU transfer bag, Grunsfeld performs thereplacement RSU-1 video close-out.

While positioned at the Aft Shroud, Smith andGrunsfeld then perform the NICMOS valvereconfiguration. They remove two “CoolantIn” and “Coolant Out” bayonet caps and openthe “Coolant In” and “Coolant Out” valves.Grunsfeld performs the NICMOS valve recon-figuration video close-out.

Grunsfeld partially closes the –V3 AftShroud doors as Smith ingresses the PFR.With Smith’s assistance, Grunsfeld closes theleft and right doors, engages the four doorlatches, checks the door seals, and extends thetwo FHST light seals. Smith egresses thePFR/Articulating Socket, removing it fromHST and restowing it in cargo bay. Grunsfeldmaneuvers to the COPE and stows the oldRSU-1 in the transport module, closes thetransport module lid, then fully closes theCOPE lid, engaging all three latches.

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Smith and Grunsfeld now prepare to installVIKs on the HST batteries. Smith takesGrunsfeld’s place on the MFR and Grunsfeldbecomes the free floater after the MFR swap.

Grunsfeld translates to airlock, retrieves theVIK Caddy, translates to Bay 3, and transfersthe VIK Caddy to Smith. Smith (in the MFR)opens the Bay 3 door by disengaging six

J-hooks. Grunsfeld retrieves the handrail covercaddy, inspects the handrails around Bays 2and 3, and installs the handrail covers ifneeded. Smith demates three Bay 3 battery con-nectors (one at a time) and installs a VIK in-linewith each battery connector (see Fig. 2-9).Smith performs the Bay 3 VIK video close-out,closes the door, and engages the six doorJ-hooks.

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Smith then opens Bay 2 door by disengagingsix J-hooks. He demates three Bay 2 batteryconnectors (one at a time) and installs a VIK in-line with each battery connector. He performsthe Bay 2 VIK video close-out, closes the door,engages the six door J-hooks, and stows theVIK Caddy on the MFR.

For the daily close-out, Grunsfeld removes thecenter pins on the BSP, inspects the FSS mainumbilical, and retracts the TAs while Smithprepares the CATs installed on the MFRhandrail for return into the airlock.Additionally, Smith releases the MFR safetytether from the grapple fixture for contingencyEarth return and releases the lower CTVCcable. After the completion of EVA Day 1, bothastronauts return to the airlock with the Day 1CATs installed on the MFR handrail.

EVA Day 2: Replace DF-224 computer withAdvanced Computer and install Bay 1 NOBL.Change out FGS-2.

During EVA Day 2, EVA astronauts Nicollier(EV1) and Foale (EV2) are scheduled to replaceHST’s DF-224 computer with a faster, morepowerful unit called the Advanced Computer.They will also change out a degraded FGS inposition number 2 (see Fig. 2-10).

Fewer daily setup tasks are required for EVADay 2 than for EVA Day 1. Foale exits the air-lock with the EVA Day 2 required CATsinstalled on the MFR handrail. Nicollier recon-nects the safety strap on the MFR and connectsthe CTVC cable to the RMS end effector. Foalethen installs the MFR handrail and the CTVCon the RMS. Nicollier deploys the TAs andinstalls the BSP center PIP pins.

The astronauts’ first servicing task for the dayis to change out the DF-224 computer with the

new Advanced Computer. Foale (in the MFR)maneuvers to the COPE Converter TransportModule and retrieves the Connector ConverterCaddy. Foale then maneuvers to the Bay 1worksite and installs handrail covers on thehandrails adjacent to the Bay 1 door if hedeems them necessary.

Meanwhile, Nicollier (free floating) translatesto the LOPE and opens the LOPE lid by dis-engaging four J-hook bolts. He removes theY-harness and mates one end of it to theAdvanced Computer. Nicollier also preparesthe Advanced Computer for Foale to removefrom the LOPE , releasing five of its six J-hooksand temporarily closing the LOPE lid andengaging a single J-hook.

Foale opens the Bay 1 door by disengaging sixJ-hooks and sets the integral door stay.Nicollier translates back to Bay 1 to assist Foalewith the computer swap. Foale removes theDF-224 computer by demating its nine electri-cal connectors and disengaging six J-hooks. Hetransfers it to Nicollier positioned at the MFRstanchion. Foale then installs the seven connec-tor converters on the HST harnesses.

Nicollier transfers the DF-224 computer toFoale for the maneuver to the LOPE worksite.Foale transfers the DF-224 computer back toNicollier and opens the LOPE lid by disengag-ing one J-hook. Foale removes the AdvancedComputer from the LOPE by disengaging oneJ-hook, then maneuvers back to Bay 1.

Meanwhile, Nicollier installs the DF-224 com-puter in the LOPE, engaging six J-hooks, thencloses the LOPE lid and engages four J-bolts.Nicollier translates back to the Bay 1 worksiteto assist Foale. Foale installs the AdvancedComputer in Bay 1 by engaging six J-hooks,mating two remaining Y-harness connectors,

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and mating seven harnesses. He then performsthe Advanced Computer video close-out,releases the door stay, and closes the Bay 1door, engaging six J-hooks.

After the computer change-out, Nicollier andFoale open the NPE, retrieving the Bay 1 NOBLand NOBL plug stringer from the NPE and

closing it. They install the Bay 1 NOBL on theBay 1 door by installing four NOBL vent holeplugs and connecting the NOBL ground cap tothe door J-bolt.

As the astronauts prepare to undertake theFGS-2 change-out, Nicollier replaces Foale onthe MFR and Foale becomes the free floater.

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Fig. 2-10 Fine Guidance Sensor change-out

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Nicollier and Foale retrieve a PFR/PFRExtender and set it up on the HST in FootRestraint 11. Nicollier retrieves the outboardFGS handhold from the ORUC forward fixtureand installs handrail covers at FGS-2 worksiteas needed. Foale deploys the aft fixture.

The astronauts open and secure the FGS baydoors and demate the FGS connectors andground strap. Nicollier installs four guidestuds on the FGS, then installs the FGS hand-hold and loosens the A latch. The FGS is nowready to be removed.

With Foale in the PFR and Nicollier holding theFGS handhold, the team removes the FGS asFoale gives clearance instructions. To ensuresuccessful installation of the replacement FGS,the team may practice reinsertion of the oldFGS back into the HST guiderails withoutlatching the latches or mating the connectorsfor mass handling evaluation.

After this practice insertion, Nicollier removesthe FGS and stows it on the aft fixture. Foalethen conducts a bay inspection. Next, Foaletranslates to the ORUC FSIPE, opens the lid,latches the lid in the open position, anddemates and secures the ground strap. In par-allel, Nicollier retrieves the other FGS hand-hold from the ORUC forward fixture andpositions himself for its installation on thereplacement FGS in the FSIPE.

After installing the handhold on the replace-ment FGS, Nicollier loosens the A latch. Then,with instructions and assistance from Foale,Nicollier removes the replacement FGS fromthe FSIPE and translates with it to the HST.Foale closes the FSIPE lid and engages one lidlatch. He then ingresses the PFR mounted onthe HST and prepares to remove the FGS mir-ror cover. Nicollier presents the FGS to Foale so

that the FGS mirror cover can be removed.Foale tethers to the cover, releases the slide locklever, and operates the handle to remove themirror cover. When the mirror cover isremoved, IVA repositions Nicollier on the MFRso the FGS can be installed.

Nicollier inserts the FGS with assistance fromFoale (see Fig. 2-10). Nicollier tightens the Alatch, removes the FGS handhold, and stows ittemporarily on the MFR, then installs the groundstrap and electrical connectors. After the electri-cal connectors are mated, IVA informs MissionControl to perform an aliveness test. Nicolliertakes the close-out photographs (video), closesand latches the doors, and checks the door seals.He then retrieves the HST PFR and stows it forlater reinstallation in the cargo bay. Foale stowsthe mirror cover on the ORUC.

Nicollier retrieves the FGS from the aft fixture.Foale stows the aft fixture, translates to theFSIPE, and opens the lid. Nicollier translates tothe FSIPE with the FGS. Foale assists Nicollierduring the FGS insertion into FSIPE. After fullinsertion, Nicollier tightens the A latch whileFoale mates the ground strap. Foale releasesthe FSIPE on-orbit latches and closes andsecures the lid. Nicollier stows the replace-ment FGS handhold in the forward fixture.Foale retrieves the PFR that was used for theFGS change-out and reinstalls it on the Shuttlebay adaptive payload carrier.

For the EVA Day 2 daily close-out, Foaleremoves the center pins on the BSP, inspects theFSS main umbilical mechanism, and retracts theTAs while Nicollier prepares the CATs installedon the MFR handrail for return to the airlock.Foale releases the MFR safety tether from theRMS in the event of contingency Earth return.Both astronauts return to the airlock with theMFR handrail and its installed CATs.

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EVA Day 3: Mate additional OCE-EK connec-tors for the new FGS-2. Change out SSAT.Replace E/STR #3 with SSR #3. Repair MLI.

On EVA Day 3, EVA astronauts Smith andGrunsfeld are scheduled to mate additionalOCE-EK connectors to allow for the on-orbitalignment optimization of the replacementFGS adjustable, articulated, fold flat #3 mirror.They will replace an S-band single access trans-mitter (SSAT) that failed in 1998. An identicalbackup transmitter has functioned perfectlyand HST’s observing program has not beenaffected. The astronauts also will install an SSRin place of an E/STR. Finally, they will under-take MLI repairs over the doors on Bays 5through 10.

The daily setup for EVA Day 3 is identical toEVA Day 2. Smith still exits the airlock with therequired CATs and CTVC installed on the MFRhandrail and transfers them to Grunsfeld. Hereconnects the safety strap on the MFR/RMSand connects the CTVC to the RMS end-effec-tor. Grunsfeld installs the MFR handrail andthe CTVC on the RMS. Smith deploys the TAand installs the BSP center PIP pins.

The first task of the day is the mating of theOCE-EK connectors. Grunsfeld on the RMStranslates to the Optical Telescope Assembly(OTA) Bay C door and opens the door via thethree J-hooks. He then demates the P11 connec-tor from the OCE, and mates the OCE-EK J/P11A with the OCE J11 and HST P11 connectors.The close-out photographs are taken and theBay C door is secured with its three J-hooks.

Grunsfeld on the MFR then opens the Bay 5 doorby disengaging six J-hooks, then secures thedoor in the open position. He prepares the oldSSAT-2 for removal by demating two circularconnectors and three coaxial connectors. He

installs a fastener retention block, removes eightnon-captive bolts/washers, and removes the oldSSAT-2. Grunsfeld transfers the SSAT-2 to Smith,who is free floating.

Smith brings the old SSAT-2 to the COPE, opensit, and retrieves the replacement SSAT. Hestows the old SSAT in the COPE, closes it, andtranslates back to Bay 5. Grunsfeld receives thereplacement SSAT-2 from Smith, installs it onthe Bay 5 door by engaging seven bolts, remov-ing the two connector caps, mating two circularconnectors, and mating three coaxial connec-tors (see Fig. 2-11). Grunsfeld then performs thevideo close-out. Because the SSR installation isthe next task, the Bay 5 door is left open.

The next change-out task is replacement of theE/STR-3 with a newer technology SSR-3.Grunsfeld demates the T-harness installed dur-ing SM2 and two other E/STR-3 connectors,being careful not to allow the powered-on P1connector to touch structure ground. Hesecures the P1 connector behind the cable har-ness along the side wall, demates the four key-hole fasteners, and removes the E/STR.

Smith and Grunsfeld translate to the COPE withthe E/STR-3. Smith releases the COPE lid latch-es and opens the lid. Smith and Grunsfeldremove SSR-3 from the COPE transport moduleand install the E/STR-3 into the COPE transportmodule. Grunsfeld translates back to the Bay 5worksite. Smith stows the T-harness in the COPElid, retrieves the J-harness, then closes and latch-es the lid. He then translates with the J-harness toBay 5 to assist with the SSR installation.

In parallel to Smith’s closure of the COPE,Grunsfeld installs the SSR into the #3 position(see Fig. 2-12). SSR-3 is mounted via four key-hole bolts. The SSR-3 connector caps areremoved and the connectors mated, then the

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J-harness is mated to the J-4 connectors onSSR-3 and SSR-1. The J-harness providesincreased capability for the SSR and HST.Grunsfeld takes the close-out video. With theassistance of Smith, Grunsfeld closes the doorand secures it with the six J-hooks.

The crewmembers now swap roles: Smith movesto the MFR and Grunsfeld becomes free floating.

Grunsfeld retrieves the MLI recovery bagsfrom ORUC ATM. Smith opens the NPE andretrieves the NOBLs for Bays 5 and 6 and a sin-gle NOBL plug stringer. He temporarily closesthe NPE by securing one latch.

Grunsfeld assists Smith throughout the NOBLinstallation process. Smith installs the Bay 6NOBL to the door stop and secures the handle

Fig. 2-11 S-Band Single Access Transmitter change-out

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attachment to the door handle. He removes thetwo Bay 7 tie wraps and stows them in thetrash bag, then removes the two Bay 8 patchesand stows them in an MLI recovery bag. Thepatches were installed during SM2.

Smith removes the original Bay 5 door MLIand stows it in an MLI recovery bag. He

installs the Bay 5 NOBL and secures it with thefour NOBL plugs, then connects the NOBLground cap to a J-bolt (see Fig. 2-13).

Smith retrieves the Bay 7 and Bay 8 NOBL andtwo vent plug stringers from the NPE andtemporarily closes the NPE, securing a singlelatch. He installs the Bay 7 NOBL, securing it

Fig. 2-12 Installation of Solid State recorder

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with the four NOBL plugs, then connects theNOBL ground cap to a J-bolt.

Smith installs the Bay 8 NOBL and secures itwith the NOBL plugs, then connects theNOBL ground cap to a J-bolt. He removes thetwo MLI patch kits installed on Bay 10 dur-ing SM2 and stows them in an MLI recoverybag, then removes the two original Bay 10

door MLI pieces and stows them in an MLIrecovery bag.

Smith retrieves the Bay 9 and Bay 10 NOBLand NOBL plugs from the NPE and closes theNPE, securing all three latches. He installs theBay 9 NOBL to the door stop and secures thehandle attachment to the door handle. Smiththen installs the Bay 10 NOBL and secures it

Fig. 2-13 New outer blanket layer installation

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with the four NOBL plugs, then connects theNOBL ground cap to a J-bolt.

For the Day 3 EVA close-out, Grunsfeldremoves the center pins on the BSP, inspectsthe FSS main umbilical mechanism, andretracts the TAs while Smith prepares the CATsinstalled on the MFR handrail for return intothe airlock. Smith releases the MFR safetytether from the RMS in the event of contin-gency Earth return. After completion of theEVA Day 3 close-outs, both astronauts return tothe airlock with the MFR handrail and itsinstalled CATs.

EVA Day 4: EVA Day 4 consists of a series ofscheduled and optional tasks: Install SSRFson the forward shell/light shield. Installhandrail covers on the Bays A and J handrails.Install Aft Shroud Latch Repair (ASRL) Kitson the +V2 aft shroud doors.

EVA crewmembers Foale and Nicollier arescheduled to install new insulation material onthe outer surfaces of the Telescope where MLIhas become degraded. Optional tasks are toinstall handrail covers where the paint cover-ing has debonded and install repair kits tofunctionally replace the degraded aft shrouddoor latches.

EVA Day 4 has a daily setup similar to that ofthe prior EVA days. Nicollier starts the day inthe MFR. Foale exits the airlock with therequired CATs installed on the MFR handrailand the CTVC and transfers them to Nicollier.Nicollier reconnects the safety strap on theMFR/RMS and connects the CTVC cable to theRMS end-effector. Nicollier then installs theMFR handrail and the CTVC to the RMS.Foale deploys the TAs and installs the BSPcenter PIP pins.

Foale opens the ORUC ASIPE by disengagingthe five lid latches. Nicollier opens the lid.Foale retrieves the five SSRF rib clamps andNicollier retrieves SSRFs 5, 6, and 7. Nicolliercloses the ASIPE lid temporarily by engaging asingle lid latch.

Foale and Nicollier install the five SSRF ribclamps on the HST station 358 rib. Working as ateam, Foale and Nicollier install SSRFs 5, 6, and 7.

Nicollier opens the ASIPE and retrieves SSRFs1, 2, 3, and 4. He then fully closes the ASIPE,engaging the five lid latches. Nicollier andFoale translate to HST and installs SSRFs 1 and 2.

When that task is complete, the IV crew rotatesthe FSS to the HST 30-degree position. Nicollierand Foale install SSRFs 3 and 4. Both theninstall two ASLR Kits on the two degraded +V2door latches if needed. The handrail coversalso can be installed, if needed, at this time.

For the final daily close-out, Foale stows theTAs, removes and stows the LGAPC, inspectsthe FSS main umbilical mechanism, andinspects the P105/P106 connector covers.Nicollier installs the CATs on the MFR handrailfor return to the cabin and demates and stowsthe MFR from the RMS. When the tasks arecomplete, both crewmembers enter the airlockwith the CATs-laden MFR handrail.

EVA Contingency Day. An unscheduled EVAday has been allocated for enhancing payloadmission success and for any payload require-ments on the HST redeployment day.

Redeploying the Telescope. The day followingEVA Day 4 will be devoted to any unscheduledEVA tasks and redeployment of the HST intoEarth orbit (see Fig. 2-14).

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The SAs are slewed to the Sun to generate elec-trical power for the Telescope and to charge thebatteries, and HGAs are commanded to theirdeployed position. When the battery chargingis complete, the RMS operator guides therobotic arm to engage HST’s grapple fixture.The ground crew commands Hubble to switchto internal power. This accomplished, crewmembers command Discovery’s electricalumbilical to demate from Hubble and open theberthing latches on the FSS. If any Telescopeappendages fail to deploy properly, two mis-

sion specialists can perform EVA tasks on theredeployment day, manually overriding anyfaulty mechanisms.

2.8 Future Servicing Plans

As the Hubble Space Telescope enters the 21stcentury, other enhancements are planned. Athird-generation instrument, the AdvancedCamera for Surveys (ACS), will greatly enhanceHST’s imaging capabilities. Shuttle astronautsplan to install the camera during SM3B.

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Fig. 2-14 Redeploying the Space Telescope

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ACS is truly an advanced camera, withpredicted performance improvementsone to two orders of magnitude over cur-rent Hubble science instruments. Itsunique characteristics and dramaticallyimproved efficiencies will exploit the full

potential of the Telescope to serve theneeds of the science community.

Periodic upgrades and servicing will ensurethat Hubble continues to yield remarkableadvances in our knowledge of the universe.

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golden era of space exploration anddiscovery began April 24, 1990, with thelaunch and deployment of NASA’s

Hubble Space Telescope (HST). During nearly10 years of operation, Hubble’s rapid-fire rateof unprecedented discoveries has invigoratedastronomy. Not since the invention of the tele-scope nearly 400 years ago has our vision of theuniverse been so revolutionized over such ashort stretch of time.

As the 12.5-ton Earth-orbiting observatorylooks into space unburdened by atmosphericdistortion, new details about planets, stars, andgalaxies come into crystal clear view. TheTelescope has produced a vast amount of infor-mation and a steady stream of images thathave astounded the world’s astronomical andscientific communities. It has helped confirmsome astronomical theories, challenged others,and often come up with complete surprises forwhich theories do not yet even exist.

Hubble was designed to provide three basiccapabilities:

• High angular resolution – the ability toimage fine detail

• Ultraviolet performance – the ability toproduce ultraviolet images and spectra

• High sensitivity – the ability to detect veryfaint objects.

Each year NASA receives over a thousand newobserving proposals from astronomers aroundthe world. Observing cycles are routinely over-subscribed by a factor of three or greater.

The Telescope is extremely popular because itallows scientists to get their clearest view everof the cosmos and to obtain information on thetemperature, composition, and motion of celes-

tial objects by analyzing the radiation emittedor absorbed by the objects. Results of HSTobservations are being presented regularly inscientific papers at meetings of the AmericanAstronomical Society and other major scientificconferences.

Although Hubble’s dramatic findings to dateare too numerous to be described fully in thisMedia Reference Guide, the following para-graphs highlight some of the significant astro-nomical discoveries and observations in threebasic categories:

• Planets• Formation and evolution of stars and planets• Galaxies and cosmology.

For further detailed information, visit theSpace Telescope Science Institute website at:http://oposite.stsci.edu.

3.1 Planets

Since 1990 Hubble has kept an “eye” on oursolar system planets: a comet slamming intoJupiter, clouds on Uranus, monster storms onNeptune, auroras on Jupiter and Saturn, andunusual weather on Mars.

Mars. The Telescope captured images of anenormous cyclonic storm system raking thenorthern polar regions of Mars on April 27,1999 (see Fig. 3-1). Nearly four times the size ofTexas, the storm consists of water ice cloudslike those found in storm systems on Earthrather than the dust typically found on Mars.Although similar to so-called “spiral” stormsobserved more than 20 years ago by NASA’sViking Orbiter spacecraft, this Martian storm isnearly three times the size of the largest previ-ously detected Martian storm system.

AHUBBLE SPACE TELESCOPE SCIENCE AND DISCOVERIES

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The Telescope monitored the Red Planet’sweather in the spring and summer of 1997,providing detailed reports to help plan thelanding of NASA’s Mars Pathfinder and thearrival of Mars Global Surveyor. Pictures takenabout a week before the landing of thePathfinder spacecraft show a dust stormchurning through the deep canyons of VallesMarineris, just 600 miles (1,000 kilometers)south of the landing site. Remarkable changesin the behavior of dust and water ice clouds onthe planet were recorded from July 9 to 11,indicating that weather changes on Mars arevery rapid – possibly chaotic.

Hubble took pictures of the entire planet ofMars during its closest approach to Earth ineight years: 54 million miles (87 million kilo-meters). Using the HST images, astronomers

made a full-color global map of Mars (seeFig. 3.2). Latitudes below about 60 degreessouth were not viewed because the planet’snorth pole was tilted towards Earth duringthat time.

Saturn and Jupiter. Astronomers usedHubble’s ultraviolet-light camera, the SpaceTelescope Imaging Spectrograph (STIS), toprobe auroras – curtains of light that seem todance above the north and south poles ofSaturn and Jupiter.

Saturn’s auroras rise more than 1,000 milesabove the cloud tops (see Fig. 3-3). Like thoseon Earth, these auroral displays are caused byan energetic wind from the Sun that sweepsover the planet. But unlike Earth, Saturn’sauroras have been seen only in ultraviolet light.

Fig. 3-1 On April 27, 1999, Hubble took pictures of a Martian storm more than 1000 miles(1600 km) across. Left: an image of the polar storm as seen in blue light (410 nm).Upper right: a polar view of the north polar region, showing the location of the stormrelative to the classical bright and dark features in this area. Lower right: an enhancedview of the storm processed to bring out additional detail in its spiral cloud structures.

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The auroras are primarily shaped and poweredby a continual tug-of-war between Saturn’smagnetic field and the flow of charged particlesfrom the Sun.

The Telescope has also taken many images ofJupiter’s aurora in ultraviolet light. Jovianauroral storms develop when electricallycharged particles trapped in the magnetic fieldsurrounding the planet spiral inward at highenergies toward the north and south magneticpoles. As these particles strike the upperatmosphere, they excite atoms and moleculesthere, causing them to glow.

Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) hasprovided a multicolor view of Saturn,giving detailed information on the clouds

and hazes in the planet’s atmosphere (seeFig. 3-4).

The blue colors in the NICMOS image of Saturnindicate a clear atmosphere down to a maincloud layer. Most of the northern hemispherevisible above the rings is relatively clear. Thedark region around the south pole correspondsto the region of auroral activity and displaysdifferent reflective properties. The green andyellow colors define a haze above the maincloud layer; the haze is thin where the colorsare green and thick where they are yellow. Dueto Saturn’s east-west winds, these layers arealigned along latitude lines. The red andorange colors indicate clouds reaching highinto the atmosphere. Red clouds are higherthan orange clouds. The densest regions of twostorms near Saturn’s equator appear white.

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Fig. 3-2 The HST WFPC2 captured these images between April 27 and May 6, 1999, whenMars was 54 million miles (87 million kilometers) from Earth. From this distance thetelescope could see Martian features as small as 12 miles (19 kilometers) wide.

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Fig. 3-3 This is the first image of Saturn’s ultraviolet aurora taken by the STIS in October 1997,when Saturn was 810 million miles (1.3 billion kilometers) from Earth.

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The Telescope captured an invasion of Jupiterin 1994 when 21 fragments of the cometShoemaker-Levy 9 slammed into the giantplanet. As each comet fragment crashed intoJupiter, mushroom-shaped plumes wereexpelled from the planet. The largest frag-ment impacts created Earth-sized “bull’s-eye” patterns on Jupiter. Hubble’s record of

the comet’s bombardment, combined withresults from other space-borne and Earth-based telescopes, sheds new light on Jupiter’satmospheric winds and its immensemagnetic field.

Uranus. Astronomers used Hubble images,taken from 1994 to 1998, to create a time-

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Fig. 3-4 Saturn viewed in the infrared shows atmospheric clouds and hazes.K9322-304

lapse movie that shows, for the first time,seasonal changes on Uranus. Once considereda bland-looking planet, Uranus is nowrevealed as a dynamic world with rapidlychanging bright clouds – some circling theplanet at more than 300 miles per hour – anda fragile ring system that wobbles like anunbalanced wagon wheel. The clouds prob-ably are made of crystals of methane, whichcondense as warmer bubbles of gas well upfrom deep in the planet’s atmosphere. Themovie clearly shows for the first time thewobble in the ring system, comprisingbillions of tiny pebbles. The wobble may becaused by Uranus’s shape, which is like aslightly flattened globe, and the gravitationaltug from its many moons.

Uranus is tilted completely over on its side,giving rise to 20-year-long seasons andunusual weather. For nearly a quarter of theUranian year, the Sun shines directly overeach pole, leaving the other half of theplanet in a long, dark, frigid winter. Nowthe northern hemisphere of Uranus iscoming out of the grip of its two-decadewinter.

Neptune. The unusual weather patterns ofNeptune also are the subject of a time-lapserotation movie using HST images. Themovie shows that the planet has some of thewildest, weirdest weather in the solarsystem. The Telescope captured the mostinsightful images to date of a planet whoseblustery weather – monster storms andequatorial winds of 900 miles per hour – isstill a mystery to scientists.

Pluto. Even the outermost planet in our solarsystem hasn’t escaped Hubble’s scrutiny. TheTelescope unveiled the never-before-seensurface of Pluto, orbiting at the dim outerreaches of the solar system nearly threebillion miles (five billion kilometers) from theSun. Pluto is only two-thirds the size of theEarth’s moon but is 12,000 times fartheraway.

The Faint Object Camera (FOC) imaged nearlythe entire surface of Pluto as it rotated throughits 6.4-day period in late June and early July1994. The images, made in blue light, showthat Pluto is an unusually complex object,with large-scale contrast.

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3.2 Formation and Evolution of Stars and Planets

Planets form from leftover star-making material,the pancake-shaped disks of dust and gasswirling around nascent stars. Hubble hasprovided evidence that these disks are commonaround developing stars. The Telescope also

has expanded astronomers’ knowledge aboutthe life cycle of stars. It has peeked behind thedusty veil of star birth regions to chronicle thegenesis of stars and captured the colorfulshrouds around dying stars.

The giant galactic nebula NGC 3603 (see Fig. 3-5)contains a community of stars in various stages

Fig. 3-5 The crisp resolution of the Telescope reveals various stages of the life cycle of stars inthis single view of the giant galactic nebula NGC 3603.

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of life. Dark clouds in the upper right are so-called Bok globules, dense clouds of dust andgas that harbor embryonic stars. Giant pillars ofgas (below and right of the blue cluster of stars),composed of cold molecular hydrogen, serve asan incubator for new stars. Ultraviolet light fromthe nearby star cluster is sculpting the pillars,which are reminiscent of the famous pillars theHubble photographed in the M16 Eagle Nebula.

Below the cluster are two compact, tadpole-shaped protoplanetary disks. Near the center ofthe image is a starburst cluster dominated byyoung, hot Wolf-Rayet stars and early O-typestars. A torrent of ionizing radiation and faststellar winds from these massive stars hasblown a large cavity around the cluster. To theupper left of center is the aging blue supergiantSher 25. The star has a unique ring of glowinggas that is a galactic twin to the famous ringaround the exploding star, supernova 1987A.

Are there other planetary systems like our own?If so, how are they created? Astronomers are onthe hunt to find out. They are studying debrisdisks, composed of dust and gas, whirlingaround developing stars. This dust and gascould eventually clump together to formplanets. Hubble has imaged such disks aroundseveral stars ranging in age from 1 million to 10million years. The Telescope’s arsenal ofcameras has recorded the disks in light fromultraviolet to near infrared. By chronicling thedisks at different stages of a star’s early life,astronomers are adding information to theplanet-making recipe.

Hubble’s sharp vision has analyzed disksaround a group of nearby young stars in theconstellation Taurus. Images taken in the near-infrared as well as visible light reveal howinfalling material forms first into a thin diskaround the young stars. Then a portion of the

disk material is shot back into the interstellarmedium in the form of opposing jets that emergeperpendicularly from the center of the disk. Theaccretion disks are huge, averaging more thanten times the diameter of our solar system.

The Wide Field and Planetary Camera 2(WFPC2) spotted the first example of an edge-on disk in a young double-star system. Theimages offer further evidence that planet forma-tion should be possible in binary star systems.Theory holds that gravitational forces betweentwo stars in a binary system tend to tear apartfragile planet-forming disks, but the Hubbleimages reveal evidence of dust clumpingtogether in the disks – a first step on the longroad to planet formation.

Looking at a disk around a slightly older star,astronomers using STIS found evidence of moreclumping of material. This two- to four-million-year-old star, called AB Aurigae, is in theconstellation Auriga. The clumps are muchfarther away from the star than is Pluto, theoutermost solar system planet, from our Sun.

Turning its gaze to a fully developed star,HR 4796A, about 10 million years old, NICMOScaptured images of a wide dust ring (6.5 billionmiles). The rings around the star resemblethose of Saturn, but on a grander scale. Gapsbetween the rings could be the result of unseenbodies sweeping out lanes. The star is 220 light-years away in the constellation Centaurus.

Hubble also has provided images of what maybe the most luminous known star, called thePistol Star, which is big enough to fill the diam-eter of Earth’s orbit. This celestial mammothreleases up to 10 million times the power of theSun, unleashing as much energy in threeseconds as our Sun does in one year. TheNICMOS image also reveals a bright nebula

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created by massive stellar eruptions. The nebulais so big (four light-years across) that it wouldspan nearly the distance from the Sun to AlphaCentauri, the nearest star to Earth’s solar system.

Astronomers estimate that when the Pistol Starwas formed one to three million years ago, it mayhave weighed up to 200 times the mass of the Sunbefore shedding much of its bulk in violent erup-tions. The star is approximately 25,000 light-yearsfrom Earth near the center of the Milky Way.

A star like our Sun spends its embryonic yearsaccumulating mass until it reaches maturity.

When it nears the end of its life, it sheds some ofits mass as it contracts to a white dwarf. As mate-rial ejected from the star is driven outward intospace, radiation from the star causes it to glow.The illuminated stellar remains surrounding adying star are called a planetary nebula. Anebula will shine for about 10,000 years.

Hubble has provided a gallery of these nebulae,which come in many shapes and glow in akaleidoscope of colors. The Ring Nebula (M57)is a stunning example of a dying central starfloating in a blue haze of hot gas (see Fig. 3-6).The colorful nebula surrounding the star

Fig. 3-6 In this October 1998 image of the Ring Nebula (M57), Hubble looks down abarrel of gas cast off by a dying star thousands of years ago.

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Fig. 3-7 Hubble sees supersonic exhaust from nebula M2-9, a striking example of a “butterfly”or bipolar planetary nebula.

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appears ring-shaped, but only because of theviewing angle. Many astronomers believe thatthe nebula is actually a cylinder and theHubble picture supports this view.

The Telescope’s image of the Ring Nebulashows numerous small, dark clouds of dustthat have formed in the gas flowing out fromthe star. They are silhouetted against moredistant, bright gas. These finger-like cloudsappear only in the outer portions of the RingNebula; none are seen in the central region.This suggests that they are not distributed in auniform sphere but are instead located only onthe walls of the barrel, which is a light-year indiameter. The nebula is 2,000 light-years fromEarth in the constellation Lyra.

Another striking planetary nebula is M2-9,called a “butterfly” or bipolar planetary nebulabecause of its shape (see Fig. 3-7). Another

more revealing name might be the “Twin JetNebula.” A companion star orbiting perilouslyclose to its dying mate may have caused theunusual shape. Astronomers suspect thegravity of one star pulls weakly bound gasfrom the surface of the other and flings it into athin, dense disk surrounding both stars. Thehigh-speed wind of gas from one of the starsslams into the disk, which serves as a nozzle.Deflected in a perpendicular direction, thewind forms a pair of jets speeding at 200 milesper second. The nebula is 2,100 light-yearsaway in the constellation Ophiuchus.

Unlike lighter-weight stars that quietly endtheir lives by forming planetary nebulae,massive stars die in mammoth explosions. HSThas been monitoring one such explosion, super-nova 1987A – 167,000 light-years away in theLarge Magellanic Cloud. Scientists first viewedthe star’s self-destruction on February 23, 1987,

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Fig. 3-8 A bright knot appears in the Supernova 1987A Ring.K9322-308

in a naked-eye observation. In July 1997, STIScaptured the first images of material ejectedby the exploding star ramming into an innerring around the dying object (see Fig. 3-8).

Shocked by the 40-million-miles-per-hour sledge-hammer blow, a 100-billion-mile-wide knot of gasin a piece of the ring has already begun to “lightup” as its temperature surges from a few thou-sand to a million degrees Fahrenheit. Byanalyzing the glowing ring, astronomers mayfind clues to many of the supernova’s unan-swered mysteries: What was the progenitor star?Was it a single star or a binary system? What wasthe process that created a ring that formed 20,000years before the star exploded?

3.3 Galaxies and Cosmology

Galaxies are the largest assemblages of stars inthe universe. In a galaxy, billions of stars arebound together by the mutual pull of gravity.The Sun resides in the Milky Way Galaxy.

The study of galaxies falls into the realm ofcosmology, the science of the evolution of theuniverse on the largest scale. Edwin P. Hubblediscovered the expansion of the universe bymeasuring the redshifts of distant galaxies. Hedemonstrated that the galaxies are rushingaway from us, with their velocities increasingproportionally to their distances. The constantof proportionality is known as the Hubbleconstant. The reciprocal of the Hubble constantis an index to the age of the universe, or thetime since the Big Bang.

Measuring the Hubble constant was a majorgoal for the Hubble Space Telescope before itwas launched in 1990. In May 1999, theTelescope key project team announced that ithad completed its efforts to measure precisedistances to far-flung galaxies, an ingredientneeded to determine the age, size, and fate ofthe universe. The team measured the Hubbleconstant at 70 km/sec/Mpc, with an uncer-tainty of 10 percent. This means that a galaxy

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appears to be moving 160,000 miles per hourfaster for every 3.3 million light-years awayfrom Earth.

Using the Telescope, the team observed 18galaxies, some as far away as 65 million light-years. The team was looking for Cepheid vari-able stars, a special class of pulsating star usedfor accurate distance measurements. Almost800 Cepheids were discovered. But the teamcould utilize only Cepheids in nearby andintermediate-distance galaxies. To calculatedistances to galaxies farther away, the teamused so-called “secondary” distance measure-ments, such as a special class of exploding starcalled a Type Ia supernova. Combining theHubble constant measurement with estimatesfor the density of the cosmos, the team hasdetermined that the universe is approximately12 billion years old and that it contains insuffi-cient mass to halt the expansion of space. Thiswas, perhaps, the most important astronomicaldiscovery of the decade.

In December 1995 the Telescope providedhumankind’s deepest, most detailed visible-lightview of the heavens called the Hubble DeepField. In 1998 Hubble’s penetrating vision wasturned toward the southern skies. The Telescopepeered down a narrow, 11-billion-light-year-longcorridor loaded with thousands of never-before-seen galaxies. The observation, called the HubbleDeep Field South (HDF-S), doubles the numberof far-flung galaxies available for decipheringthe history of the universe. In each “deep field”view, astronomers counted about 3,000galaxies. Both views also show that galaxieswere smaller in the past and have evolved tothe giant spirals and ellipticals of today.

The southern “far-look” complements theoriginal Hubble Deep Field, for whichHubble was aimed at a small patch of sky

near the Big Dipper. The southern region is inthe constellation Tucana, near the south celes-tial pole. The 10-day HDF-S observation tookplace in October 1998. At first glance HDF-Sappears to validate the common assumptionthat the universe should look largely thesame in any direction (see Fig. 3-9).

The two deep fields give astronomers two“core samples” of the universe to use for betterunderstanding the history of the cosmos. All ofHubble’s cameras and other instruments weretrained on the sky at the same time for theobservations. STIS dissected light from aquasar (the bright, active core of a distantgalaxy). This light, which had traveled nearlythree-quarters of the way across the heavens,provides a powerful three-dimensional probeof the universe’s hidden structure. Invisibleclouds of primeval hydrogen gas strung alongbillions of light-years between HST and thequasar are detectable in the signature of thelight. The quasar is so brilliant it is like asearchlight shining through haze.

The Hubble Deep Field uncovered a plethoraof odd-shaped, disrupted-looking galaxies.They offer direct evidence that galaxy colli-sions were more the rule than the exception inthe early universe. Because collisions of distantgalaxies are too faint and too small to study inmuch detail, the Telescope turned its gaze to agalactic wreck closer to home – the Antennaegalaxies. A pair of long tails of luminous matterformed by a collision resembles an insect’santennae (see Fig. 3-10). The galaxies are 63million light-years from Earth in the southernconstellation Corvus.

At the heart of these colliding galaxies, theTelescope uncovered more than 1,000 bright,young star clusters bursting to life in a brief,intense, brilliant “fireworks show.” By probing the

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Fig. 3-9 In an observation called the Hubble Deep Field South (HDF-S), the Telescope peered downan 11-billion-light-year-long corridor loaded with thousands of never-before seen galaxies.

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Antennae and other colliding galaxies, Hubblehas given astronomers a variety of surprises:

• Globular star clusters are not necessarilyrelics of the earliest generations of star forma-tion in a galaxy. They may provide fossilrecords of more recent encounters.

• The “seeds” for star clusters appear to begiant molecular clouds of cold hydrogengas, tens to hundreds of light-years across.The clouds are squeezed by surrounding hotgas heated during the galaxy collisions, thencollapse under their own gravity. Like astring of firecrackers ignited by the collision,

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Fig. 3-10 This HST image provides a detailed look at a “fireworks show” in the center of acollision between two galaxies.

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the reservoirs of gas light up in a great burstof star formation.

• The ages of the resulting clusters provide aclock for estimating the age of a collision.This offers an unprecedented opportunityfor understanding, step by step, the complexsequence of events that occurs during anencounter and possibly even the evolutionof spiral galaxies into elliptical galaxies.

Sometimes the debris from galaxy collisions isfodder for supermassive black holes. HST hasgiven astronomers a peek at a feeding frenzy asa supermassive black hole in a nearby galaxydevours a smaller galaxy after the two collide.Such fireworks were common in the earlyuniverse, as galaxies formed and evolved, butare rare today.

The WFPC2 image of Centaurus A, also calledNGC 5128, shows in sharp clarity a dramaticdark lane of dust girdling the colliding galaxy(see Fig. 3-11). Blue clusters of newborn starsare clearly resolved, and silhouettes of dustfilaments are interspersed with blazing orange-glowing gas. Located 10 million light-yearsaway, this peculiar-looking galaxy contains theclosest active galactic nucleus to Earth and haslong been considered an example of an ellip-tical galaxy disrupted by a recent collision witha smaller companion spiral galaxy.

Using NICMOS, astronomers have penetrated thiswall of dust for the first time to see a twisted disk ofhot gas swept up in the black hole’s gravitationalwhirlpool. The suspected black hole is so dense itcontains the mass of perhaps a billion stars,

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compacted into a small region of space notmuch larger than our solar system.

Resolving features as small as seven light-years across, the Hubble has shown that thehot gas is tilted in a direction different fromthe axis of the black hole – like a wobblywheel around an axle. This gas disk presum-ably fueling the black hole may have formedso recently that it has not yet become alignedwith the black hole’s spin axis, or it maysimply be influenced more by the gravita-tional tug of the galaxy than by that of theblack hole.

Still, scientists do not know if the black holewas always present in the host galaxy, if itbelonged to the spiral galaxy that fell into thecore, or if it is the product of the merger of apair of smaller black holes that lived in the twoonce-separate galaxies.

From invisible black holes to the mysteriousflashes of high-energy radiation called gammaray bursts, Hubble is on the trail. These brightbursts of energy appear from random regions inspace and typically last just a few seconds. U.S.Air Force Vela satellites first discovered gammaray bursts in the 1960s. Since then, numerous

Fig. 3-11 Hubble offers an unprecedented close-up view of a turbulent firestorm of starbirthalong a nearly edge-on dust disk girdling Centaurus A.

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theories of their origins have been proposed,but their causes remain unknown. TheTelescope has helped astronomers trace themback to distant galaxies. The principal limitationin understanding the bursts was the difficultyin pinpointing their direction in the sky. Unlikevisible light, gamma rays are exceedingly diffi-cult to observe with a telescope, and their shortduration exacerbates the problem.

Hubble has teamed up with several observato-ries, including X-ray satellites, to collect infor-mation on gamma ray bursts. The most ener-getic burst, GRB 971214, was detectedDecember 14, 1997. Astronomers measured thedistance to a faint galaxy from which the burstoriginated. Using the Italian/Dutch satelliteBeppoSAX, they pinpointed the direction of theburst, which permitted follow-up observationswith the world’s most powerful telescopes.

The follow-up observations tracked GRB971214’s “afterglow” in radio waves, X-ray,visible, and infrared light. While gamma raybursts last only a few seconds, their afterglowscan be studied for several months. By analyzingafterglows, astronomers have discovered thatthe bursts do not originate within our owngalaxy, the Milky Way, but are associated withextremely distant galaxies. The Hubble imagesof GRB 971214 confirmed the association of theburst’s afterglow with a faint galaxy.

Astronomers still don’t understand the originsof gamma ray bursts. Theories suggest theyhappen where vigorous star formation takesplace. Whatever are the origins of gamma raybursts, the Telescope has observed many ofthem, and in nearly every case they have beenfound to be associated with faint host galaxies.

3.4 Summary

The Hubble Space Telescope has establisheditself as a premier astronomical observatorythat continues to make dramatic observa-tions and discoveries at the forefront ofastronomy. Following the successful Firstand Second Servicing Missions, theTelescope has achieved all of its originalobjectives. Among a long list of achieve-ments, Hubble has:

• Improved our knowledge of the size andage of the universe

• Provided decisive evidence of the exis-tence of supermassive black holes at thecenters of galaxies

• Clearly revealed the galactic environ-ments in which quasars reside

• Detected objects with coherent structure(protogalaxies) close to the time of theorigin of the universe

• Provided unprecedentedly clear imagesand spectra of the collision of CometShoemaker-Levy 9 with Jupiter

• Detected a large number of protoplane-tary disks around stars

• Elucidated the various processes bywhich stars form

• Provided the first map of the surface ofPluto

• Routinely monitored the meteorology ofplanets beyond the orbit of Earth

• Made the first detection of an ultraviolethigh-energy laser in Eta Carinae.

After the Servicing Missions 3A and 3B, theHubble Space Telescope will view theuniverse anew with significantly expandedscientific and technological capabilities.

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hree instruments are currently in activescientific use on HST – the Wide Fieldand Planetary Camera 2 (WFPC2), the

Space Telescope Imaging Spectrograph (STIS)and Fine Guidance Sensor 1R (FGS1R), whichhas been designated as the prime FGS forastrometric science. Other instrument bays areoccupied by the Near Infrared Camera andMulti-Object Spectrometer (NICMOS), whichis now dormant due to the depletion of its solidnitrogen cryogen; the Faint Object Camera(FOC), which is obsolete and has been decom-missioned; and the corrective optical devicecalled COSTAR, which is no longer needed.

During HST Servicing Mission 3B (SM3B) anexperimental mechanical cooling system willbe attached to NICMOS to determine if it canbe brought back into operation. During thesame mission the FOC will be replaced by theAdvanced Camera for Surveys (ACS).COSTAR will be removed during the final HSTservicing mission (SM4) to make room for theCosmic Origins Spectrograph (COS).

The Fine Guidance Sensors (FGSs) are under-going a systematic program of refurbishmentand upgrading. In “round-robin” fashion, oneFGS per servicing mission is being replaced,returned to the ground, disassembled andrefurbished, and then taken back to HST on thenext servicing mission to become the replace-ment unit for the next FGS to be serviced. Inthis manner, by the conclusion of SM4, all threeFGSs will have been brought up to optimumcondition.

4.1 Space Telescope ImagingSpectrograph

STIS was developed under the direction of theprincipal investigator, Dr. Bruce E. Woodgate,jointly with Ball Aerospace (see Fig. 4-1). The

spectrograph was designed to be versatile andefficient, taking advantage of modern tech-nologies to provide a new two-dimensionalcapability to HST spectroscopy. The twodimensions can be used either for “long slit”spectroscopy, where spectra of many differentpoints across an object are obtained simultane-ously, or in an echelle mode to obtain morewavelength coverage in a single exposure. STISalso can take both UV and visible imagesthrough a limited filter set.

STIS was designed to replace many of the capa-bilities of the instruments it succeeded on SM2 –the Goddard High Resolution Spectrographand the Faint Object Spectrograph. However, ithas additional enhanced capabilities. STIS coversa broader wavelength range with two-dimensional capability, adds a coronagraphcapability, and has a high time-resolutioncapability in the UV. It also can image andcan provide objective prism spectra in theintermediate UV. STIS carries its own aberration-correcting optics.

4.1.1 Physical Description

STIS resides in an axial bay behind the HST mainmirror. Externally, the instrument measures 7.1 x2.9 x 2.9 ft (2.2 x 0.98 x 0.98 m) and weighs 825 lb

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TSCIENCE INSTRUMENTS

Fig. 4-1 Space Telescope Imaging Spectrograph

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(374 kg). Internally, STIS consists of a carbonfiber optical bench, which supports thedispersing optics and three detectors.

STIS has been designed to work in threedifferent wavelength regions, each with itsown detector. Some redundancy is built intothe design with overlap in the detectorresponse and backup spectral modes. To selecta wavelength region or mode, a single mecha-nism, called the mode selection mechanism(MSM), is used. The MSM has 21 opticalelements: 16 first-order gratings (six of whichare order-sorting gratings used in the echellemodes), an objective prism, and four mirrors.The optical bench supports the input correctoroptics, focusing and tip/tilt motions, the inputslit and filter wheels, and the MSM.

Light from the HST main mirror is first correctedand then brought to a focus at the slit wheel.After passing through the slit, it is collimatedby a mirror onto one of the MSM opticalelements. A computer selects the mode andwavelength. The MSM rotates and nutates toselect the correct optical element, grating,mirror, or prism, and points the beam along theappropriate optical path to the correct detector.

In the case of first-order spectra, a first-ordergrating is selected for the wavelength anddispersion. The beam then is pointed to acamera mirror, which focuses the spectrumonto the detector, or goes directly to thedetector itself.

For an echelle spectrum, an order-sortinggrating that directs the light to one of the fourfixed echelle gratings is selected, and thedispersed echellogram is focused via a cameramirror onto the appropriate detector. Thedetectors are housed at the rear of the bench, sothey can easily dissipate heat through an outer

panel. The optical bench is thermallycontrolled. An onboard computer controls thedetectors and mechanisms.

Each of the three detectors has been optimizedfor a specific wavelength region. Band 1, from115 to 170 nm, uses a Multi-AnodeMicrochannel Plate Array (MAMA) with acesium iodide (CsI) photocathode. Band 2,from 165 to 310 nm, also uses a MAMA butwith a cesium telluride (CsTe) photocathode.Bands 3 and 4, covering the wavelengths from305 to 555 nm and 550 to 1000 nm, use the samedetector, a charge-coupled device (CCD).Figure 4-2 shows the instrument schematically.

Entrance Apertures. After the light beam passesthrough the corrector, it enters the spectro-graph through one of several slits. The slits aremounted on a wheel and the slit or entranceaperture can be changed by wheel rotation.

The first-order spectral imaging modes canselect slits 50 arcsec long and from 0.05 to 2arcsec wide. Three slits have occulting bars thatcan block out a bright star in the field. Four slitsare tilted to an angle of 45 degrees for planetaryobservations. For echelle spectroscopy, 16 slitsranging in length from 0.10 to 1 arcsec areavailable. The slit length is short to control theheight of the echelle spectra in the image planeand avoid spectral order overlap. The echelleslits have widths of 0.05, 0.10, 0.12, 0.20, and 0.5arcsec. There also are camera apertures of 50 x50 and 25 x 25 arcsec. Some of the apertureshave occulting bars incorporated. The tele-scope can be positioned to place bright starsbehind the occulting bars to allow viewing andobservation of faint objects in the field of view.In addition, there is a special occulting mask orcoronagraph, which is a finger in the aperturethat can be positioned over a bright star toallow examination of any faint material nearby.

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It can be thought of as a simulation of a totaleclipse on a nearby star. This mode is particu-larly useful to search for faint companion starsor planetary disks around stars.

Mode Selection Mechanism. The mode selec-tion mechanism (MSM) is a rotating wheel thathas 16 first-order gratings, an objective prism,and four mirrors mounted to it. The MSM axisis a shaft with two inclined outer sleeves, onesleeve fitting inside the other. The sleeves areconstructed so that rotation of one sleeverotates a wheel to orient the appropriate opticinto the beam. Rotation of the second sleevechanges the inclination of the axis of the wheel

or tilt of the optic to select the wavelengthrange and point the dispersed beam to thecorresponding detector. One of three mirrorscan be selected to take an image of an object.The objective prism is used exclusively withthe Band 2 MAMA (see Fig. 4-3). Of the 16gratings, six are cross dispersers that directdispersed light to one of the four echelle gratingsfor medium- and high-resolution modes.

Multi-Anode Microchannel Plate ArrayDetectors. For UV modes, two types of MAMAdetectors are employed on STIS. A photo-cathode optimizes each detector to its wave-length region. Each detector’s photocathode

Fig. 4-2 STIS components and detectors

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provides maximum sensitivity in the wave-length region selected, while it rejects visiblelight not required for the observations. TheBand 1 MAMA uses a CsI cathode, which issensitive to the wavelength region from 115 to170 nm and very insensitive to wavelengths inthe visible. Similarly, the Band 2 MAMA uses aCsTe photocathode, which is sensitive to thewavelength region from 120 to 310 nm.

The heart of each MAMA detector is amicrochannel plate (MCP) – a thin disk ofglass approximately 1.5 mm thick and 5 cm indiameter that is honeycombed with small(12.5-micron) holes or pores. The front andback surfaces are metal coated. With a voltageapplied across the plate, an electron enteringany pore is accelerated by the electric field, andit eventually collides with the wall of the pore,giving up its kinetic energy to liberate two ormore secondary electrons. The walls are treatedto enhance the secondary electron productioneffect. The secondary electrons continue downthe pore and collide with the wall to emit moreelectrons, and so the process continues,

producing a cascade of a million electrons atthe end of the pore.

In the Band 1 tube, shown in Fig. 4-4, UVphotons enter and hit the CsI photocathodethat is deposited on the front surface of theMCP. The cathode produces an electron when aphoton hits it and the electron is acceleratedinto the MCP pores. The MCP amplifies thenumber of electrons, which fall as a showeronto the anode array as they leave the MCP.

The anode array is a complex fingerlike pattern.When electrons strike certain anodes, a signalis sent to the computer memory indicating theposition and time of arrival of the photon.Figure 4-5 shows the detection scheme insimplified form.

The anode array has been designed so that only132 circuits are required to be able to read outall 1024 x 1024 pixels. As the MAMA recordsthe arrival of each photon, it can provide a timesequence. For instance, if an object is varying intime, like a pulsar, the data can be displayed to

Fig. 4-3 STIS spectroscopic modes

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show if there is any periodicity. Similarly, tocreate an image, the data must be integrated inthe computer memory before it is displayed.The MAMA data is recorded to a time resolu-tion of 125 microseconds.

In the case of Band 2, the CsTe photocathode isdeposited on the inside surface of the frontwindow as a semi-transparent film. Photonspass through the window, and some arestopped in the cathode film where they generateelectrons, which are amplified and detected inthe same manner as the Band 1 detector.

When used in the normal mode, each detector

has 1024 x 1024 pixels, each 25 x 25 micronssquare. However, data received from the anodearray can be interpolated to give a higher reso-lution, splitting each pixel into four 12.5 x 12.5micron pixels. This is known as the high-reso-lution mode; however, data taken in this modecan be transformed to normal resolution ifrequired. The high-resolution mode provideshigher spatial resolution for looking at finestructural details of an object and ensures fullsampling of the optical images and spectra.

Charge-Coupled Detector. The STIS CCD wasdeveloped with GSFC and Ball input atScientific Imaging Technologies (SITe).

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Fig. 4-4 Multi-Anode Microchannel Plate Array (MAMA) detector

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Fabricated using integrated circuit technology,the detector consists of light-sensitive pictureelements (pixels) deposited onto a thin waferof crystalline silicon. Each element is 21 x 21microns. The elements are arranged 1024 to arow, in 1024 columns. The 1024 x 1024 formatcontains 1,048,576 pixels.

Each element acts as a small capacitance. Aslight falls on a pixel, it liberates electrons,which effectively charge the capacitance. Thenumber of electrons stored is then proportionalto the intensity or brightness of the lightreceived. The charge in each pixel can be readout by applying a small voltage across the chip.

The CCD is most sensitive to red light, but theSTIS chip has been enhanced through what isknown as a “backside treatment” to provide ausable sensitivity in the near-ultraviolet. TheCCD is sensitive from approximately 200 nmto the near infrared at 1000 nm. The violet

extension allows the CCD to overlap with theBand 2 MAMA sensitivity and can serve as abackup detector.

To reduce thermionic noise generated in theCCD, the detector is integrated into a housingand cooled to below -80°C. The cooling isprovided by a thermoelectric cooler, which isbonded onto the back of the CCD. The heatextracted from the CCD is dissipated through aradiative cooling panel on the outside of STIS.The housing has a front window made fromfused silica and is kept close to 20°C, the designtemperature of the optical bench.

The CCD can make exposures ranging from 0.1seconds to 60 minutes. In space, above Earth’sprotective atmosphere, radiation from cosmicrays is higher than at Earth’s surface. CCDs aresensitive to cosmic rays, which can producelarge numbers of electrons in the pixels. For thisreason, two shorter exposures of up to 1 hour

Fig. 4-5 Simplified MAMA system

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are made and comparison of the frames allowscosmic ray effects to be subtracted.

The CCD is a 16-bit device, allowing a dynamicrange from 1 to 65,535 to be recorded.Changing the gain can further extend thedynamic range. The gain is commandable for1 electron/bit to 8 electrons/bit.

Another useful feature is called binning, inwhich pixels are merged on the chip. Typically,binning is 2 x 2 for imaging, making the pixelslarger, which can reduce the noise per pixeland increase sensitivity at the cost of resolu-tion. Binning can be used to look for extendedfaint objects such as galaxies. Another binningapplication is in the long-slit mode onextended faint objects. In this mode, binningalong the slit by 1 x 4, for instance, wouldmaintain the spectral resolution but sum thespectra from different parts of an object seenalong the slit to increase signal or detectivity.

Finally, to increase the CCD’s performance atlow light levels, the chip has incorporated aminichannel. The main problem with readingout a signal from a CCD is that a charge gener-ated by light must be dragged across the chip,through all its adjacent pixels, to be read out ofone corner. In so doing, the charge meetsspurious defects in each pixel that add noise.Because the noise can be very path dependent,the minichannel ion implant is designed torestrict the path taken at low signal levels toimprove CCD performance.

4.1.2 Spectra Operational Modes

Figure 4-3 shows the spectral operationalmodes. Two numbers, W and R, describe eachinstrument mode, where W refers to the wave-length range and R to the resolving power.

The low-resolution, or spectral-imaging mode,is R~500 to 1,000, and can be carried out in allfour bands using a long slit. The medium reso-lution mode, R~5,000 to 10,000, is a spectralimaging mode that can be carried out in allfour bands using long slits. However, asdispersion increases, not all of the spectrumfalls on the detector. Obtaining an entire spec-tral range may require moving the spectrumand taking another image. Figure 4-3 indicatesthe number of exposures to cover the wholewavelength range.

The medium-resolution echelle spectroscopywith R~24,000 uses short slits and is availablein the UV only. Band 2 requires two exposuresto cover the whole wavelength region.

High-resolution echelle spectroscopy, withR~100,000, uses short slits and is available in theUV only. Both Bands 1 and 2 require multipleexposures.

Objective spectroscopy, R~26 (at 300 nm) and930 (at 121 nm), is available using the Band 2detector only. This mode uses a prism insteadof a grating. The prism dispersion, unlike agrating, is not uniform with wavelength. Thelow-resolution gratings and the prism also canprovide imaging spectroscopy of emission lineobjects such as planetary nebulae, supernovaremnants, or active galaxies.

Imaging Operational Modes. STIS can be usedto acquire an image of an object in UV orvisible light. To do this, an open aperture isselected and a mirror placed in the beam by theMSM. The instrument has nine filters that canbe selected (see Fig. 4-6). The cameras for theCCD and the MAMAs have different magnifi-cation factors. The field of view is 25 x 25 arcsecfor the MAMAs and 50 x 50 arcsec for the CCD.

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Target Acquisition. Normally an object isacquired using the CCD camera with a 50 x 50-arcsec field. Two short exposures are taken toenable subtraction of cosmic rays. The HSTFGSs have a pointing accuracy of ±2 arcsec,and the target usually is easily identifiable inthe field. Once identified, an object is posi-tioned via small angle maneuvers to thecenter of the chosen science mode slit posi-tion. Two more exposures are made, and thenthe calibration lamp is flashed through the slitto confirm the exact slit position. A furtherpeak up on the image is then performed.Acquisition can be expected to take up toapproximately 20 minutes.

Data Acquisition. The MAMAs take data inthe high-resolution mode. For normalimaging and spectroscopy, the data will beintegrated in the onboard computer andstored in this format on the solid-staterecorders for later downlink. The MAMAsalso have a time-tag mode, where eachphoton is stored individually with its arrivaltime and location (x, y, t). The data is storedin a 16-Mb memory and as the memory fills,

the data is dumped into the onboard recorder.The time-tag mode has a time resolution of125 microseconds.

4.1.3 STIS Specifications

Figure 4-7 shows STIS specifications.

4.1.4 Observations

Scientists using STIS focus their science onmany areas, including:

• Search for massive black holes by studyingstar and gas dynamics around the centers ofgalaxies

• Measurement of the distribution of matter inthe universe by studying quasar absorptionlines

• Use of the high sensitivity and spatial resolu-tion of STIS to study stars forming in distantgalaxies

• Mapping – giving fine details of planets,nebulae, galaxies, and other objects

• Coronagraphic capability may enable it toimage Jupiter-sized planets around nearby stars.

STIS also can provide physical diagnostics, suchas chemical composition, temperature, density,and velocity of rotation or internal mass motionsin planets, comets, stars, interstellar gas,nebulae, stellar ejecta, galaxies, and quasars.

Fig. 4-6 STIS filter set

Fig. 4-7 STIS specifications

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Studies of Black Holes in Centers of Galaxies.A black hole was discovered in the center ofM87 using FOS. The black hole’s massprovides the gravity required to hold in orbitgas and stars that are rapidly rotating about thegalactic nucleus. From the spectra of starssurrounding the nucleus, a measure of howrapidly the velocity changes from one side ofthe nucleus to the other can be determined bymeasuring the Doppler shift. STIS’s long-slitmode is particularly well suited for this type ofmeasurement because all the spatial positionsrequired can be measured along the slit in asingle exposure. With STIS, a study of blackholes can be made easily for many galaxies andcompared from one galaxy to another.

Abundances and Dynamics of theIntergalactic Medium. STIS is well suited toobserve the spectra of distant galaxies andquasars. Absorption lines from interveningmaterial in the spectra of these objects give ameasure of the dynamics and abundances ofspecific elements. However, because theseobjects are distant, we effectively are lookingback in time to an earlier stage of the universewhen the chemical composition was differentfrom that seen in the vicinity of our Sun today.Measuring these differences can provideimportant clues to how the universe hasevolved over time.

Abundances in the Interstellar Medium. TheSTIS high-resolution mode is particularly wellsuited to measurements of the spectral absorp-tion lines created in the interstellar mediumand seen against distant O- and B-type stars.From the Doppler shift, a measure of gas speedis obtained. Temperature and density andchemical composition also can be measured.The interstellar medium is thought to play animportant role in when star formation occurs ingalaxies. Current theories point to hot material

being expelled from supernovas into a galaxy’ssurrounding halo. Later, after cooling, thematerial returns to the galactic plane and even-tually forms new stars. How quickly the inter-stellar medium circulates can be a clue to whenstar formation occurs.

Search for Protoplanetary Disks. STIS’s coro-nagraphic mode can be used to image nearbystars and search for protoplanetary disks.These observations provide complementarydata to NICMOS, which was to peer throughthe thick dust around young stars. Theseobservations shed light on how planets form,what type of stars have planetary disks, andhow quickly the disks evolve into planets.

4.2 Wide Field and PlanetaryCamera 2

Hubble’s “workhorse” camera is WFPC2. Itrecords two-dimensional images at two magni-fications through a selection of 48 color filterscovering a spectral range from far-ultravioletto visible and near-infrared wavelengths. Itprovides pictorial views of the celestialuniverse on a grander scale than any otherinstrument flown to date. Like its predecessorWFPC1, WFPC2 was designed and built atNASA’s Jet Propulsion Laboratory (JPL),which is operated by the California Institute ofTechnology. Professor James A. Westphal ofCaltech was the principal investigator forWFPC1. Dr. John T. Trauger of JPL is theprincipal investigator for WFPC2.

WFPC1, the first-generation instrument, waslaunched with the Telescope in 1990 and func-tioned flawlessly. The second-generationinstrument, WFPC2, was already underconstruction when the Hubble Telescope waslaunched. Its original purpose was to provide abackup for WFPC1 with certain enhancements,

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including an upgraded set of filters, advanceddetectors, and improved UV performance.With modifications introduced after 1990,WFPC2 also provided built-in compensationfor the improper curvature of the Telescope’sprimary mirror so as to achieve the originallyspecified imaging performance of theTelescope in the WFPC2 field of view.

WFPC2 has four CCD cameras arranged torecord simultaneous images in four separatefields of view at two magnifications.

In three Wide Field Camera fields, each detectorpicture element (pixel) occupies 1/10th arcsec,and each of the three detector arrays covers asquare 800 pixels on a side (or 80 arcsec, slightlymore than the diameter of Jupiter when it isnearest the Earth). The Telescope is designed toconcentrate 70 percent of the light of a starimage into a circle 0.2 arcsec (or two WideField Camera pixels) in diameter. This three-field camera (which operates at a focal ratio off/12.9) provides the greatest sensitivity for thedetection of faint objects. Stars as faint as 29thmagnitude are detectable in the longest expo-sures (29th magnitude is over one billion timesfainter than can be seen with the naked eye).

The Planetary Camera provides a magnifica-tion about 2.2 times larger, in which each pixeloccupies only 0.046 arcsec, and the singlesquare field of view is only 36.8 arcsec on aside. It operates at a focal ratio of f/28.3.Originally incorporated for studying thefinest details of bright planets, the PlanetaryCamera actually provides the optimumsampling of the Telescope’s images at visiblewavelengths and is used (brightness permit-ting) whenever the finest possible spatial reso-lution is needed, even for stars, stellarsystems, gaseous nebulae, and galaxies.

With its two magnifications and its built-incorrection for the Telescope’s spherical aberra-tion, WFPC2 can resolve the fine details andpick out bright stellar populations of distantgalaxies. It can perform precise measurementsof the brightness of faint stars, and study thecharacteristics of stellar sources even incrowded areas such as globular clusters –ancient swarms of as many as several hundredthousand stars that reside within a hugespherical halo surrounding the Milky Way andother galaxies – that could not be studied effec-tively with WFPC1 because of the aberration.WFPC2’s high-resolution imagery of theplanets within our solar system allowscontinued studies of their atmospheric compo-sition as well as discovery and study of time-varying processes on their surfaces.

4.2.1 Physical Description

The WFPC2 occupies one of four radial bays inthe focal plane structure of the HST. The otherthree radial bays support the FGSs, which areused primarily for controlling the pointing ofthe Telescope.

The WFPC2’s field of view is located at thecenter of the Telescope’s field of view, wherethe telescopic images are nearly on axis andleast affected by residual aberrations (fieldcurvature and astigmatism) that are inherent inthe Ritchey-Chretien design.

Because the focal plane is shared by the otherinstruments, WFPC2 is equipped with a flatpickoff mirror located about 18 in. ahead ofthe focal plane and tipped at almost 45degrees to the axis of the Telescope. Thepickoff mirror is attached to the end of a stifftruss, which is rigidly fastened to WFPC’sprecisely located optical bench. The pickoff

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mirror reflects the portion of the Telescope’sfocal plane belonging to WFPC2 into a nearlyradial direction, from which it enters the frontof the instrument, allowing light falling onother portions of the focal plane to proceedwithout interference.

WFPC2 is shaped somewhat like a piece ofpie, the pickoff mirror lying at the point of thewedge, with a large, white-painted cylin-drical panel 2.6 ft (0.8 m) high and 7 ft (2.2 m)wide at the wide end. The panel forms part ofthe curved outer skin of the Support SystemsModule (SSM) and radiates away the heatgenerated by WFPC’s electronics. The instru-ment is held in position by a system of latchesand is clamped in place by a threadedfastener at the end of a long shaft that pene-trates the radiator and is accessible to theastronauts.

WFPC2 weighs 619 lb (281 kg). The camerascomprise four complete optical subsystems,four CCDs, four cooling systems using ther-moelectric heat pumps, and a data-processingsystem to operate the instrument and senddata to the SI C&DH unit. Figure 4-8 showsthe overall configuration of the instrument.

Optical System. The WFPC2 optical systemconsists of the pickoff mirror, an electricallyoperated shutter, a selectable optical filterassembly, and a four-faceted reflectingpyramid mirror used to partition the focalplane to the four cameras. Light reflected bythe pyramid faces is directed by four “fold”mirrors into each of four two-mirror relaycameras. The relays re-image the Telescope’soriginal focal plane onto the four detectorarrays while providing accurate correction forthe spherical aberration of the Telescope’s

Fig. 4-8 Wide Field and Planetary Camera (WFPC) overall configuration

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primary mirror. Figure 4-9 shows the lightpath from the Telescope to the detectors.

As in an ordinary camera, the shutter is usedto control the exposure time, which can rangefrom about 1/10th second to 28 hours. Typicalexposure times are 45 minutes, about the timerequired for the Telescope to complete half anorbit.

WFPC2’s pickoff mirror and three of its fourfold mirrors are equipped with actuators thatallow them to be controlled in two axes (tipand tilt) by remote control from the ground.The actuators ensure that the spherical aberra-tion correction built into WFPC2 is accuratelyaligned relative to the Telescope in all fourchannels.

The Selectable Optical Filter Assembly (SOFA)consists of 12 independently rotatable wheels,each carrying four filters and one clearopening, for a total of 48 filters. These can beused singly or in certain pairs. Some of theWFPC2’s filters have a patchwork of areas

with differing properties to provide versatilityin the measurement of spectral characteristicsof sources.

WFPC2 also has a built-in calibration channel,in which stable incandescent light sources serveas references for photometric observations.

Charge-Coupled Detectors. A CCD is a devicefabricated by methods developed for themanufacture of integrated electronic circuits.Functionally, it consists of an array of light-sensitive picture elements (pixels) built upon athin wafer of crystalline silicon. Complex elec-tronic circuits also built onto the wafer controlthe light-sensitive elements. The circuitsinclude low-noise amplifiers to strengthensignals that originate at the light sensors. Aslight falls upon the array, photons of lightinteract with the sensor material to createsmall electrical charges (electrons) in thematerial. The charge is very nearly propor-tional to the number of photons absorbed. Thebuilt-in circuits read out the array, sending asuccession of signals that will allow later

Fig. 4-9 WFPC optics design

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reconstruction of the pattern of incoming lighton the array. Figure 4-10 illustrates the process.

The CCDs used in WFPC2 consist of 800 rowsand 800 columns of pixels, 640,000 pixels ineach array. The pixels can be thought of as tinysquares side by side, 15 microns (about6/10,000 in.) on a side. Their sensitivity to lightis greatest at near-infrared and visible wave-lengths, but in WFPC2 it is extended to the UVby coating them with a thin fluorescent layerthat converts UV photons to visible ones.

To achieve a very low-noise background thatdoes not interfere with measurements of faintastronomical light sources, the CCDs must beoperated at a low temperature, approximately-50 to -70°C (-8 to -130°F). This is accomplishedby an electrically operated solid-state coolingsystem that pumps heat from the cold CCDs tothe warmer external radiator by means of heatpipes. The radiator faces away from the Earthand Sun so that its heat can be effectively radi-ated into the cold vacuum of space.

CCDs are much more sensitive to light thanphotographic film and many older forms ofelectronic light sensors. They also have finer

resolution, better linearity, and ability toconvert image data directly into digital form.As a result, CCDs have found many astronom-ical and commercial applications followingtheir early incorporation in WFPC1.

Processing System. A microprocessor controlsall of WFPC2’s operations and transfers data tothe SI C&DH unit. Commands to controlvarious functions of the instrument (includingfilter and shutter settings) are sent by radiouplink to the Telescope in the form of detailedencoded instructions originated at the SpaceTelescope Science Institute (STScI) in Baltimore,Maryland. Because the information rate of theTelescope’s communication system is limited,the large amount of data associated with evenone picture from WFPC2 is digitally recordedduring the CCD readout. The data then is trans-mitted at a slower rate via a communicationssatellite that is simultaneously in Earth orbit.

4.2.2 WFPC2 Specifications

Figure 4-11 shows the WFPC2 specifications.

4.2.3 Observations

The WFPC2 can perform several tasks whileobserving a single object. It can focus on anextended galaxy and take a wide-field picture

Fig. 4-11 WFPC2 specifications

Fig. 4-10 WFPC2 imaging

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of the galaxy, then concentrate on the galaxynucleus to measure light intensity and takephotographic closeups of the center. In addi-tion, the WFPC2 can measure while otherinstruments are observing.

Specific applications of this camera range fromtests of cosmic distance scales and universeexpansion theories to specific star, supernova,comet, and planet studies. Important searchesare being made for black holes, planets in otherstar systems, atmospheric storms on Mars, andthe connection between galaxy collisions andstar formation.

4.3 Astrometry (Fine GuidanceSensors)

When two FGSs lock on guide stars to providepointing information for the Telescope, thethird FGS serves as a science instrument tomeasure the position of stars in relation toother stars. This astrometry helps astronomersdetermine stellar masses and distances.

Fabricated by Raytheon Optical Systems Inc., thesensors are in the focal plane structure, at rightangles to the optical path of the Telescope and 90degrees apart. They have pickoff mirrors todeflect incoming light into their apertures, asshown in Fig. 4-12. (See para 5.3 for more details.)

Each refurbished FGS has been upgraded bythe addition of an adjustable fold mirror(AFM). This device allows HST’s optical beamto be properly aligned to the internal optics ofthe FGS by ground command. The first-gener-ation FGSs did not contain this feature andtheir optical performance suffered as a conse-quence. During Servicing Mission 2, FGS 1was removed from HST and replaced withFGS 1R, the first FGS to feature this activealignment capability. Now with its optical

system properly aligned, FGS1R performssuperbly and is the prime instrument on HSTfor astrometric science observations.

4.3.1 Fine Guidance SensorSpecifications

Figure 4-13 shows FGS specifications.

4.3.2 Operational Modes forAstrometry

Once the two target-acquisition FGSs lock ontoguide stars, the third sensor can perform astro-metric operations on targets within the field of

Fig. 4-13 FGS specifications

Fig. 4-12 Fine Guidance Sensor (FGS)

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view set by the guide stars’ positions. Thesensor should be able to measure stars as faintas 18 apparent visual magnitude.

There are three operational modes for astro-metric observations: position, transfer-func-tion, and moving-target. Position modeallows the astrometric FGSs to calculate theangular position of a star relative to theguide stars. Generally, up to 10 stars will bemeasured within a 20-minute span.

In the transfer-function mode, sensorsmeasure the angular size of a target, eitherthrough direct analysis of a single-pointobject or by scanning an extended target.Examples of the latter include solar systemplanets, double stars, and targets surroundedby nebulous gases.

Astrometric observations of binary stars provideinformation about stellar masses which is impor-tant to understanding the evolution of stars.

In moving-target mode, sensors measure arapidly moving target relative to other targetswhen it is impossible to precisely lock onto themoving target; for example, measuring theangular position of a moon relative to itsparent planet.

4.3.3 Fine Guidance Sensor FilterWheel

Each FGS has a filter wheel for astrometricmeasurement of stars with different brightnessand to classify the stars being observed. Thewheel has a clear filter for guide-star acquisitionand faint-star (greater than 13 apparent visualmagnitude) astrometry. A neutral-density filter isused for observation of nearby bright stars; andtwo colored filters are utilized for estimating atarget’s color (chemical) index, increasing

contrast between close stars of different colors, orreducing background light from star nebulosity.

4.3.4 Astrometric Observations

Astronomers measure the distance to a star bycharting its location on two sightings fromEarth at different times, normally 6 monthsapart. The Earth’s orbit changes the perceived(apparent) location of the nearby star, and theparallax angle between the two locations canlead to an estimate of the star’s distance.Because stars are so distant, the parallax angleis very small, requiring a precise field of view tocalculate the angle. Even with the precision ofthe FGSs, astronomers cannot measuredistances by the parallax method beyondnearby stars in our galaxy.

An important goal of the FGS astrometry projectis to obtain improved distances to fundamentaldistance calibrators in the universe, for instanceto the Hyades star cluster. This is one of thefoundations of the entire astronomical distancescale. An accurate distance to the Hyades wouldmake it possible for astronomers to infer accu-rate distances to similar stars that are too distantfor the direct parallax method to work.

Astronomers have long suspected that somestars might have a planetary system like thataround our Sun. Unfortunately, the greatdistance of stars and the faintness of anypossible planet make it very difficult to detectsuch systems directly. It may be possible todetect a planet by observing nearby stars andlooking for the subtle gravitational effects thata planet would have on the star it is orbiting.

Astronomers use the FGS in two modes of opera-tion to investigate known and suspected binarystar systems. Their observations lead to the deter-mination of the orbits and parallaxes of the binary

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stars and therefore to the masses of these systems.For example, 40 stars in the Hyades cluster wereobserved with the FGS. Ten of the targets werediscovered to be binary star systems and one ofthem has an orbital period of 3.5 years.

Other objects, such as nearby M dwarf starswith suspected low-mass companions, arebeing investigated with the FGS with the hopeof improving the mass/luminosity relationshipat the lower end of the main sequence.

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These generate electrical power and chargeonboard batteries and communicationsantennas to receive commands and sendtelemetry data from the HST. Figure 5-1 showsthe HST configuration.

The Telescope performs much like a groundobservatory. The SSM is designed to supportfunctions required by any ground astronomicalobservatory. It provides power, points theTelescope, and communicates with the OTA, SIC&DH unit, and instruments to ready anobservation. Light from an observed targetpasses through the Telescope and into one ormore of the science instruments, where the lightis recorded. This information goes to onboardcomputers for processing, and then it is eithertemporarily stored or sent to Earth in real time,via the spacecraft communication system.

The Telescope completes one orbit every 97minutes and maintains its orbital position along

HUBBLE SPACE TELESCOPE SYSTEMS

Fig. 5-1 Hubble Space Telescope – exploded view

MAGNETIC TORQUER (4)


SUPPORT SYSTEMS MODULE FORWARD SHELL

OPTICAL TELESCOPE ASSEMBLYSECONDARY MIRROR ASSEMBLY

SECONDARY MIRROR BAFFLE

CENTRAL BAFFLE

OPTICAL TELESCOPE ASSEMBLYPRIMARY MIRROR AND MAIN RING

FINE GUIDANCE OPTICALCONTROL SENSOR (3)

OPTICAL TELESCOPEASSEMBLY FOCALPLANE STRUCTURE

AXIAL SCIENCEINSTRUMENTMODULE(4)

SUPPORT SYSTEMS MODULEAFT SHROUD

FIXED HEAD STAR TRACKER (3)AND RATE SENSOR UNIT (3)

RADIAL SCIENCEINSTRUMENT MODULE (1)

OPTICAL TELESCOPE ASSEMBLYEQUIPMENT SECTION

SUPPORT SYSTEMS MODULEEQUIPMENT SECTION

OPTICAL TELESCOPE ASSEMBLYMETERING TRUSS

MAIN BAFFLE SOLAR ARRAY (2)

APERTUREDOOR

LIGHT SHIELD

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T he Hubble Space Telescope (HST) hasthree interacting systems:

• The Support Systems Module (SSM), an outerstructure that houses the other systems andprovides services such as electrical power,data communications, and pointing controland maneuvering

• The Optical Telescope Assembly (OTA),which collects and concentrates the incominglight in the focal plane for use by the scienceinstruments

• Eight major science instruments, four housedin an aft section focal plane structure (FPS)and four placed along the circumference ofthe spacecraft. With the exception of the FineGuidance Sensors (FGS), the ScienceInstrument Control and Data Handling (SIC&DH) unit controls all.

Additional systems that also support HSToperations include two Solar Arrays (SA).

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three axial planes. The primary axis, V1, runsthrough the center of the Telescope. The othertwo axes parallel the SA masts (V2) and theHigh Gain Antenna (HGA) masts (V3) (seeFig. 5-2). The Telescope points and maneuversto new targets by rotating about its body axes.Pointing instruments use references to theseaxes to aim at a target in space, position the SA,or change Telescope orientation in orbit.

5.1 Support Systems Module

The design features of the SSM include:

• An outer structure of interlocking shells• Reaction wheels and magnetic torquers to

maneuver, orient, and attitude stabilize theTelescope

• Two SAs to generate electrical power• Communication antennas• A ring of Equipment Section bays that

contain electronic components, such asbatteries, and communications equipment.(Additional bays are provided on the +V3side of the spacecraft to house OTAelectronics as described in para 5.2.4.)

• Computers to operate the spacecraft systemsand handle data

• Reflective surfaces and heaters for thermalprotection

• Outer doors, latches, handrails, and footholdsdesigned for astronaut use during on-orbitmaintenance.

Figure 5-3 shows some of these features.

Major component subsystems of the SSM are:

• Structures and mechanisms• Instrumentation and communications• Data management• Pointing control• Electrical power• Thermal control• Safing (contingency) system.

5.1.1 Structures and MechanismsSubsystem

The outer structure of the SSM consists ofstacked cylinders, with the aperture door on topand the aft bulkhead at the bottom. Fittingtogether are the light shield, the forward shell,the SSM Equipment Section, and the aftshroud/bulkhead – all designed and built byLockheed Martin Missiles & Space (see Fig. 5-4).

Aperture Door. The aperture door,approximately 10-ft (3 m) in diameter, coversthe opening to the Telescope’s light shield. Thedoor is made from honeycombed aluminumsheets. The outside is covered with solar-reflecting material, and the inside is paintedblack to absorb stray light.

The door opens a maximum of 105 degrees fromthe closed position. The Telescope apertureallows for a 50-degree field of view (FOV)centered on the +V1 axis. Sun-avoidance sensorsprovide ample warning to automatically closethe door before sunlight can damage theTelescope’s optics. The door begins closingFig. 5-2 Hubble Space Telescope axes

+V3

–V2

+V3

–V1+V2

+V2

–V3

+V1

SOLAR ARRAY

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when the sun is within ±35 degrees of the +V1axis and is closed by the time the sun reaches20 degrees of +V1. This takes no more than 60seconds.

The Space Telescope Operations Control Center(STOCC) can override the protective door-closing mechanism for observations that fallwithin the 20-degree limit. An example isobserving a bright object, using the dark limb(edge) of the Moon to partially block the light.

Light Shield. The light shield (see Fig. 5-4)blocks out stray light. It connects to both theaperture door and the forward shell. On theouter skin of the Telescope on opposite sides arelatches to secure the SAs and HGAs when theyare stowed. Near the SA latches are scuff plates,

HIGH GAIN ANTENNA

FORWARDSHELL

CREWHANDRAILS

APERTUREDOOR

LIGHT SHIELD

MAGNETICTORQUERS

SOLAR ARRAY

EQUIPMENTSECTION

BATTERIESAND CHARGECONTROLLER

EQUIPMENT BAY

DIGITALINTERFACE UNIT

REACTIONWHEEL ASSEMBLY

COMMUNICATION SYSTEM

COMPUTER

LOW GAINANTENNA

UMBILICALINTERFACE

LATCH PINASSEMBLY

SUN SENSORS (3)

AFTSHROUD

ACCESS DOOR K70110-503

Fig. 5-3 Design features of Support Systems Module

APERTUREDOOR

LIGHTSHIELDMAGNETIC

TORQUER (4)

HIGH-GAIN ANTENNA (2)

FORWARD SHELL

EQUIPMENTSECTION

AFT SHROUD K70110-504

Fig. 5-4 Structural components of Support SystemsModule

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large protective metal plates on struts thatextend approximately 30 in. from the surface ofthe spacecraft. Trunnions lock the Telescope intothe Shuttle cargo bay by hooking to latches inthe bay. The light shield supports the forwardLow Gain Antenna (LGA) and its communi-cations waveguide, two magnetometers, andtwo sun sensors. Handrails encircle the lightshield, and built-in foot restraints support theastronauts working on the Telescope.

Figure 5-5 shows the aperture door and lightshield. The shield is 13-ft (4 m) long, with aninternal diameter of 10-ft (3 m). It is machinedfrom magnesium, with a stiffened, corrugated-skin barrel covered by a thermal blanket.Internally the shield has 10 light baffles, paintedflat black to suppress stray light.

Forward Shell. The forward shell, or centralsection of the structure, houses the OTA mainbaffle and the secondary mirror (see Fig. 5-6).When stowed, the SAs and HGAs are latchedflat against the forward shell and light shield.Four magnetic torquers are placed 90 degrees

apart around the circumference of the forwardshell. The outer skin has two grapple fixturesnext to the HGA drives, where the Shuttle’sRemote Manipulator System can attach to theTelescope. The forward shell also has handholds,footholds, and a trunnion, which is used to lockthe Telescope into the Shuttle cargo bay.

The forward shell is 13-ft (4 m) long and 10-ft(3 m) in diameter. It is machined from alumi-num plating, with external reinforcing rings andinternal stiffened panels. The rings are on theoutside to ensure clearance for the OTA inside.Thermal blankets cover the exterior.

Equipment Section. This section is a ring ofstorage bays encircling the SSM. It containsabout 90 percent of the electronic componentsthat run the spacecraft, including equipmentserviced during extravehicular activities (EVA)by Space Shuttle astronauts.

The Equipment Section is a doughnut-shapedbarrel that fits between the forward shell and aftshroud. This section contains 10 bays forequipment and two bays to support aft trunnionpins and scuff plates. As shown in Fig. 5-7,

APERTUREDOOR

APERTUREDOOR HINGE

LIGHT SHIELD

MAGNESIUMMONOCOQUESKIN

SOLAR ARRAYLATCHES

HIGH-GAIN ANTENNAAND SOLAR ARRAYLATCH SUPPORT RING

LIGHT SHIELDASSEMBLY/FORWARD SHELLATTACH RING

INTERNALBAFFLE RINGS

HIGH GAINANTENNA LATCHES

INTEGRALLYSTIFFENED SKIN

APERTURE DOORHINGE SUPPORT

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Fig. 5-5 Aperture door and light shield

STOWEDHIGH GAINANTENNAS

MAGNETICTORQUERS

CREW AIDS

STOWEDSOLAR ARRAYS

EQUIPMENT SECTION/FORWARD SHELLINTERFACE RING

INTEGRALLYSTIFFENEDSKIN PANELS

FORWARD SHELLREINFORCINGRINGS

HIGH GAINANTENNA MAST

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Fig. 5-6 Support Systems Module forward shell

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Fig

. 5-7

Sup

port

Sys

tem

s M

odul

e E

quip

men

t Sec

tion

bays

and

con

tent

s

1 2 3 4 5

8 9 106

1

3

2

–V2

10 –V2

10

9

87

+V

3

–V3

3

4

–V2

4

+V

2

6

57

K70

110-

507

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Fig. 5-8 Support Systems Module aft shroudand bulkhead

going clockwise from the +V3 (top) position, thebays contain:

1. Bay 8 – pointing control hardware2. Bay 9 – Reaction Wheel Assembly (RWA)3. Bay 10 – SI C&DH unit4. Unnumbered trunnion support bay5. Bay 1 – data management hardware6. Bay 2 through Bay 4 – electrical power

equipment7. Unnumbered trunnion support bay8. Bay 5 – communication hardware9. Bay 6 – RWA10. Bay 7 – mechanism control hardware.

The cross section of the bays is shaped like atrapezoid, with the outer diameter (the door) –3.6-ft (1 m) – greater than the inner diameter –2.6-ft (0.78 m). The bays are 4-ft (1.2 m) wideand 5-ft (1.5 m) deep. The Equipment Section isconstructed of machined and stiffenedaluminum frame panels attached to an inneraluminum barrel. Eight bays have flathoneycombed aluminum doors mounted withequipment. In Bays 6 and 9, thermal-stiffenedpanel doors cover the reaction wheels. Aforward frame panel and aft bulkhead enclosethe SSM Equipment Section. Six mounts on theinside of the bulkhead hold the OTA.

Aft Shroud and Bulkhead. The aft shroud (seeFig. 5-8) houses the FPS containing the axialscience instruments. It is also the location of theCorrective Optics Space Telescope AxialReplacement (COSTAR) unit.

The three FGSs and the Wide Field and PlanetaryCamera 2 (WFPC2) are housed radially near theconnecting point between the aft shroud andSSM Equipment Section. Doors on the outside ofthe shroud allow shuttle astronauts to removeand change equipment and instruments easily.Handrails and foot restraints for the crew runalong the length and circumference of the

shroud. During maintenance or removal of aninstrument, interior lights illuminate thecompartments containing the scienceinstruments. The shroud is made of aluminum,with a stiffened skin, internal panels andreinforcing rings, and 16 external and internallongeron bars for support. It is 11.5-ft (3.5 m) longand 14-ft (4.3 m) in diameter.

The aft bulkhead contains the umbilicalconnections between the Telescope and theshuttle, used during launch/deployment andon-orbit maintenance. The rear LGA attaches tothe bulkhead, which is made of 2-in.-thickhoneycombed aluminum panels and has threeradial aluminum support beams.

The shroud and bulkhead support a gas purgesystem that was used to prevent contaminationof the science instruments before launch. Allvents used to expel gases are light tight. Thus,stray light is prevented from entering the OTAfocal plane.

Mechanisms. Along the SSM structure aremechanisms that perform various functions,including:

EQUIPMENTSECTION/AFT SHROUDINTERFACERING

CREW AIDS

INTEGRALLY STIFFENEDSKIN PANELS

LOW GAIN ANTENNA

HONEYCOMB LAMINATE PANELS

FLIGHT SUPPORT SYSTEMPIN SUPPORT BEAM

FLIGHT SUPPORT SYSTEM PINVENTS

ELECTRICALUMBILICALS

REINFORCING RINGS

+V2 +V1

+V3

RADIAL SCIENCE INSTRUMENT DOORS

AXIAL SCIENCE INSTRUMENT DOORS

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• Latches to hold antennas and SAs• Hinge drives to open the aperture door and

erect arrays and antennas• Gimbals to move the HGA dishes• Motors to power the hinges and latches and

to rotate arrays and antennas.

There are nine latches: four for antennas, fourfor arrays, and one for the aperture door. Theylatch and release using four-bar linkages and aredriven by stepper motors called Rotary DriveActuators (RDA).

There are three hinge drives, one for each HGAand one for the door. The hinges also use anRDA. Both hinges and latches have hex-wrenchfittings so an astronaut can manually operatethe mechanism to deploy the door, antenna, orarray if a motor fails.

5.1.2 Instrumentation andCommunications Subsystem

This subsystem provides the communicationsloop between the Telescope and the Trackingand Data Relay Satellites (TDRS), receivingcommands and sending data through the HGAsand LGAs. All information is passed throughthe Data Management Subsystem (DMS).

The HGAs achieve a much higher RF signalgain, which is required, for example, whentransmitting high-data-rate scientific data. Theseantennas require pointing at the TDRSs becauseof their characteristically narrow beam widths.On the other hand, the LGAs provide sphericalcoverage (omnidirectional) but have a muchlower signal gain. The LGAs are used for low-rate-data transmission and all commanding ofthe Telescope.

S-Band Single Access Transmitter (SSAT).The Telescope is equipped with two SSATs.“S-Band” identifies the frequency at which the

science data is transmitted, and “Single Access”specifies the type of antenna on the TDRSsatellite to which the data is sent.

High Gain Antennas. Each HGA is a parabolicreflector (dish) mounted on a mast with a two-axis gimbal mechanism and electronics to rotateit 100 degrees in either direction (see Fig. 5-9).General Electric designed and made the antennadishes. They are manufactured fromhoneycomb aluminum and graphite-epoxyfacesheets.

Each antenna can be aimed with a one-degreepointing accuracy. This accuracy is consistentwith the overall antenna beam width of overfour degrees. The antennas transmit over twofrequencies: 2255.5 MHz or 2287.5 MHz (plus orminus 10 MHz).

Low Gain Antennas. The LGAs receive groundcommands and transmit engineering data. Theyare set 180 degrees apart on the light shield andaft bulkhead of the spacecraft. Each antenna isa spiral cone that can operate over a frequencyrange from 2100 MHz to 2300 MHz.Manufactured by Lockheed Martin, the LGAs

Fig. 5-9 High Gain Antenna

HIGH GAINANTENNA

TWIN-AXISGIMBALS

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are used for all commanding of the Telescopeand for low-data-rate telemetry, particularlyduring Telescope deployment or retrieval onorbit, or during safemode operations.

5.1.3 Data ManagementSubsystem

The DMS receives communicationscommands from the STOCC and data fromthe SSM systems, OTA, and scienceinstruments. It processes, stores, and sendsthe information as requested. Subsystemcomponents are:

• DF-224 computer (will be replaced on SM3Awith the Advanced Computer)

• Data Management Unit (DMU)• Four Data Interface Units (DIU)• Three engineering/science data recorders• Two oscillators (clocks).

The components are located in the SSMEquipment Section, except for one DIU storedin the OTA Equipment Section.

The DMS receives, processes, and transmits fivetypes of signals:

1. Ground commands sent to the HST systems2. Onboard computer-generated or computer-

stored commands3. Scientific data from the SI C&DH unit4. Telescope engineering status data for

telemetry5. System outputs, such as clock signals and

safemode signals.

Figure 5-10 is the subsystem functionaldiagram.

DF-224 Computer. The DF-224 computer is ageneral-purpose digital computer for onboard

Fig. 5-10 Data Management Subsystem functional block diagram

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engineering computations. It executes storedcommands; formats status data (telemetry);performs all Pointing Control Subsystem (PCS)computations to maneuver, point, and attitudestabilize the Telescope; generates onboardcommands to orient the SAs toward the Sun;evaluates the health status of the Telescopesystems; and commands the HGAs. TheAdvanced Computer will replace the DF-224 onSM3A and will assume its functions.

Advanced Computer. The Advanced Computeris based on the Intel 80486 microchip. It operates20 times faster and has six times as muchmemory as the DF-224.

The Advanced Computer was designed usingcommercially developed components. A batteryof mechanical, electrical, radiation and thermaltests were performed at GSFC to assure itssurvival in the space environment. A successfulflight test of the hardware was carried outaboard the space shuttle Discovery on STS-95 inOctober 1998.

The Advanced Computer is configured as threeindependent single-board computers. Eachsingle-board computer (SBC) has twomegabytes of fast static random access memoryand one megabyte of non-volatile memory.

The Advanced Computer communicates withthe HST by using the direct memory accesscapability on each SBC through the DataManagement Unit (DMU). Only one SBC maycontrol the Telescope at a time. The other SBCscan be off, in an idle state, or performing internaltasks.

Upon power on, each SBC runs a built-in self-test and then copies the operating software fromslower non-volatile memory to faster randomaccess memory. The self-test is capable ofdiagnosing any problems with the Advanced

Computer and reporting them to the ground.Fast static random access memory is used in theAdvanced Computer to eliminate wait statesand allow it to run at its full-rated speed.

The Advanced Computer measures 18.8 x 18 x 13inches (0.48 x 0.46 x 0.33 m) and weighs 70.5 lb(32 kg). It will be located in Bay 1 of the SSMEquipment Section (see Fig. 5-11).

Data Management Unit. The DMU links withthe computer. It encodes data and sendsmessages to selected Telescope units and allDMS units, powers the oscillators, and is thecentral timing source. The DMU also receivesand decodes all incoming commands, thentransmits each processed command to beexecuted.

The DMU receives science data from the SIC&DH unit. Engineering data, consisting ofsensor and hardware status readings (such astemperature or voltages), comes from eachTelescope subsystem. The data can be stored inthe onboard data recorders if direct telemetryvia a TDRS is unavailable.

Fig. 5-11 Advanced computer

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The DMU is an assembly of printed-circuitboards, interconnected through a backplate andexternal connectors. The unit weighs 83 lb (37.7kg), measures 26 x 30 x 7 in. (60 x 70 x 17 cm),and is attached to the door of EquipmentSection Bay 1 (see Fig. 5-12).

Data Interface Unit. The four DIUs provide acommand and data link between DMS andother Telescope electronic boxes. The DIUsreceive commands and data requests from theDMU and pass data or status information backto the DMU. The OTA DIU is located in the OTAEquipment Section; the other units are in Bays3, 7, and 10 of the SSM Equipment Section. Asa safeguard, each DIU is two complete units inone; either part can handle the unit’s functions.Each DIU measures 15 x 16 x 7 in. (38 x 41 x 18cm) and weighs 35 lb (16 kg).

Engineering/Science Data Recorders. The DMSincludes three data recorders that store

engineering or science data that cannot betransmitted to the ground in real time. Therecorders, which are located in EquipmentSection Bays 5 and 8, hold up to 12 billion bitsof information. Two recorders are used innormal operations; the third is a backup. Eachrecorder measures 12 x 9 x 7 in. (30 x 23 x 18cm) and weighs 20 lb (9 kg).

Solid State Recorder. During SM2 a Solid StateRecorder (SSR) was installed, replacing a reel-to-reel tape recorder. A second state-of-the-artSSR will be installed during SM3A. This digitalrecorder will replace one of the two remainingreel-to-reel recorders on HST. The Solid StateRecorders have an expected on-orbit life of atleast eight years. They can record two datastreams simultaneously, allowing both scienceand engineering data to be captured on a singlerecorder. In addition, data can be recorded andplayed back at the same time.

The SSR has no reels or tape, and no movingparts to wear out and limit lifetime. Data isstored digitally in computer-like memory chipsuntil HST’s operators at GSFC command theSSR to play it back. Although they are the samesize as the reel-to-reel recorders, the SSRs canstore over 10 times more data — 12 gigabitsversus only 1.2 gigabits for the tape recordersthey replace.

Oscillator. The oscillator provides a highlystable central timing pulse required by theTelescope. It has a cylindrical housing 4 in. (10 cm)in diameter and 9 in. (23 cm) long and weighs3 lb (1.4 kg). The oscillator and a backup aremounted in Bay 2 of the SSM EquipmentSection.

5.1.4 Pointing Control Subsystem

A unique PCS maintains Telescope pointingstability and aligns the spacecraft to point to

K70110-512

POWER SUPPLY NO. 2(CDI DC-DCCONVERTER)

TOP COVER(WITH COMPRESSION PAD)

PC BOARDSTANDARD(5 X 7 in.)

CARDGUIDE

ENCLOSURE(DIP-BRAZEDAL.)

POWER SUPPLYNO. 1(DMU DC-DCCONVERTER)

BOTTOM COVER

COAX CONNECTOR

INTERFACECONNECTOR

MATRIXCONNECTOR

HEATSHIELDPARTITION(TYP)

Fig. 5-12 Data Management Unit configuration

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and remain locked on any target. The PCS isdesigned for pointing to within 0.01 arcsec andholding the Telescope in that orientation with0.007-arcsec stability for up to 24 hours whilethe Telescope continues to orbit the Earth at17,500 mph. If the Telescope were in LosAngeles, it could hold a beam of light on a dimein San Francisco without the beam strayingfrom the coin’s diameter.

Nominally, the PCS maintains the Telescope’sprecision attitude by locating guide stars intotwo FGSs and controlling the Telescope to keepit in the same position relative to these stars.When specific target requests requirerepositioning the spacecraft, the pointingsystem selects different reference guide starsand moves the Telescope into a new attitude.

The PCS encompasses the Advanced Computer,various attitude sensors, and two types ofdevices, called actuators, to move the spacecraft(see Fig. 5-13). It also includes the Pointing/Safemode Electronics Assembly (PSEA) and theRetrieval Mode Gyro Assembly (RMGA); bothused by the spacecraft safemode system. Seepara 5.1.7 for details.

Sensors. The five types of sensors used by thePCS are the Coarse Sun Sensors (CSS), theMagnetic Sensing System (MSS), the Rate GyroAssemblies (RGA), the Fixed Head Star Trackers(FHST), and the FGSs.

The CSSs measure the Telescope’s orientation tothe Sun. They are used to calculate the initialdeployment orientation of the Telescope,determine when to begin closing the aperturedoor, and point the Telescope in special sun-orientation modes during contingencyoperations. Five CSSs are located on the lightshield and aft shroud. CSSs also provide signalsto the PSEA, located in Bay 8 of the SSMEquipment Section.

The MSS measures the Telescope’s orientationrelative to Earth’s magnetic field. The systemconsists of magnetometers and dedicatedelectronic units that send data to the AdvancedComputer and the Safemode ElectronicAssembly. Two systems are provided. Both arelocated on the front end of the light shield.

Three RGAs are provided on the Telescope.Each assembly consists of a Rate Sensor Unit(RSU) and an Electronics Control Unit (ECU).An RSU contains two rate-sensing gyroscopes,each measuring attitude rate motion about itssensitive axis. This output is processed by itsdedicated electronics, which are contained inthe ECU. Each unit has two sets of electronics.The RSUs are located behind the SSM EquipmentSection, next to the FHSTs in the aft shroud. TheECUs are located inside Bay 10 of the SSMEquipment Section. The RGAs provide input tothe PCS to control the orientation of the Tele-scope’s line of sight and to provide the attitudereference when maneuvering the Telescope.

Four of the original six rate gyros were replacedduring the First Servicing Mission. All six rategyros are planned for replacement duringSM3A. Three of six gyroscopes are required tocontinue the Telescope science mission.

An FHST is an electro-optical detector thatlocates and tracks a specific star within its FOV.Three FHSTs are located in the aft shroudbehind the FPS, next to the RSUs. STOCC usesstar trackers as an attitude calibration devicewhen the Telescope maneuvers into its initialorientation. The trackers also calculate attitudeinformation before and after maneuvers to helpthe FGS lock onto guide stars.

Three FGSs, discussed in more detail in para 5.3,provide angular position with respect to thestars. Their precise fine-pointing adjustments,accurate to within a fraction of an arcsecond,

5-12K9322-05M

K70110-513

MAGNETICTORQUERS (4)

MAGNETOMETER (2)

COARSE SUN SENSORS (2)

REACTION WHEELS (4)

COMPUTER

EQUIPMENTSECTION

SCIENCEINSTRUMENTS

FIXED HEADSTAR TRACKER (3)

COARSE SUNSENSORS (2)

RATE SENSORUNIT (3)

FINE GUIDANCESENSOR (3)

BAY 8 – POINTING CONTROL AND INSTRUMENTS– RETRIEVAL MODE GYRO ASSEMBLY– POINTING AND SAFEMODE

ELECTRONIC ASSEMBLY

BAY 9 – REACTION WHEEL (2)NO. 3 FORWARDNO. 4 AFT

BAY 10 – SCIENCEINSTRUMENTCONTROLAND DATAHANDLING

BAY 1 – DATA MANAGEMENTCOMPUTER

BAY 7 – MECHANISM CONTROL

BAY 6 – REACTION WHEELREACTION WHEEL ASSEMBLY (2)NO. 1 FORWARDNO. 2 AFT

+ V2

5

6

7

+ V38

9

10

– V2

1

– V33

4

RWA

RWA

LOOKING FORWARD2

Fig. 5-13 Location of Pointing Control Subsystem equipment

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pinpoint the guide stars. Two of the FGSsperform guide-star pointing, while the third isavailable for astrometry, the positionalmeasurement of specific stars.

Pointing Control Subsystem Software. PCSsoftware accounts for a large percentage of theflight code executed by the Hubble’s maincomputer. This software translates groundtargeting commands into reaction wheel torqueprofiles that reorient the spacecraft. All motionof the spacecraft is smoothed to minimize jitterduring data collection. The software alsodetermines Telescope orientation, or attitude,from FHST or FGS data and commands themagnetic torquer bars so that reaction wheelspeeds are always minimized. In addition, thesoftware provides various telemetry formats.

Since the Telescope was launched, majormodifications have been made to the PCS. Adigital filtering scheme, known as Solar ArrayGain Augmentation (SAGA) was incorporatedto mitigate the effect of any SA vibration or jitteron pointing stability. Software also was used toimprove FGS performance when the Telescopeis subjected to the same disturbances. Thisalgorithm is referred to as the FGS Re-CenteringAlgorithm.

Software is used extensively to increase Telescoperobustness when hardware failures areexperienced. Two additional software safemodeshave been provided. The spin-stabilized modeprovides pointing of the Telescope -V1 axis to theSun with only two of the four RWAs operating.The other mode allows Sun pointing of theTelescope without any input from the RGA;magnetometer and CSS data is used to derive allreference information needed to maintain Sunpointing (+V3 and -V1 are options).

A further software change “refreshes” the FGSconfiguration. This is achieved by maintaining

data in the Advanced Computer memory so itcan be sent periodically to the FGS electronics,which are subject to single-event upsets (logicstate change) when transitioning through theSouth Atlantic Anomaly.

Actuators. The PCS has two types of actuators:RWAs and magnetic torquers. Actuators movethe spacecraft into commanded attitudes andprovide required control torques to stabilize theTelescope’s line of sight.

The reaction wheels work by rotating a largeflywheel up to 3000 rpm or braking it toexchange momentum with the spacecraft. Thewheel axes are oriented so that the Telescopecan provide science with only three wheelsoperating. Wheel assemblies are paired, twoeach in Bays 6 and 9 of the SSM EquipmentSection. Each wheel is 23 in. (59 cm) in diameterand weighs about 100 lb (45 kg). Figure 5-14shows the RWA configuration.

Magnetic torquers create torque on the space-craft and are primarily used to manage reactionwheel speed. The torquers react against Earth’smagnetic field. The torque reaction occurs in thedirection that reduces the reaction wheel speed,managing the angular momentum.

The magnetic torquers also provide backupcontrol to stabilize the Telescope’s orbitalattitude during the contingency modes, asdescribed in para 5.1.2. Each torquer, locatedexternally on the forward shell of the SSM, is 8.3

K70110-514

RWA RWA

Fig. 5-14 Reaction Wheel Assembly

5-14K9322-05M

ft (2.5 m) long and 3 in. (8 cm) in circumferenceand weighs 100 lb (45 kg).

Pointing Control Operation. To point precisely,the PCS uses the gyroscopes, reaction wheels,magnetic torquers, star trackers, and FGSs. TheFGSs provide the precision reference point fromwhich the Telescope can begin repositioning.Flight software commands the reaction wheelsto spin, accelerating or decelerating as requiredto rotate the Telescope toward a new target. Rategyroscopes sense the Telescope’s angularmotion and provide a short-term attitudereference to assist fine pointing and spacecraftmaneuvers. The magnetic torquers reducereaction wheel speed.

As the Telescope nears the target area, startrackers locate preselected reference stars thatstand out brightly in that region of the sky. Oncethe star trackers reduce the attitude error below60 arcsec, the two FGSs take over the pointingduties. Working with the gyroscopes, the FGSsmake possible pointing the Telescope to within0.01 arcsec of the target. The PCS can maintainthis position, wavering no more than 0.005arcsec, for up to 24 hours to guarantee faint-object observation.

5.1.5 Electrical Power Subsystem

Power for the Telescope and scienceinstruments comes from the Electrical PowerSubsystem (EPS). The major components aretwo SA wings and their electronics, sixbatteries, six Charge Current Controllers(CCC), one Power Control Unit (PCU), andfour Power Distribution Units (PDU). Allexcept the SAs are located in the bays aroundthe SSM Equipment Section.

During the servicing mission, the Shuttle willprovide the electrical power. After deployment,the SAs again begin converting solar radiation

into electricity. Energy will be stored in nickel-hydrogen (NiH2) batteries and distributed by thePCUs and PDUs to all Telescope components asshown in Fig. 5-15. The Telescope will not bereleased until the batteries are fully charged.

Solar Arrays. The SA panels, discussed later inthis section, are the primary source of electricalpower. Each array wing has a solar cell blanketthat converts solar energy into electrical energy.Electricity produced by the solar cells chargesthe Telescope batteries.

Each array wing has associated electronics.These consist of a Solar Array Drive Electronics(SADE) unit, which transmits positioningcommands to the wing assembly; a DeploymentControl Electronics Unit, which controls thedrive motors extending and retracting thewings; and diode networks to direct theelectrical current flow.

Batteries and Charge Current Controllers.Developed for the 1990 deployment mission, theTelescope’s batteries were NASA’s first flightNiH2 batteries. They provide the observatorywith a robust, long-life electrical energy storagesystem.

Six NiH2 batteries support the Telescope’selectrical power needs during three periods:when demand exceeds SA capability, whenthe Telescope is in Earth’s shadow, and duringsafemode entry. The batteries reside in SSMEquipment Section Bays 2 and 3. These unitshave extensive safety and handling provisionsto protect the Shuttle and its astronauts. The de-sign and operation of these batteries, along withspecial nondestructive inspection of each cell,have allowed these units to be “astronaut-rated”for replacement during a servicing mission. Tocompensate for the effects of battery aging,SM3A astronauts will install a Voltage/Tem-perature Improvement Kit (VIK) on each of

5-15K9322-05M

Hubble’s six batteries. The VIK provides batterythermal stability by precluding battery over-charge when the HST enters safemode, effec-tively lowering the Charge Current Controller(CCC) recharge current.

Each battery consists of 22 cells in series alongwith heaters, heater controllers, pressuremeasurement transducers and electronics, andtemperature-measuring devices and theirassociated electronics. Three batteries arepackaged into a module measuring roughly 36by 36 by 10 in. (90 x 90 x 25 cm) and weighingabout 475 lb (214 kg). Each module is equippedwith two large yellow handles that astronautsuse to maneuver the module in and out of theTelescope in space.

The SAs recharge the batteries every orbitfollowing eclipse (the time in the Earth’sshadow). The recharge current is controlled bythe CCCs. Each battery has its own CCC thatuses voltage-temperature measurements tocontrol battery recharge.

Fully charged, each battery contains more than75 amp-hours. This is sufficient energy to sustainthe Telescope in normal science operations modefor 7.5 hours or five orbits. The batteries providean adequate energy reserve for all possiblesafemode contingencies and all enhancementsprogrammed into the Telescope since launch.

Power Control and Distribution Units. ThePCU interconnects and switches current flowingamong the SAs, batteries, and CCCs. Located inBay 4 of the Equipment Section, the PCUprovides the main power bus to the four PDUs.The PCU weighs 120 lb (55 kg) and measures43 x 12 x 8 in. (109 x 30 x 20 cm).

Four PDUs, located on the inside of the door toBay 4, contain the power buses, switches, fuses,and monitoring devices for electrical power dis-tribution to the rest of the Telescope. Two busesare dedicated to the OTA, science instruments,and SI C&DH; two supply the SSM. Each PDUmeasures 10 x 5 x 18 in. (25 x 12.5 x 45 cm) andweighs 25 lb (11 kg).

Fig. 5-15 Electrical Power Subsystem functional block diagram

K70110-515

SOLAR ARRAYELECTRONICS

CONTROLASSEMBLY

POWER

SOLAR ARRAYMECHANISMS

SO

LAR

AR

RA

YS

OLA

R A

RR

AY

POWER

COMMANDS

TELEMETRY

POWERPOWER

POWER

POWERPOWER

POWER

POWER

POWER

POWER

POWER

POWERCONTROL

UNITCOMMANDS

TELEMETRY

BATTERYNO. 1

23

45

6

PWRDISTRUNIT 1

23

4

SSMSASIs

OTA

SIC&DH

TELEMETRY

ORBITER POWER(PREDEPLOYMENT)

COMMANDS

POWER

COMMANDS

5-16K9322-05M

5.1.6 Thermal Control

Multilayer insulation (MLI) covers 80 percent ofthe Telescope’s exterior, and supplementalelectric heaters maintain its temperatures withinsafe limits. The insulation blankets are 15 layersof aluminized Kapton, with an outer layer ofaluminized Teflon flexible optical solar reflector(FOSR). Aluminized or silvered flexible reflectortape covers most of the remaining exterior.These coverings protect against the cold ofspace and reflect solar heat. In addition,reflective or absorptive paints are used.

The SSM Thermal Control Subsystem (TCS)maintains temperatures within set limits for thecomponents mounted in the Equipment Sectionand structures interfacing with the OTA andscience instruments. The TCS maintains safecomponent temperatures even for worst-caseconditions such as environmental fluctuations,passage from “cold” Earth shadow to “hot”solar exposure during each orbit, and heatgenerated from equipment operation.

Specific thermal-protection features of the SSMinclude:

• MLI thermal blankets for the light shield andforward shell

• Aluminum FOSR tape on the aperture doorsurface facing the sun

• Specific patterns of FOSR and MLI blanketson the exteriors of the Equipment Section baydoors, with internal MLI blankets on thebulkheads to maintain thermal balancebetween bays

• Efficient placement of equipment and use ofequipment bay space to match temperaturerequirements, such as placing heat-dissipat-ing equipment on the side of the EquipmentSection mostly exposed to orbit shadow

• Silvered FOSR tape on the aft shroud and aftbulkhead exteriors

• Radiation shields inside the aft shroud doorsand MLI blankets on the aft bulkhead and shroudinteriors to protect the science instruments

• More than 200 temperature sensors and ther-mistors placed throughout the SSM, exter-nally and internally, to monitor individualcomponents and control heater operations.

Figure 5-16 shows the location and type ofthermal protection used on the SSM. SM2observations identified degradations of all of theMLI. Additional material will be installedduring SM3A to cover some of the degradedmaterial and restore the external layer surfaceproperties. The additional material has beenlife-tested to an equivalent of 10 years.

The layer being added to the SSM EquipmentSection is a composite-coated (silicone dioxide)stainless steel layer, known as the New OuterBlanket Layer (NOBL). The light shield/forward shell material is Teflon with a scrimbacking for durability.

5.1.7 Safing (Contingency)System

Overlapping or redundant Telescope equipmentsafeguards against any breakdown.Nonetheless, a contingency or Safing Systemexists for emergency operations. It uses manypointing control and data managementcomponents as well as dedicated PSEAhardware. This system maintains stableTelescope attitude, moves the SAs for maximumSun exposure, and conserves electrical power byminimizing power drain. The Safing System canoperate the spacecraft indefinitely with nocommunications link to ground control.

During scientific observations (normal mode),the Safing System is relegated to monitorautomatically Telescope onboard functions. Thesystem sends Advanced-Computer-generated

5-17K9322-05M

K70110-516

SILVERIZED FOSR90-DEGMULTILAYERINSULATION(MLI)

90-DEG MLI

MLIMLI

MLI

MLIMLIFOSR

LIGHTSHIELD

FORWARDSHELL

ALUMINIZEDFOSR

ALUMINIZED FOSRSILVERIZED

FOSR

AFT SHROUDEQUIPMENTSECTION

OPTICALBLACK

+V1

BLACKCHEMGLAZE

APERTUREDOOR

ALUMINIZED FLEXIBLE OPTICAL SOLAR REFLECTOR (FOSR)

malfunction before any science operations ornormal functions can be resumed.

Since deployment of the Telescope in 1990, theSafing System has seen additionalimprovements to increase its robustness tosurvive hardware failures and still protect theTelescope. Paragraph 5.1.4 describes thesefeatures.

For the modes described above, the SafingSystem operates through computer software. Ifconditions worsen, the system turns overcontrol to the PSEA in Hardware Sun PointMode. Problems that could provoke this actioninclude any of the following:

• Computer malfunction• Batteries losing more than 50 percent of their

charge• Two of the three RGAs failing• DMS failing.

If these conditions occur, the AdvancedComputer stops sending keep-alive signals. Thisis the “handshake” mechanism between theflight software and the PSEA.

In the Hardware Sun Point Mode, the PSEAcomputer commands the Telescope and turns

“keep-alive” signals to the PSEA that indicateall Telescope systems are functioning. Entry intothe Safemode is autonomous once a failure isdetected.

The Safing System is designed to follow aprogression of contingency operating modes,depending on the situation aboard theTelescope. If a malfunction occurs and does notthreaten the Telescope’s survival, the SafingSystem moves into a Software Inertial HoldMode. This mode holds the Telescope in the lastposition commanded. If a maneuver is inprogress, the Safing System completes themaneuver, then holds the Telescope in thatposition, suspending all science operations.Only ground control can return to scienceoperations from Safemode.

If the system detects a marginal electrical powerproblem, or if an internal PCS safety check fails,the Telescope enters the Software Sun PointMode. The Safing System maneuvers theTelescope so the SAs point toward the Sun tocontinuously generate solar power. Telescopeequipment is maintained within operatingtemperatures and above survival temperatures,anticipating a return to normal operations. TheSTOCC must intercede to correct the

Fig. 5-16 Placement of thermal protection on Support Systems Module

5-18K9322-05M

off selected equipment to conserve power.Components shut down include theAdvanced Computer and, within two hours,the SI C&DH. Before this, a payload(instruments) safing sequence begins and, ifit has not already done so, the Telescope turnsthe SAs toward the Sun, guided by the CSSs.The PSEA removes operating power fromequipment not required for Telescopesurvival.

Once ground control is alerted to a problem,NASA management of the STOCC convenes afailure analysis team to evaluate the problemand seek the best and safest corrective actionwhile the Safing System maintains control of theTelescope.

The failure analysis team is led by a seniormanagement representative from NASA/GSFCwith the authority not only to call upon theexpertise of engineers and scientists employedby NASA or its support contractors, but alsoto draft support from any organizationpreviously affiliated with the TelescopeProject. The failure analysis team is charteredto identify the nature of the anomaly and torecommend corrective action. This recom-mendation is reviewed at a higher managementlevel of NASA/GSFC. All changes to theTelescope’s hardware and all software con-figurations require NASA Level I concurrenceas specified in the HST Level I OperationsRequirements Document.

Pointing/Safemode Electronics and RetrievalMode Gyro Assemblies. The PSEA consists of40 electronic printed-board circuits withredundant functions to run the Telescope, evenin the case of internal circuit failure. It weighs86 lb (39 kg) and is installed in the EquipmentSection Bay 8. A backup gyroscope package, theRMGA, is dedicated for the PSEA and is alsolocated in Bay 8. The RMGA consists of three

gyroscopes. These are lower quality rate sensorsthan the RGAs because they are not intendedfor use during observations.

5.2 Optical Telescope Assembly

The OTA was designed and built by the Perkin-Elmer Corporation (Raytheon Optical Systems,Inc.). Although the OTA is modest in size byground-based observatory standards and has astraightforward optical design, its accuracy –coupled with its place above the Earth’satmosphere – renders its performance superior.

The OTA uses a “folded” design, common tolarge telescopes, which enables a long focallength of 189 ft (57.6 m) to be packaged into asmall telescope length of 21 ft (6.4 m). (Severalsmaller mirrors in the science instruments aredesigned similarly to lengthen the light pathwithin the particular science instrument.) Thisform of telescope is called a Cassegrain, and itscompactness is an essential component of anobservatory designed to fit inside the Shuttlecargo bay.

Conventional in design, the OTA isunconventional in other aspects. Largetelescopes at ground-based sites are limited intheir performance by the resolution attainablewhile operating under the Earth’s atmosphere,but the HST orbits high above the atmosphereand provides an unobstructed view of theuniverse. For this reason the OTA was designedand built with exacting tolerances to providenear-perfect image quality over the broadestpossible region of the spectrum.

The OTA is a variant of the Cassegrain, called aRitchey-Chretien, in which both the mirrors arehyperboloidal in shape (having a deepercurvature than a parabolic mirror). This form iscompletely corrected for coma (an imageobservation having a “tail”) and spherical

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K70110-517

FINE GUIDANCESENSORS (3)

AXIAL SCIENCEINSTRUMENTS (4)

PRIMARYMIRROR

FOCAL PLANE(IMAGE FORMED HERE)

RADIAL SCIENCEINSTRUMENTS

SUPPORTSYSTEMSMODULE

STRAY-LIGHTBAFFLES

INCOMING LIGHT

SECONDARY MIRRORAPERTURE DOOR

Fig. 5-17 Light path for the main Telescope

aberrations to provide an aplanatic system inwhich aberrations are correct everywhere in theFOV. The only residual aberrations are fieldcurvature and astigmatism. Both of these arezero exactly in the center of the field andincrease toward the edge of the field. Theseaberrations are easily corrected within theinstrument optics. For example, in the FaintObject Camera (FOC) there is a small telescopedesigned to remove image astigmatism.

Figure 5-17 shows the path of a light ray froma distant star as it travels through the Telescopeto the focus. Light travels down the tube, pastbaffles that attenuate reflected light fromunwanted bright sources, to the 94.5-in. (2.4-m)primary mirror. Reflecting off the front surfaceof the concave mirror, the light bounces back upthe tube to the 12-in. (0.3-m)-diameter convexsecondary mirror. The light is now reflected andconverged through a 23.5-in. (60-cm) hole in theprimary mirror to the Telescope focus, 3.3 ft(1.5 m) behind the primary mirror.

Four science instruments and three FGSs sharethe focal plane by a system of mirrors. A small“folding” mirror in the center of the FOV directs

light into the WFPC2. The remaining “science”field is divided among three axial scienceinstruments, each receiving a quadrant of thecircular FOV. Around the outside of the sciencefield, a “guidance” field is divided among thethree FGSs by their own folding mirrors. EachFGS receives 60 arcmin2 of field in a 90-degreesector. Figure 5-18 shows instrument/sensorfields of view.

The OTA hosts the science instruments andFGSs in that it maintains the structural supportand optical-image stability required for theseinstruments to fulfill their functions (see Fig.5-19). Components of the OTA are the primarymirror, the secondary mirror, the FPS, and theOTA Equipment Section. Perkin-ElmerCorporation designed and built all the opticalassemblies; Lockheed Martin built the OTAequipment section.

5.2.1 Primary Mirror Assemblyand Spherical Aberration

As the Telescope was put through its paces onorbit in 1990, scientists discovered its primarymirror had a spherical aberration. The outer

5-20K9322-05M

edge of the 8-foot (2.4-m) primary mirror wasground too flat by a width equal to 1/50 thethickness of a sheet of paper (about 2 microns).After intensive investigation, the problem wastraced to faulty test equipment used to defineand measure mirror curvature. The opticalcomponent of this test equipment was slightlyout of focus and, as a result, had shown themirror to be ground correctly. After thediscovery, Ball Aerospace scientists andengineers built the Corrective Optics SpaceTelescope Axial Replacement (COSTAR). TheCOSTAR was installed during the FirstServicing Mission in December 1993 andbrought the Telescope back to its originalspecifications.

The primary mirror assembly consists of themirror supported inside the main ring, whichis the structural backbone of the Telescope, andthe main and central baffles shown in Fig. 5-20.

This assembly provides the structural couplingto the rest of the spacecraft through a set ofkinematic brackets linking the main ring to theSSM. The assembly also supports the OTAbaffles. Its major parts are:

• Primary mirror• Main ring structure• Reaction plate and actuators• Main and central baffles.

Primary Mirror. The primary mirror blank, aproduct of Corning Glass Works, is known asultralow-expansion (ULE) glass. It was chosenfor its very low-expansion coefficient, whichensures the Telescope minimum sensitivity totemperature changes. The mirror is of a“sandwich” construction: two lightweightfacesheets separated by a core, or filling, of glasshoneycomb ribs in a rectangular grid (see Fig.5-21). This construction results in an 1800-lb

Fig. 5-18 Instrument/sensor field of view

SPACE TELESCOPEIMAGING SPECTROGRAPH

OPTICAL CONTROLSENSOR (3)

FINEGUIDANCESENSOR 3

NEAR INFRAREDCAMERA ANDMULTI-OBJECTSPECTROMETER

2 DETECTORS,SEPARATE APERTURES

FINE GUIDANCE SENSOR 2

CORRECTIVE OPTICSSPACE TELESCOPEAXIAL REPLACEMENT

FINEGUIDANCESENSOR 1

FAINT OBJECTCAMERA

+V3 AXIS

WIDE FIELD AND PLANETARY CAMERA 2

INCOMING IMAGE(VIEW LOOKING FORWARD

+V1 AXIS INTO PAGE)

14.1 arcminWFC

#1#2 #3

K70110-518

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Fig. 5-19 Optical Telescope Assembly components

Fig. 5-20 Primary mirror assembly

(818-kg) mirror instead of an 8000-lb solid-glassmirror.

Perkin-Elmer (now Raytheon Optical Systems,Inc.) ground the mirror blank, 8 ft (2.4 m) indiameter, to shape in its large optics fabricationfacility. When it was close to its finalhyperboloidal shape, the mirror was transferredto Perkin-Elmer ’s computer-controlledpolishing facility.

After being ground and polished, the mirrorwas coated with a reflective layer of aluminumand a protective layer of magnesium fluorideonly 0.1 and 0.025 micrometer thick,

K70110-520

MAIN BAFFLE

PRIMARY MIRROR

MIRROR MOUNT

ACTUATOR(TYPICAL)

REACTION PLATE

CENTRALBAFFLE

MAIN RING

SUPPORT SYSTEMSMODULE ATTACHMENTBRACKETS

PRIMARY MIRROR ASSEMBLY

SECONDARYMIRRORASSEMBLY GRAPHITE EPOXY

METERING TRUSS

CENTRAL BAFFLE

SUPPORT

FINE GUIDANCESENSORS (3)

FOCAL PLANESTRUCTURE

AXIALSCIENCEINSTRUMENTS(4)

ALUMINUMMAIN BAFFLE

ELECTRONIC BOXES

PRIMARY MIRROR

MAIN RING

RADIAL SCIENCEINSTRUMENT (1)

FIXED-HEADSTAR TRACKER

K70110-519

5-22K9322-05M

respectively. The fluoride layer protects thealuminum from oxidation and enhancesreflectance at the important hydrogen emissionline known as Lyman-Alpha. The reflectivequality of the mirror is better than 70 percent at1216 angstroms (Lyman-Alpha) in theultraviolet spectral range and better than 85percent for visible light.

The primary mirror is mounted to the main ringthrough a set of kinematic linkages. Thelinkages attach to the mirror by three rods thatpenetrate the glass for axial constraint and bythree pads bonded to the back of the glass forlateral support.

Main Ring. The main ring encircles the primarymirror; supports the mirror, the main baffle andcentral baffle, and the metering truss; andintegrates the elements of the Telescope to thespacecraft. The titanium ring is a hollow box

beam 15 in. (38 cm) thick, weighing 1200 lb(545.5 kg), with an outside diameter of 9.8 ft(2.9 m) (see Fig. 5-22). It is suspended inside theSSM by a kinematic support.

Reaction Plate. The reaction plate is a wheel ofI-beams forming a bulkhead behind the mainring, spanning its diameter. It radiates from acentral ring that supports the central baffle. Itsprimary function is to carry an array of heatersthat warm the back of the primary mirror,maintaining its temperature at 70 degrees. Madeof lightweight, stiff beryllium, the plate alsosupports 24 figure-control actuators attached tothe primary mirror and arranged around thereaction plate in two concentric circles. Thesecan be commanded from the ground, ifnecessary, to make small corrections to theshape of the mirror.

Baffles. The baffles of the OTA prevent stray lightfrom bright objects, such as the Sun, Moon, andEarth, from reflecting down the Telescope tubeto the focal plane. The primary mirror assem-bly includes two of the three assembly baffles.

Attached to the front face of the main ring, theouter, main baffle is an aluminum cylinder 9 ft(2.7 m) in diameter and 15.7 ft (4.8 m) long.Internal fins help it attenuate stray light. Thecentral baffle is 10 ft (3 m) long, conical in shape,

FRONTFACESHEET

INNEREDGEBAND

LIGHTWEIGHTCORE

OUTEREDGEBAND

REARFACESHEET

MIRRORCONSTRUCTION

THE MIRROR IS MADE OF CORNING CODE 7971ULTRA-LOW EXPANSION (ULE) SILICA GLASS

K70110-521

Fig. 5-21 Primary mirror construction

RADIALI-BEAMS (15)

FIGURE CONTROLACTUATORS (24)

MAIN RING

MIRROR PADS(24)

FLEXUREMOUNTS(15)

AXIALMOUNTS(15)

INTERCOSTALRIBS (48)

CENTRALRING

CENTRAL BAFFLE BRACKETS (5)

K70110-522

Fig. 5-22 Main ring and reaction plate

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and attached to the reaction plate through a holein the center of the primary mirror. It extendsdown the centerline of the Telescope tube. Thebaffle interiors are painted flat black tominimize light reflection.

5.2.2 Secondary Mirror Assembly

The Secondary Mirror Assembly cantilevers offthe front face of the main ring and supports thesecondary mirror at exactly the correct positionin front of the primary mirror. This positionmust be accurate within 1/10,000 in. wheneverthe Telescope is operating. The assemblyconsists of the mirror subassembly, a light baffle,and an outer graphite-epoxy metering trusssupport structure (see Fig. 5-23).

The Secondary Mirror Assembly contains themirror, mounted on three pairs of alignmentactuators that control its position andorientation. All are enclosed within the centralhub at the forward end of the truss support.

The secondary mirror has a magnification of10.4X, converting the primary-mirror

converging rays from f/2.35 to a focal ratiosystem prime focus of f/24 and sending themback toward the center of the primary mirror,where they pass through the central baffle to thefocal point. The mirror is a convex hyperboloid12 in. (0.3 m) in diameter and made of Zerodurglass coated with aluminum and magnesiumfluoride. Steeply convex, its surface accuracy iseven greater than that of the primary mirror.

Ground command adjusts the actuators to alignthe secondary mirror to provide perfect imagequality. The adjustments are calculated fromdata picked up by tiny optical control systemsensors located in the FGSs.

The principal structural element of the SecondaryMirror Assembly is the metering truss, a cage with48 latticed struts attached to three rings and acentral support structure for the secondary mirror.The truss, 16 ft (4.8 m) long and 9 ft (2.7 m) indiameter, is a graphite, fiber-reinforced epoxystructure. Graphite was chosen for its highstiffness, light weight, and ability to reduce thestructure’s expansiveness to nearly zero. This isvital because the secondary mirror must stayperfectly placed relative to the primary mirror,accurate to within 0.0001 in. (2.5 micrometers)when the Telescope operates.

The truss attaches at one end to the front faceof the main ring of the Primary MirrorAssembly. The other end has a central hub thathouses the secondary mirror and baffle alongthe optical axis. Aluminized mylar MLI in thetruss compensates for temperature variations ofup to 30 degrees Fahrenheit when the Telescopeis in Earth’s shadow so the primary andsecondary mirrors remain aligned.

The conical secondary mirror subassembly lightbaffle extends almost to the primary mirror. Itreduces the stray bright-object light fromsources outside the Telescope FOV.

BASE PLATE

ACTUATOR PAIR

SECONDARY MIRROR

SECONDARYMIRROR BAFFLE

ACTUATORPAIR

CLAMP

FLEXURE

SHROUD

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Fig. 5-23 Secondary mirror assembly

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5.2.3 Focal Plane StructureAssembly

The FPS is a large optical bench that physicallysupports the science instruments and FGSs andaligns them with the image focal plane of theTelescope. The -V3 side of the structure, awayfrom the Sun in space, supports the FHSTs andRSUs (see Fig. 5-24). It also provides facilities foron-orbit replacement of any instruments andthermal isolation between instruments.

The structure is 7 ft (2.1 m) by 10 ft (3.04 m) longand weighs more than 1200 lb (545.5 kg).Because it must have extreme thermal stabilityand be stiff, lightweight, and strong, the FPS isconstructed of graphite-epoxy, augmented withmechanical fasteners and metallic joints atstrength-critical locations. It is equipped withmetallic mounts and supports for OrbitalReplacement Units (ORU) used duringmaintenance.

The FPS cantilevers off the rear face of the mainring, attached at eight flexible points that adjustto eliminate thermal distortions. The structureprovides a fixed alignment for the FGSs. It has

guiderails and latches at each instrumentmounting location so Shuttle crews can easilyexchange science instruments and otherequipment in orbit.

5.2.4 OTA Equipment Section

The Equipment Section for the OTA is a largesemicircular set of compartments mountedoutside the spacecraft on the forward shell ofthe SSM (see Fig. 5-25). It contains the OTAElectrical Power and Thermal ControlElectronics (EP/TCE) System, Fine GuidanceElectronics (FGE), Actuator Control Electronics(ACE), Optical Control Electronics (OCE), andthe fourth DMS DIU. The OTA EquipmentSection has nine bays: seven for equipmentstorage and two for support. All bays haveoutward-opening doors for easy astronautaccess, cabling and connectors for theelectronics, and heaters and insulation forthermal control.

The EP/TCE System distributes power from theSSM EPS and the OTA system. Thermostatsregulate mirror temperatures and prevent mirrordistortion from the cold of space. The electricaland thermal electronics also collect thermalsensor data for transmission to the ground.

The three FGE units provide power, commands,and telemetry to each FGS. The electronicsperform computations for the sensor andinterface with the spacecraft pointing system foreffective Telescope line-of-sight pointing andstabilization. There is a guidance electronicsassembly for each guidance sensor.

The ACE unit provides the command andtelemetry interface to the 24 actuators attachedto the primary mirror and to the six actuatorsattached to the secondary mirror. Theseelectronics select which actuator to move andmonitor its response to the command.

K70110-524

SCIENCEINSTRUMENTCONNECTORPANEL

AXIAL SCIENCEINSTRUMENT

FOCAL PLANESTRUCTURE

PRIMARY MIRRORMAIN RING

RATE SENSINGUNIT FOR RATE GYROASSEMBLIES (3)

CREWSYSTEMHANDLESFIXED-HEADSTAR TRACKERS (3)

SCIENCE INSTRUMENTLATCHING MECHANISM

V2

V1

V3

Fig. 5-24 Focal plane structure

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Positioning commands go from the ground tothe electronics through the DIU.

The OCE unit controls the optical controlsensors. These white-light interferometersmeasure the optical quality of the OTA and sendthe data to the ground for analysis. There is oneoptical control sensor for each FGS, but the OCEunit runs all control sensors. The DIU is anelectronic interface between the other OTAelectronics units and the Telescope commandand telemetry system.

5.3 Fine Guidance Sensor

The three FGSs are located at 90-degreeintervals around the circumference of the focalplane structure, between the structure frameand the main ring. Each sensor is 5.4 ft (1.5 m)long and 3.3 ft (1 m) wide and weighs 485 lb(220 kg).

Each FGS enclosure houses a guidance sensorand a wavefront sensor. The wavefront sensorsare elements of the optical control sensor usedto align and optimize the optical system of theTelescope.

The Telescope’s ability to remain pointing at adistant target to within 0.005 arcsec for longperiods of time is due largely to the accuracy ofthe FGSs. They lock on a star and measure any

apparent motion to an accuracy of 0.0028 arcsec.This is equivalent to seeing from New York Citythe motion of a landing light on an aircraftflying over San Francisco.

When two sensors lock on a target, the thirdmeasures the angular position of a star, aprocess called astrometry. Sensor astrometricfunctions are discussed in Section 4. DuringSM2 a re-certified FGS (S/N 2001) was installedas a replacement in the HST FGS Bay 1. DuringSM3A a re-certified FGS (S/N 2002) will beinstalled in the HST FGS Bay 2.

5.3.1 Fine Guidance SensorComposition and Function

Each FGS consists of a large structure housinga collection of mirrors, lenses, servos to locatean image, prisms to fine-track the image, beamsplitters, and four photomultiplier tubes, asshown in Fig. 5-26. The entire mechanismadjusts to move the Telescope into precisealignment with a target star. Each FGS has alarge (60 arcmin2) FOV to search for and trackstars, and a 5.0 arcsec2 FOV used by the detectorprisms to pinpoint the star.

The sensors work in pairs to aim the Telescope.The Guide Star Selection System, developed bythe Science Institute, catalogs and charts guidestars near each observation target to make it

K70110-525

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Fig. 5-25 Optical Telescope Assembly Equipment Section

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easier to find the target. First, one sensorsearches for a target guide star. After the firstsensor locks onto a guide star, the second sensorlocates and locks onto another target guide star.The guide stars, once designated and located,keep the image of the observation target in theaperture of the selected science instrument.

Each FGS uses a 90-degree sector of theTelescope’s FOV outside the central “science”field. This region of the FOV has the greatestastigmatic and curvature distortions. The size ofthe FGS’s FOV was chosen to heighten theprobability of finding an appropriate guide star,even in the direction of the lowest starpopulation near the galactic poles.

An FGS “pickoff” mirror intercepts theincoming stellar image and projects it into thesensor ’s large FOV. Each FGS FOV has 60arcmin2 available. The guide star of interestcan be anywhere within this field, so the FGSwill look anywhere in that field to find it.After finding the star, the sensor locks onto itand sends error signals to the Telescope,telling it how to move to keep the star imageperfectly still.

The FGS can move its line of sight anywherewithin its large FOV using a pair of star selectorservos. Each can be thought of as an opticalgimbal: One servo moves in a north-southdirection, the other east and west. They steer thesmall FOV (5 arcsec2) of the FGS detectors toany position in the sensor field. Encoders withineach servo system send back the exact coordinatesof the detector field centers at any point.

Because the exact location of a guide star maybe uncertain, the star selector servos also cancause the detector to search the region aroundthe most probable guide star position. Itsearches in a spiral pattern, starting at the centerand spiraling out until it finds the guide star itseeks. Then the detectors are commanded to gointo fine-track mode and hold the star imageexactly centered in the FOV, while the starselector servo encoders send information aboutthe position of the star to the spacecraft PCS.

The detectors are a pair of interferometers,called Koester ’s prisms, coupled tophotomultiplier tubes (see Fig. 5-27). Eachdetector operates in one axis, so two detectorsare needed. Operating on the incomingwavefront from the distant guide star, theinterferometers compare the wave phase at oneedge of the Telescope’s entrance aperture with

Fig. 5-26 Cutaway view of Fine Guidance Sensor

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STARSELECTORMIRRORS

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Fig. 5-27 Optical path of Fine Guidance Sensor

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the phase at the opposite edge. When the phasesare equal, the star is exactly centered. Any phasedifference shows a pointing error that must becorrected.

Along the optical path from Telescope todetector are additional optical elements thatturn or fold the beam to fit everything inside theFGS enclosure, and to correct the Telescope’sastigmatism and field curvature. All opticalelements are mounted on a temperature-controlled, graphite-epoxy composite opticalbench.

5.3.2 Articulated Mirror System

Analysis of the FGS on-orbit data revealed thatminor misalignments of the optical pupilcentering on Koester’s prism interferometer inthe presence of spherical aberration preventedthe FGS from achieving its optimumperformance. During the recertification of FGS(S/N 2001), fold flat #3 in the radial bay moduleoptical train was mechanized to allow on-orbitalignment of the pupil.

Implementation of this system utilized existingsignals and commands by rerouting them witha unique interface harness enhancement kit(OCE-EK) interfacing the OCE, the DIU, and theFine Guidance System/Radial Bay Module(FGS/RBM). The OCE-EK was augmented withthe Actuator Mechanism Electronics (AME) andthe fold flat #3 Actuator Mechanism Assembly(AMA) located internal to the FGS/RBM.Ground tests indicate a substantial increase inperformance of the FGS with this innovativedesign improvement.

5.4 Solar Array and JitterProblems

From the beginning, in the late 1970s, the SAs– designed by the European Space Agency and

built by British Aerospace, Space Systems –have been scheduled for replacement because oftheir power loss from radiation exposure inspace. However, as engineers put the Telescopethrough its paces in April 1990, they discoveredtwo problems: a loss of focus and images thatjittered briefly when the Telescope flew into andout of Earth’s shadow. The jitter problem wastraced to the two large SAs. Abrupt temperaturechanges, from -150 to 200 degrees Fahrenheitduring orbit, cause the panels to distort twiceduring each orbit. As a temporary fix, engineerscreated software that commanded the PCS tocompensate for the jitter automatically. Theproblem was mitigated during SM1 by thereplacement of the old arrays with new onesthat had been modified to reduce thermalswings of the bi-stems.

5.4.1 Configuration

The SAs are two large rectangular wings ofretractable solar cell blankets fixed on a two-stem frame. The blanket unfurls from a cassettein the middle of the wing. A spreader bar ateach end of the wing stretches the blanket andmaintains tension. For the replacement SAsdelivered to the Telescope during the FirstServicing Mission, the spring-loaded rollerassembly was replaced by a series of springsconnecting the spreader bar to the blanket. Thischange eliminated the jitter induced into theTelescope as it passed from eclipse (night) intosunlight (day) each orbit.

The wings are on arms that connect to a driveassembly on the SSM forward shell at one endand to the secondary deployment mechanism(blankets and bistems) on the other end. Thetotal length of the cassette, arm, and drive is15.7 ft (4.8 m) (see Fig. 5-28).

Each wing has 10 panels that roll out from thecassette. The panels are made of 2438 solar cells

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attached to a glass-fiber/Kapton surface, withsilver mesh wiring underneath, covered byanother layer of Kapton. The blankets are lessthan 500 micrometers thick, so they roll uptightly when the wings are stowed. Each wingweighs 17 lb (7.7 kg) and, at full extension, is40 ft (12.1 m) long and 8.2 ft (2.5 m) wide.

5.4.2 Solar Array Subsystems

The SA subsystems include the primary andsecondary deployment mechanisms, theirdrives, and associated electronics.

The primary deployment mechanism raises theSA mast from the side of the SSM to a standingposition perpendicular to the Telescope. Thereare two mechanisms, one for each wing. Eachmechanism has motors to raise the mast andsupports to hold it in place when erect.

An astronaut can raise the array mast manuallyif the drive power fails. Using a wrench fittingon the deployment drive, the astronaut hand-cranks the mast after releasing the latches.

Once the SA is raised, the secondarydeployment mechanism unfurls the wingblankets. Each wing has a secondary

mechanism assembly: a cassette drum to holdsolar panels, a cushion to protect the blanket,and motors and subassemblies. The assemblyrolls out the blanket, applies tension evenly sothe blankets stretch, and transfers data andpower along the wing assembly. The blanketcan roll out completely or part way. Thesecondary deployment mechanism also has amanual override (see Fig. 5-29).

A SA drive at the base of each mast rotates thedeployed array toward the Sun, turning ineither direction. Each drive has a motor thatrotates the mast on command and a brake tokeep the array in a fixed position with respectto the Telescope. The drive can move and lockthe SA into any position.

Each drive has a clamp ring that acts as a releasemechanism if opened. This allows a crewmember to jettison the entire SA if necessary.

Two electronics assemblies (boxes) – the SolarArray Deployment Electronics and the SolarArray Drive Electronics – control and monitorall functions of each SA. They provide theelectronic interface to the other Telescopesystems and generate the commands for theprimary and secondary deploymentmechanisms and the SA drive.

FITTING FORMANUAL DEPLOYMENT

MAST

SECONDARY DRIVEMECHANISM

WING

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Fig. 5-29 Fitting for Solar Array manual deployment

COMPENSATOR RAIL

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BISTEMWITH NEWTHERMALSHIELD

SPREADERBAR

PRIMARYDEPLOYMENTMECHANISM

SOLAR ARRAY DRIVEDEPLOYMENT ARM

BISTEM CASSETTE

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Fig. 5-28 Solar Array wing detail

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5.4.3 Solar Array Configurationfor Servicing Mission 3A

The Solar Array wings will remain deployedduring servicing. This will allow the Telescope’sbatteries to remain fully charged during themission and will not impact servicing activities.

5.5 Science Instrument Controland Data Handling Unit

The SI C&DH unit keeps all science instrumentsystems synchronized. It works with the DMUto process, format, temporarily store on the datarecorders, or transmit all science and engineer-ing data created by the instruments to theground. Fairchild Camera and Instrument Cor-poration and IBM built this unit.

5.5.1 Components

The SI C&DH unit is a collection of electroniccomponents attached to an ORU tray mountedon the door of Bay 10 in the SSM EquipmentSection (see Fig. 5-30). Small Remote InterfaceUnits (RIU), also part of the system, provide theinterface to individual science instruments.Components of the SI C&DH unit are the NASAStandard Spacecraft Computer (NSCC-I), twostandard interface circuit boards for the com-puter, two control units/science data formatterunits, two CPU modules, a PCU, two RIUs, andvarious memory, data, and command commu-nications lines (buses) connected by couplers.The SI C&DH components are redundant so thesystem can recover from any single failure.

NASA Computer. The NSSC-I has a CPU andeight memory modules, each holding 8,192eighteen-bit words. One embedded softwareprogram (the “executive”) runs the computer. Itmoves data, commands, and operation pro-grams (called applications) for individual sci-ence instruments in and out of the processingunit. The application programs monitor and

control specific instruments and analyze andmanipulate the collected data.

The memory stores operational commandsfor execution when the Telescope is not incontact with the ground. Each memory unithas five areas reserved for commands andprograms unique to each science instru-ment. The computer can be reprogrammedfrom the ground for future requests or forworking around failed equipment.

Standard Interface Unit. The standard in-terface board is the communications bridgebetween the computer and the CU/SDF.

Control Unit/Science Data Formatter. Theheart of the SI C&DH unit is the CU/SDF.It formats and sends all commands anddata to designated destinations such as theDMU of the SSM, the NASA computer, andthe science instruments. The unit has a mi-croprocessor for control and formattingfunctions.

Fig. 5-30 Science Instrument Control and DataHandling unit

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EXTRAVEHICULAR ACTIVITYHANDLE

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The CU/SDF receives ground commands, datarequests, science and engineering data, andsystem signals. Two examples of system signalsare “time tags,” clock signals that synchronizethe entire spacecraft, and “processor interfacetables,” or communications codes. The CU/SDFtransmits commands and requests afterformatting them so that the specific destinationunit can read. For example, ground commandsand SSM commands are transmitted withdifferent formats. Ground commands use 27-bitwords, and SSM commands use 16-bit words.The formatter translates each command signalinto a common format. The CU/SDF alsoreformats and sends engineering and sciencedata. Onboard analysis of the data is an NSSC-Ifunction.

Power Control Unit. The PCU distributes andswitches power among components of the SIC&DH unit. It conditions the power required byeach unit. For example: The computer memoryboards typically need +5 volts, -5 volts, and +12volts; the CU/SDF, on the other hand, requires+28 volts. The PCU ensures that all voltagerequirements are met.

Remote Interface Units. RIUs transmitcommands, clock and other system signals, andengineering data between the scienceinstruments and the SI C&DH unit. The RIUsdo not send science data. There are six RIUs inthe Telescope: five attached to the scienceinstruments and one dedicated to the CU/SDFand PCUs in the SI C&DH unit. Each RIU canbe coupled with up to two expander units.

Communications Buses. The SI C&DH unitcontains data bus lines that pass signals anddata between the unit and the scienceinstruments. Each bus is multiplexed: one linesends system messages, commands, andengineering data requests to the module units,and a reply line transmits requested information

and science data back to the SI C&DH unit. Acoupler attaches the bus to each remote unit.This isolates the module if the RIU should fail.The SI C&DH coupler unit is on the ORU tray.

5.5.2 Operation

The SI C&DH unit handles science instrumentsystem monitoring (such as timing and systemchecks), command processing, and dataprocessing.

System Monitoring. Engineering data tells themonitoring computer whether instrumentsystems are functioning. At regular intervals,varying from every 500 milliseconds to every 40seconds, the SI C&DH unit scans all monitoringdevices for engineering data and passes data tothe NSCC-I or SSM computer. The computersprocess or store the information. Any failureindicated by these constant tests could initiatea “safing hold” situation (see para 5.1.7), andthus a suspension of science operations.

Command Processing. Figure 5-31 shows theflow of commands within the SI C&DH unit.Commands enter the CU/SDF (bottom right inthe drawing) through the SSM Command DIU(ground commands) or the DIU (SSMcommands). The CU/SDF checks and reformatsthe commands, which then go either to the RIUsor to the NSCC-I for storage. “Time-tagged”commands, stored in the computer’s memory(top right of drawing), also follow this process.

Each command is interpreted as “real time,” asif the SI C&DH just received it. Manycommands actually are onboard storedcommands activated by certain situations. Forexample, when the Telescope is positioned fora programmed observation using the SpaceTelescope Imaging Spectrograph, that programis activated. The SI C&DH can issue certainrequests to the SSM, such as to execute a limited

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number of pointing control functions to makesmall Telescope maneuvers.

Science Data Processing. Science data can comefrom all science instruments at once. The CU/SDF transfers incoming data through computermemory locations called packet buffers. It fillseach buffer in order, switching among them asthe buffers fill and empty. Each data packet goesfrom the buffer to the NSCC-I for furtherprocessing, or directly to the SSM for storage inthe data recorders or transmission to theground. Data returns to the CU/SDF aftercomputer processing. When transmitting, theCU/ SDF must send a continuous stream ofdata, either full packet buffers or empty bufferscalled filler packets, to maintain a synchronizedlink with the SSM. Special checking codes

(Reed-Solomon and pseudo-random noise) canbe added to the data as options. Figure 5-32shows the flow of science data in the Telescope.

5.6 Space Support Equipment

The Hubble Space Telescope was designed to bemaintained, repaired, and enhanced while inorbit, extending its life and usefulness. Forservicing, the Space Shuttle will capture andposition the Telescope vertically in the aft end ofthe cargo bay, and the crew will performmaintenance and replacement tasks. The SpaceSupport Equipment (SSE) to be used during themission provides a maintenance platform to holdthe Telescope, provides electrical support of theTelescope during servicing, and provides storagefor replacement components known as ORUs.

Fig. 5-31 Command flow for Science Instrument Control and Data Handling unit

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The major SSE items used for the ServicingMission 3A are the Flight Support System(FSS) and the ORU Carrier (ORUC).Additionally, crew aids and tools will be usedduring servicing.

5.6.1 Flight Support System

The FSS provides the platform that holds theTelescope during servicing (see Fig. 5-33). TheFSS has been used in different configurations forthe HST First and Second Servicing Missions,the Solar Maximum Repair Mission, and theUpper Atmospheric Research Satellite DeployMission. The FSS consists of two majorcomponents: a horseshoe-shaped cradle and asupporting latch beam providing a structuraland electrical interface with the Shuttle. Acircular ring called the Berthing and PositioningSystem (BAPS) interfaces with the Telescope,allowing it to pivot or rotate during the mission.

The BAPS ring is pivoted down and locked forliftoff. During the mission, the ring is pivoted

up to a horizontal position. A closed-circuittelevision camera mounted to the FSS helpsastronauts guide the Telescope onto the ring.Three remote-controlled latches on the ring graband hold three towel-rack-like pins on the rearof the Telescope. A remote-controlled electricalumbilical connector on the FSS engages theTelescope, providing it with orbiter powerthrough the FSS. This power helps relieve thedrain on the Telescope’s batteries during themission. Radio communications providetelescope telemetry data and control.

Once the Telescope is berthed to the ring (seeFig. 5-34), the FSS can pivot (tilt) or rotate theTelescope. This positions the appropriate regionof the Telescope for access during extravehicularactivity. Additionally, the ring can pivot theTelescope to an appropriate attitude for orbiterreboost.

During the first EVA, crew members will installthe BAPS Support Post (BSP). The BSP providesan additional linkage to support and isolate the

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Fig. 5-32 Flow of science data in the Hubble Space Telescope

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Fig. 5-33 Flight Support System configuration

5.6.2 Orbital Replacement UnitCarrier

An ORUC is used to carry replacements intoorbit and to return replaced units to Earth. Thecarrier consists of a Spacelab pallet outfittedwith shelves and protective enclosures to holdthe replacement units (see Fig. 5-35). Items onthe ORUC for SM3A include (1) the AdvancedComputer and two spare VIKs in the LOPE and(2) a Solid State Recorder, an S-Band SingleAccess Transmitter, the Rate Sensor Units, andassociated flight harnesses in the COPE.

The FGS will be transported in the FGSScientific Instrument Protective Enclosure(FSIPE). A spare Advanced Computer, a spareRate Sensor Unit, and Multi-Layer Insulationpatches will be carried in the Axial ScientificInstrument Protective Enclosure (ASIPE). TheNOBL will be carried in a NOBL ProtectiveEnclosure (NPE).

All ORUs and scientific instruments are carriedwithin protective enclosures to provide them abenign environment throughout the mission.

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Telescope during EVAs and for any orbitalreboosts. The BSP will remain in position forlanding.

Astronauts remotely control all FSS mechanisms– berthing latches, umbilical connector, pivoter,BSP lock, rotator, and ring down-lock – from theorbiter’s aft flight deck, providing the crewmaximum flexibility. Besides being fullyelectrically redundant, each mechanismcontains manual overrides and backups toensure mission success and astronaut safety.

SPACETELESCOPE

BERTHING ANDPOSITIONINGRING

FLIGHT SUPPORT SYSTEM

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Fig. 5-34 Flight Support System Berthing andPositioning System ring pivoted up withTelescope berthed

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The enclosures protect the instruments fromcontamination and maintain the temperature ofthe instruments or ORUs within tight limits.

Instruments are mounted in the enclosuresusing the same manually driven latch systemthat holds instruments in the Telescope. TheASIPE and FSIPE are mounted to the pallet ona spring system that reduces the level ofvibration the instruments receive, especiallyduring liftoff and landing.

The other ORUs are carried in an additionalenclosure called the Large ORU ProtectiveEnclosure (LOPE). The enclosure also providescontamination and thermal control, though notto such stringent requirements as the SIPE. The

LOPE contains Transport Modules that aredesigned to custom fit each ORU. The transportmodules have foam or Visco-Elastic Materialthat surrounds the ORU and isolates it fromlaunch and landing vibration environments.

During the change-out process, replacedscience instruments are stored temporarily inthe ORUC. A typical change-out begins withan astronaut removing the old instrumentfrom the Telescope and attaching it to abracket on the ORUC. The astronaut thenremoves the new instrument from itsprotective enclosure and installs it in theTelescope. Finally, the astronaut places the oldinstrument in the appropriate protectiveenclosure for return to Earth.

Fig. 5-35 Orbital Replacement Unit Carrier

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The ORUC receives power for its TCS from theFSS. The carrier also provides temperaturetelemetry data through the FSS for readout inthe Shuttle and on the ground during themission.

5.6.3 Crew Aids

Astronauts perform extravehicular activitiesusing many tools to replace instruments andequipment, to move around the Telescope andthe cargo bay, and to operate manual overridedrives. Tools and equipment, bolts, connectors,and other hardware were standardized not onlyfor the Telescope but also between the Telescopeand the Shuttle. For example, grapplingreceptacles share common features.

To move around the Telescope, the crew uses225 ft of handrails encircling the spacecraft. Forvisibility, the rails are painted yellow. Inaddition, the crew can hold onto guiderails,trunnion bars, and scuff plates fore and aft.

The astronauts can install portable handholdplates where there are no permanent holds, such Fig. 5-36 Portable Foot Restraint

PITCH CONTROL

ELEVATIONCONTROL

A

A

LOCK

YAW CONTROL

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as on the FGS. Another tool is the Portable FootRestraint (PFR), shown in Fig. 5-36.

While the astronauts work, they use tethers tohook tools to their suits and tie replacementunits to the Telescope. Each crew member hasa ratchet wrench to manually crank the antennaand array masts if power for the mast drivesfails. A power wrench also is available if hand-cranking is too time consuming. Other handtools include portable lights and a jettisonhandle, which attach to sockets on the aperturedoor and to SA wings so the crew can push theequipment away from the Telescope.

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ubble Space Telescope operations areof two types: science operations thatplan and conduct the HST science

program – observing celestial objects andgathering data – and mission operations thatcommand and control HST to implement theobservation schedule and maintain theTelescope’s overall performance.

Science and mission operations often coincideand interact. For example, a science instrumentmay observe a star and calibrate incomingwavelengths against standards developedduring scientific verification. Mission opera-tions monitor observations to ensure thatTelescope subsystems have functioned correctly.

Mission operations are carried out by theTelescope ground system, which consists offacilities at the Space Telescope ScienceInstitute (STScI), and the Space TelescopeOperations Control Center (STOCC), thePacket Processing Facility (PACOR), and otherinstitutional facilities at the Goddard SpaceFlight Center (GSFC).

The STScI oversees science operations. Ithosts astronomers, evaluates and choosesobservation programs, schedules the selectedobservations, generates an overall missiontimeline and command sequences, and storesand analyzes science data from the Telescope.Meanwhile, the flight operations teamconducts mission operations from theSTOCC. The team interacts with the STScI toreceive daily mission schedules, to processengineering data and displays, and tomanage the engineering data archive.

6.1 Space Telescope Science Institute

Located in Baltimore, Maryland, STScI isresponsible to GSFC for the science programs

on the HST. It is operated by the Association ofUniversities for Research in Astronomy(AURA), a consortium of 29 United Statesuniversities that operate several national facili-ties for astronomy.

STScI solicits and reviews observationproposals and selects observations to becarried out. It schedules observations andassists guest observers in their work; generatesan integrated science and engineering timelineto support all spacecraft activities, includingany special engineering tests; and provides thefacilities and software to reduce, analyze,archive, and distribute Telescope data.

STScI also monitors the Telescope and scienceinstruments for characteristics that could affectscience data collection, such as instrumentperformance quality, pointing inaccuracies,and Telescope focus.

6.1.1 Scientific Goals

STScI helps conduct the science program tomeet the overall scientific goals of the Telescopeprogram, set by the Institute and NASA inconsultation with AURA’s Space TelescopeInstitute Council and committees representingthe international astronomical community.

6.1.2 Institute Software

Computer hardware and software play animportant role in STScI work. The STScI groundsystem consists of a planning and schedulingsystem and a science data processing system.The STScI also created the guide star catalogused to support the precise pointing require-ments of the HST pointing control subsystem.In addition, Science Data Analysis Software(SDAS) provides analytical tools forastronomers studying observational data.

HHST OPERATIONS

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STScI handles mission planning andscheduling, observation support, softwaresupport, and routine data processing.Together, these functions perform computationsneeded to run the science operations onthe Telescope.

As part of the Planning and SchedulingSystem, the STScI Guide Star SelectionSystem (GSSS) provides reference stars andother bright objects so the Fine GuidanceSensors (FGS) can point the Telescope accu-rately. This system selects guide stars thatcan be located unambiguously in the skywhen the sensors point the Telescope. Theguide star catalog has information on 20million celestial objects, created from 1,477photographic survey plates covering theentire sky.

After the STScI Science Data Pipelinecollects, edits, measures, and archivesscience data, observers can use SDAS toanalyze and interpret the data.

6.1.3 Selecting Observation Proposals

Astronomers worldwide may use theTelescope. Any scientist may submit aproposal to the STScI outlining an observingprogram and describing the scientific objec-tives and instruments required. The STScIselects observations by evaluating theserequests for technical feasibility, conductingpeer reviews, and choosing the highest rankedproposals. Because individual astronomersand astronomy teams submit many moreproposals than can possibly be accepted, ateam approach is encouraged. The final deci-sion rests with the STScI director, advised by areview committee of astronomers and scien-tists from many institutions.

6.1.4 Scheduling Selected Observations

The primary scheduling consideration isavailability of a target, limited by environmentaland stray-light constraints – for example, a faintobject that must be observed when theTelescope is in Earth’s shadow. The scheduletakes into consideration system limits, observa-tions that use more than one instrument, andrequired time for special observations.

6.1.5 Data Analysis and Storage

STScI archives calibrated science data.Computer resources include the HubbleData Archive, SDAS and other selectedcomputer facilities.

Science Data Pipeline processing receivesscience data from the PACOR at GSFC, thenautomatically formats it and verifies its quality.It calibrates data to remove the instrument’sproperties such as variation in the detector’ssensitivity across the data field. Then the soft-ware places the data on digital archive mediafrom which the data can be formatted anddistributed to the observer or archivalresearcher. The STScI processes all data within24 hours after receipt.

STScI is responsible for storing the massiveamount of data collected by the Telescope. TheHubble Data Archive catalog records the loca-tion and status of data as it pours into thestorage banks. Observers and visitingastronomers can easily retrieve the stored datafor examination or use data manipulationprocedures created by the STScI. The EuropeanSpace Agency (ESA) provides approximately15 staff members co-located with STScI staffand operates its own data analysis facility inGarching, Germany.

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In addition to science data, the STScI storesengineering data. This is important for devel-oping more efficient use of the Telescopesystems and for adjusting Telescope operationsbased on engineering findings, for example, ifan instrument provides unreliable data incertain temperature ranges.

6.2 Space Telescope OperationsControl Center

The STOCC flight operation team runsday-to-day spacecraft operations. In addition,the STOCC team works with the NASACommunications Network (NASCOM) and theTracking and Data Relay Satellite System(TDRSS) to facilitate HST data communications.

POCC uses mission-control facilities at GSFCbuilt specifically for HST operations. TheVision 2000 Control Center System (CCS) hasbeen developed to provide the next generationof operational capabilities in the STOCC. Builtspecifically to support the HST Third ServicingMissions (SM3A and SM3B), the CCS providesdistributed capabilities with a completely newuser interface that is on the forefront of space-craft operations.

STOCC has three major operational responsi-bilities:

• Spacecraft operations, including sendingcommands to the spacecraft

• Telemetry processing• Offline support.

Most specific spacecraft operations areexecuted based on time-tagged commandsmanaged by the Telescope’s onboard soft-ware. The STOCC flight operations team,using the CCS, uplinks the commands to theHST computers.

Engineering telemetry, received in the STOCCfrom the GSFC institutional communicationsystem, provides information that reflects theHST spacecraft subsystem status. For example,telemetry can verify Pointing Control Systemoperation and stability performance of theTelescope. In many cases, consultation betweenSTOCC and STScI is necessary, particularly ifthe data affects an ongoing observation.

An important part of the ground system isPACOR processing. When data arrives fromNASCOM for science handling, PACOR refor-mats the data, checks for noise or transmissionproblems, and passes each packet of data alongto the STScI with a data quality report.

Another important STScI function is to supportobservers requiring a “quick-look” analysis ofdata. STScI alerts PACOR to that need, and theincoming data can be processed for theobserver.

TDRSS has two communications relaysatellites 130 degrees apart, with a groundterminal at White Sands, New Mexico.There is a small “zone of exclusion” whereEarth blocks the Telescope signal to eitherof the satellites, but up to 91 percent of theTelescope’s orbit is within communicationscoverage. Tracking and Data RelaySatellites (TDRS) receive and send bothsingle-access (science data) and multiple-access (commands and engineering data)channels.

6.3 Operational Characteristics

Three major operational factors affect thesuccess of the Telescope: orbital characteristicsfor the spacecraft, its maneuvering characteris-tics, and communications characteristics forsending and receiving data and commands.

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6.3.1 Orbital Characteristics

The orbit of the Telescope is approximately320 nmi (593 km). The orbit inclines at a 28.5-degree angle from the equator because theShuttle launch was due east from KennedySpace Center. The chosen orbit puts the Sunin the Telescope orbital plane so that sunlightfalls more directly on the Solar Arrays. Inaddition, the orbit is high enough that aero-dynamic drag from the faint atmosphere willnot decay the Telescope’s orbit to below theminimum operating altitude.

The Telescope completes one orbit every 97minutes, passing into the shadow of theEarth during each orbit. The time in shadowvaries from 28 to 36 minutes. The variationduring a nominal 30-day period is between34.5 and 36 minutes in shadow. If Earthblocks an object from the Telescope, theTelescope reacquires the object as the space-craft comes out of Earth’s occultation. Faint-object viewing is best while the Telescope isin Earth’s shadow.

The Telescope orbit is tracked by the TDRSS,which plots the spacecraft’s orbit at leasteight times daily and sends the data to theFlight Dynamics Facility at GSFC. Althoughthis helps predict future orbits, some inaccu-racy in predicting orbital events, such as exitfrom Earth’s shadow, is unavoidable. Theenvironmental elements with greatest effecton the Telescope’s orbit are solar storms andother solar activities. These thicken theupper atmosphere and increase the dragforce on the Telescope, accelerating the orbitdecay rate considerably.

6.3.2 Celestial Viewing

The Telescope is pointed toward celestial

targets as a normal orientation to exposeinstrument detectors for up to 10 hours, ifneeded. A continuous-viewing zone exists,parallel to the orbit plane of the Telescopeand up to 18 degrees on either side of thenorth and south poles of that orbital plane(see Fig. 6-1). Otherwise, celestial viewingdepends on how long a target remainsunblocked by Earth.

The amount of shadow time available forfaint-object study also affects celestialobservations. Shadow time for an observa-tion varies with the time of year and thelocation of the target relative to the orbitplane. Astronomers use a geometricformula to decide when in a given period atarget will be most visible while theTelescope is in shadow.

Other sources affecting celestial viewing arezodiacal light and integrated or backgroundstarlight.

Fig. 6-1 “Continuous-zone” celestial viewing

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6.3.3 Solar System Object Viewing

Solar system objects also are affected by thefactors mentioned for celestial viewing. Inaddition, the Telescope works with impreciseorbit parameters for itself and objects such asthe outer planets and comets. For example,Neptune’s center may be off by 21 km whenthe sensors try to lock onto it because theTelescope is changing its position in orbit,which affects the pointing direction towardnearby objects. However, most solar systemobjects are so bright the Telescope needs only aquick snapshot of the object to fix its position.Tracking inaccuracies are more likely to cause ablurred image if they occur with long-exposureobservations of dim targets.

The Telescope’s roll attitude also may affect theview of the object and require a maneuver thatrolls the spacecraft more than the 30-degreelimit – for example, to place the image into aspectrographic slit aperture.

Tracking interior planets (Mercury and Venus)with the Telescope places the Sun within theTelescope opening’s 50-degree Sun-exclusionzone. For this reason, HST never observesMercury and has observed Venus only once,using Earth to block (occult) the Sun.

6.3.4 Natural Radiation

Energetic particles from different sourcesbombard the Telescope continuously as ittravels around the Earth. Geomagneticshielding blocks much of the solar and galacticparticle radiation. When the Telescope passesthrough the South Atlantic Anomaly (SAA), a“hole” in Earth’s magnetic field, chargedparticles can enter the Telescope and strike itsdetectors, emitting electrons and producingfalse data.

The Telescope passes through the SAA forsegments of eight or nine consecutive orbits,then has no contact with it for six or seven orbits.Each encounter lasts up to 25 minutes. In addi-tion, the SAA rotates with Earth, so it occasion-ally coincides with the Telescope as the space-craft enters Earth-shadow observation periods.Careful scheduling minimizes the effects of theanomaly, but it has some regular impact.

Solar flares are strong pulses of solar radiation,accompanied by bursts of energetic particles.Earth’s magnetic field shields the lowermagnetic latitude regions, such as theTelescope’s orbit inclination, from most ofthese charged particles. NASA regularly moni-tors the flares, and the Telescope can stop anobservation until the flares subside.

6.3.5 Maneuver Characteristics

The Telescope changes its orientation in space byrotating its reaction wheels, then slowing them.The momentum change caused by the reactionmoves the spacecraft at a baseline rate of 0.22degree per second or 90 degrees in 14 minutes.Figure 6-2 shows a roll-and-pitch maneuver.

Fig. 6-2 HST single-axis maneuvers

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When the Telescope maneuvers, it takes a fewminutes to lock onto a new target and accumu-late drift errors. This means that a larger regionof the sky must be scanned for guide stars.

One consideration with maneuvering is thedanger of moving the Solar Array wings out ofthe Sun’s direct radiation for too long.Unprotected portions of the Support SystemsModule aft shroud could be affected thermally.Therefore, maneuvers beyond a certain rangein angle and time are limited.

When the Telescope performs a pitch to atarget near the 50-degree Sun-avoidance zone,the Telescope curves away from the Sun. Forexample, if two targets are opposed at 180degrees just outside the 50-degree zone, theTelescope follows an imaginary circle of 50degrees around the Sun until it locates thesecond target (see Fig. 6-3).

6.3.6 Communication Characteristics

The Telescope communicates with theground via TDRSS. With two satellites 130degrees apart in longitude, the maximumamount of contact time is 94.5 minutes ofcontinuous communication, with only 2.5 to 7minutes in a zone of exclusion out of reach ofeither TDRS (see Fig. 6-4). However, orbitalvariations by the Telescope and communica-tions satellites affect this ideal situation toslightly widen the zone of exclusion.

The GSFC Network Control Center (NCC)schedules all TDRS communications. TheTelescope has a general orbital communicationschedule, supplemented by specific sciencerequests. The NCC prepares schedules 14 daysbefore the start of each mission week.

The backup communications link is theGround Network, which receives engineeringdata or science data if the High Gain Antennas(HGA) cannot transmit to TDRSS. The longestsingle contact time is 8 minutes. The limitingfactor of this backup system is the large gap intime between contacts with the Telescope. Inpractical terms, at least three contacts arerequired to read data from a filled science datarecorder – with gaps of up to 11 hours betweentransmissions.

Each HGA maintains continuous contact withone TDRS to avoid unnecessary gaps in

Fig. 6-4 TDRS-HST contact zones

Fig. 6-3 Sun-avoidance maneuver

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communication. Each antenna tracks thecommunication satellite, even during fine-pointing maneuvers.

Low Gain Antennas provide at least 95percent orbital coverage via a TDRS forthe minimum multiple-access commandrate used.

6.4 Acquisition and Observation

The major steps in the observation process aretarget acquisition and observation, datacollection and transmission, and data analysis.

Each science instrument has an entrance aper-ture, located in different portions of theHubble’s focal plane. The different aperturepositions make precise pointing a sometimes-lengthy procedure for the FGSs, which mustcenter the target in small apertures. Additionaltime is required to reposition the Telescope –an estimated 18 minutes to maneuver 90degrees plus the time the sensors take toacquire the guide stars. If the Telescope over-shoots its target, the Fixed Head Star Trackersmay have to make coarse-pointing updatesbefore the Telescope can use the FGSs again.

To increase the probability of a successfulacquisition, Telescope flight software allowsthe use of multiple guide-star pairs to accountfor natural contingencies that might affect aguide-star acquisition – such as a guide star

being a binary star and preventing the FGSsfrom getting a fine lock on the target.Therefore, an observer can submit a proposalthat includes a multiple selection of guide-starpairs. If one pair proves too difficult to acquire,the sensors can switch to the alternate pair.However, each observation has a limited totaltime for acquiring and studying the target. Ifthe acquisition process takes too long, theacquisition logic switches to coarse-track modefor that observation to acquire the guide stars.

Three basic modes are used to target a star.

• Mode 1 points the Telescope, then transmits acamera image, or spectrographic or photo-metric pseudo-image, to STOCC. Groundcomputers make corrections to preciselypoint the Telescope, and the coordinates passup through the Advanced Computer.

• Mode 2 uses onboard facilities, processinginformation from the larger target apertures,then aiming the Telescope to place the light inthe chosen apertures.

• Mode 3 uses the programmed target coordi-nates in the star catalog or updated acquisi-tion information to reacquire a previoustarget. Called blind pointing, this is usedmostly for generalized pointing and for theWide Field and Planetary Camera 2, whichdoes not require such precise pointing. Mode3 relies increasingly on the updated guide-star information from previous acquisitionattempts, stored in the computer system.

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ne of the most important features of theHubble Space Telescope is its ability tobe maintained and upgraded on orbit.

Every few years, a team of astronauts carries afull manifest of new equipment on the SpaceShuttle for the ultimate “tune-up” in space.

The Telescope was being designed as the SpaceShuttle was being readied for its first flights.NASA realized that if a shuttle crew could ser-vice HST, it could be upgraded and maintainedindefinitely. So from the beginning, Hubblewas designed to be modular and astronaut-friendly.

Its modular design allows NASA to equip HSTwith new, state-of-the-art scientific instrumentsevery few years – giving the Telescope excitingnew capabilities with each servicing mission(see Fig. 7-1). In addition to science upgrades,the servicing missions permit astronauts toreplace limited-life components with systemsincorporating the latest technology (see Fig. 7-2).

7.1 Cost-Effective Modular Design

In February 1997 astronauts installed two next-generation scientific instruments, giving theTelescope infrared and ultraviolet vision. Atthe same time, they installed a state-of-the-artSolid State Recorder (SSR) to increase Hubble’sobserving capability. The 1993 and 1997 servic-ing missions increased scientific exposure timeefficiencies 11-fold.

On Servicing Mission 3B (SM3B), scheduled for2001, astronauts will add a camera 10 timesmore powerful than the already extraordinarycameras on board. They also will fit theTelescope with new, super-efficient solar arraypanels that will allow simultaneous operationof scientific instruments. These technologies

were not available when Hubble was designedand launched.

The following sections identify some of theplanned upgrades to HST and their anticipatedbenefits to performance. These improvementsdemonstrate that servicing HST results in signif-icant new science data at greatly reduced cost.

7.1.1 Processor Improvements

During Servicing Mission 3A (SM3A), astro-nauts will replace Hubble’s original main com-puter, a DF-224/coprocessor combination, witha completely new computer based on the Intel80486 microchip. The new computer will be 20times faster and have six times as much mem-ory as the one it replaces (see Fig. 7-3).

In a good example of NASA’s goal of “faster,cheaper, better,” commercially developed,commonly available equipment was used tobuild the new computer at a fraction of theprice it would have cost to build a computerdesigned specifically for the spaceflightenvironment.

The greater capabilities of the new computerwill increase productivity for the Hubbleobservatory by performing more work in spaceand less work on the ground. The computersoftware uses a modern programming lan-guage, which decreases software maintenancecost.

7.1.2 Data Archiving Rate

With the addition of a second SSR on SM3A,Hubble’s data storage capability will dramati-cally increase. The science data archiving ratewill be more than 10 times greater than FirstServicing Mission (1993) rates (see Fig. 7-4).

OVALUE ADDED: The Benefits of Servicing Hubble

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7-2 ACS (= 10X WFPC2 IN RED)

WFC3 (= 20X NICMOS IN IR;35X ACS IN NUV)

STIS (= 40X ORIGINAL SPECTROGRAPHS)

COS (= 20X STIS)

NICMOS/NCS

SM2 SM3B SM4

WFPC2

SM1

NICMOS

Advanced Scientific InstrumentsPave the way to New Discoveries

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Fig. 7-1 Advanced scientific instruments installed (or to be installed) on HST

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Spacecraft Equipment Change-OutsTo Enhance Mission Life, Reliability and Productivity

Electrical•Solar Array•SADE

Electrical•SADE

Electrical•VIK

Electrical•S/A 3•Diode Box

Electrical•Batteries

Pointing& Control•RSU(2)•MSS(2)•ECU(2)

•Coprocessor

Thermal•MLI patches

•SSAT

Thermal•ASCS•NCS

•ESTR•SSR

•486•SSR

Pointing& Control•FGS•RWA

Pointing& Control•FGS•RSU(3)

Pointing& Control•FGS•RSU(3)

Thermal•NOBL•SSRF

SM1 SM2 SM3A SM3B SM4

•DIU

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DataManagement

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Fig. 7-2 Systems maintained and upgraded during each servicing mission

30

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Fig. 7-3 Processor improvements on HST

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Fig. 7-4 Data archiving rate improvements

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Prior to the Second Servicing Mission (SM2),Hubble used three reel-to-reel tape recordersdesigned in the 1970s. In February 1997 astro-nauts replaced one of the mechanical recorderswith a digital SSR. During SM3A astronautswill remove a second mechanical recorder andinstall a second SSR.

Unlike the reel-to-reel recorders they replace,the SSRs have no reels, no tape, and no movingparts that can wear out and limit lifetime. Data isstored digitally in computer-like memory chipsuntil Hubble’s operators command its playback.

Although an SSR is about the same size andshape as the reel-to-reel recorder, it can store 10times as much data: 12 gigabits of data insteadof only 1.2 gigabits. This greater storage capacityallows Hubble’s second-generation scientificinstruments to be fully productive.

7.1.3 Detector Technology

Hubble’s state-of-the-art detector technologyallows the Telescope to capture and processfaint amounts of light from the far reaches ofspace. Increased power and the resolutionrefinements achieved through advances indetector technology will greatly improveHubble’s performance and deliver even sharper,clearer, and more distinct images.

With the addition of the Advanced Camera forSurveys during SM3B, the total number ofonboard pixels will have increased 4800 per-cent from SM2 in 1997 (see Fig. 7-5).

7.1.4 Cryogenic Cooler

Installation of the Near Infrared Camera andMulti Object Spectrograph (NICMOS) Cryo-genic Cooler (NCC) during SM3B will greatlyextend the life of Hubble’s infrared cameras.

The cryogenic cooler will increase NICMOS’slife span from 1.8 years to 10 years (see Fig. 7-6).

NICMOS, which was installed on Hubble in1997, has been a spectacular success. However,in January 1999 it ran out of the coolant neces-sary for conducting scientific operations. TheNCC will preserve and extend the instrument’sunique science contribution. The cost todevelop and install the NCC is approximately$16 million, while the cost of NICMOS was$100 million. Installing a new cryocooler willincrease by sevenfold the lifetime of the instru-ment and ensure a greater scientific return onthe original investment.

HST pixels (Millions)

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Fig. 7-5 Increase in onboard pixels

HST Infrared Science Capability

SM1 SM2 SM3B

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Fig. 7-6 Increase in HST infrared capability

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7.1.5 Solar Arrays

The SM3B addition of new, rigid solar arrayswill provide substantially more energy toHubble. The increased power will enhance pro-ductivity by allowing simultaneous operation ofup to four Hubble instruments (see Fig. 7-7).

7.1.6 Simultaneous Science

One of the most exciting advances afforded byservicing has been the ability to increase simul-taneous operations of scientific instrumentsfrom two to four. Originally, the instrumentswere designed to work in pairs. FollowingSM3B, advances in solar array technology andthermal transport systems will allow four sci-ence instruments to operate at the same time,dramatically increasing Hubble’s ability tostudy the universe (see Fig. 7-8).

7.2 Accelerated Innovations

The same cutting-edge technology that allowsHubble to peer deep into the universe alsotouches life closer to home. Hubble’s innova-tive technology benefits mankind in numerousfacets of everyday life, including medicine and

manufacturing. Ultimately, these spin-off divi-dends enhance the U.S. economy and raise theAmerican standard of living, making Hubblean even better value for the investment.

By teaming with companies and universities,Hubble’s scientists and engineers push tech-nology to new levels of sophistication. Anexample is the medical application of tech-nology from the Space Telescope ImagingSpectrograph (STIS). A cancer detection applica-tion grew out of Hubble’s need for highlysophisticated imaging capability, savingpatients a significant amount of money andtrauma.

Figure 7-9 shows the projected cost savings topatients as a result of the application of STIStechnology.

7.2.1 Detecting Breast Cancer BeforeBlack Holes

With STIS, one of two next-generation instru-ments installed on Hubble in February 1997,scientists can see deeper into space than everbefore. NASA’s fastest black hole hunter, STISfinds and studies black holes, teaches us more

Solar Arrays Output Power (Min.)

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5680

Fig. 7-7 Productivity gains on HST with new solararrays

Number of Science InstrumentsAble To Operate Simultaneously

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Fig. 7-8 Simultaneous use of HST science instruments

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about how stars and planets form, and looks atvery distant, early galaxies.

However, the charge-coupled device (CCD)developed for STIS to study outer space wasused first to study “inner space,” imagingbreast tissue three years before STIS wouldever see its first black hole.

To develop STIS, NASA needed a level of imag-ing technology not available commercially.NASA funded a company called SITe, Inc. tomake a more sensitive CCD for Hubble. Thenew type of CCD found medical application aspart of the Stereo GuideTM Breast BiopsySystem manufactured by the LORAD divisionof ThermoTrex Corporation. The CCD enablesthe system to capture finely detailed digitalx-ray images of breast tissue. The images helpthe doctor guide a needle into suspicious tissueand take a biopsy.

This procedure uses about half the x-raydosage required for imaging for conventionalsurgical biopsies. It also saves patients timeand pain and eliminates scarring and disfig-urement. Patients can have the procedure

under local rather than general anesthesia andcan resume normal activities within minutes.Finally, because the new technique can be donein a doctor’s office, it is less costly to perform.With more than 500,000 women undergoingbreast biopsies each year in the U.S., radiol-ogists predict the procedure will save over$1 billion annually in national health carecosts.

In 1997 this STIS detector technology wasinducted into the United States SpaceFoundation’s Space Technology Hall of Fameand was a finalist for the prestigious DiscoverMagazine Award for TechnologicalInnovation. In 1998 it earned the FederalLaboratory Commission’s Award forTechnology Transfer.

7.2.2 Image Processing: DiagnosingCancer Earlier

The CCDs used in digital mammography arenot Hubble’s only exciting news in the fightagainst breast cancer. Techniques developedfor processing images from the Telescope maysoon help detect breast cancer in its very earlystages. The image processing techniques, usedto correct the blurry images sent back to Earthfrom Hubble before the 1993 servicing mission,have proven effective in finding microcalcifica-tions, whose presence in mammograms indi-cates breast cancer.

The Space Telescope Science Institute (STScI)developed a large repertoire of image process-ing software to compensate for the effects ofthe spherical aberration detected in Hubble’sprimary mirror soon after launch in April 1990.The software was designed to correct for theTelescope’s loss of dynamic range and spatialresolution.

$ IN

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Projected Medical SavingsAs a Result of Stereotactic Large-Core Needle

Biopsy Technology

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A group of medical and astronomicalresearchers from the STScI in Baltimore,Johns Hopkins University, and the LombardiCancer Research Center at the GeorgetownUniversity Medical Center in Washington,DC, is testing this image processing tech-nique to detect signs of breast cancer in digi-tized mammograms. The collaborative efforthas received funding from the NationalScience Foundation.

Although about one-third of breast cancer caseshave microcalcifications smaller than 50 to 100microns, current mammography can show onlythose 250 microns or larger. Image processingallows detection of smaller calcifications, there-fore earlier cancer detection and treatment. Thesooner a cancerous lesion is treated, the greaterare the odds of full recovery. Applying imageprocessing to medical treatment can save livesand preserve quality of life.

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GLOSSARY

-A-

Å Angstrom

aberration Property of an optical system that causes an image to have certain easilyrecognizable flaws. Aberrations are caused by geometrical factors suchas the shapes of surfaces, their spacing, and alignments. Image problemscaused by factors such as scratches or contamination are not calledaberrations.

ACE Actuator Control Electronics

ACS Advanced Camera for Surveys

acquisition, target Orienting the HST line of sight to place incoming target light in aninstrument’s aperture

actuator Small, high-precision, motor-driven device that can adjust the locationand orientation of an optical element in very fine steps, making fineimprovements to the focus of the image

Advanced Computer A 486-based computer that will replace the DF-224 on SM-3A. Performsonboard computations and handles data and command transmissionsbetween HST systems and the ground system.

AFM Adjustable Fold Mirror

aft Rear of the spacecraft

alignment Process of mounting optical elements and adjusting their positions andorientations so that light follows exactly the desired path through theinstrument and each optical element performs its function as planned

altitude Height in space

AMA Actuator Mechanism Assembly

AME Actuator Mechanism Electronics

aperture Opening that allows light to fall onto an instrument’s optics

aplanatic Image corrected everywhere in the field of view

apodizer Masking device that blocks stray light

arcsec A wedge of angle, 1/3600th of one degree, in the 360-degree “pie” thatmakes up the sky. An arcminute is 60 seconds; a degree is 60 minutes.

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ASCS Aft Shroud Cooling System

ASLR Aft Shroud Latch Repair (kits)

ASIPE Axial Scientific Instrument Protective Enclosure

astigmatism Failure of an optical system, such as a lens or a mirror, to image a pointas a single point

astrometry Geometrical relations of the celestial bodies and their real and apparentmotions

ATM Auxiliary Transport Module

attitude Orientation of the spacecraft’s axes relative to Earth

AURA Association of Universities for Research in Astronomy

axial science instruments Four instruments – the STIS, NICMOS, FOC, and COSTAR – locatedbehind the primary mirror. Their long dimensions run parallel to theoptical axis of the HST.

-B-

baffle Material that extracts stray light from an incoming image

BAPS Berthing and Positioning System

BPS BAPS Support Post

-C-

C Celsius

Cassegrain Popular design for large, two-mirror reflecting telescopes in which theprimary mirror has a concave parabolic shape and the secondary mirrorhas a convex hyperbolic shape. A hole in the primary allows the imageplane to be located behind the large mirror.

CAT Crew Aids and Tools

CCC Charge Current Controller

CCD Charge-coupled device

CCS Control Center System

CDI Command data interface

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change-out Exchanging a unit on the satellite

cm Centimeter

collimate To straighten or make parallel two light paths

coma Lens aberration that gives an image a “tail”

concave Mirror surface that bends outward to expand an image

convex Mirror surface that bends inward to concentrate on an image

coronograph Device that allows viewing a light object’s corona

COS Cosmic Origins Spectrograph

COSTAR Corrective Optics Space Telescope Axial Replacement

CPM Central Processor Module

CPU Central Processing Unit

CTVC Color television camera

CU/SDF Control Unit/Science Data Formatter

CSS Coarse Sun Sensor

-D-diffraction grating Device that splits light into a spectrum of the component wavelengths

DIU Data Interface Unit

DMS Data Management Subsystem

DMU Data Management Unit

drag, atmospheric Effect of atmosphere that slows a spacecraft and forces its orbit to decay

-E-ECA Electronic Control Assembly

ECU Electronics Control Unit

electron Small particle of electricity

ellipsoid Surface whose intersection with every plane is an ellipse (or circle)

EPDSU Enhanced Power Distribution and Switching Unit

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EPS Electrical Power Subsystem

EP/TCE Electrical Power/Thermal Control Electronics

ESA European Space Agency

E/STR engineering/science data recorders

EVA extravehicular activity

extravehicular Outside the spacecraft; activity in space conducted by suited astronauts

-F-

F Fahrenheit

FGE Fine Guidance Electronics

FGS Fine Guidance Sensor

FHST Fixed Head Star Tracker

FOC Faint Object Camera

focal plane Axis or geometric plane where incoming light is focused by thetelescope

FOSR Flexible optical solar reflector

FOV Field of view

FPS Focal plane structure

FPSA Focal plane structure assembly

FRB Fastener retention block

FS Forward Shell

FSIPE FGS Scientific Instrument Protective Enclosure

FSS Flight Support System

-G-

GA Gallium arsenide

G/E Graphite-epoxy

GE General Electric

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GGM Gravity Gradient Mode

GSE Ground support equipment

GSFC Goddard Space Flight Center

GSSS Guide Star Selection System

GSTDN Ground Spaceflight Tracking and Data Network

-H-

HGA High Gain Antenna

HST Hubble Space Telescope

hyperboloidal Slightly deeper curve, mathematically, than a parabola; shape of theprimary mirror

Hz Hertz (cycles per second)

-I-

IBM International Business Machines Corporation

in. Inches

interstellar Between celestial objects; often refers to matter in space that is not astar, such as clouds of dust and gas

intravehicular Inside the spacecraft

IOU Input/output unit

IR Infrared

IV Intravehicular

IVA Intravehicular activity

-J-

JPL Jet Propulsion Laboratory

JSC Johnson Space Center

-K-

k Kilo (1000)

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kB Kilobytes

kg Kilogram

km Kilometer

KSC Kennedy Space Center

-L-

Latch Mechanical device that attaches one component, such as a scienceinstrument, to the structure of the telescope and holds it in preciselythe right place

lb Pound

LGA Low Gain Antenna

LGA PC Low Gain Antenna Protective Cover

Light year The distance traveled by light in one year, approximately six trillionmiles

LMMS Lockheed Martin Missiles & Space

LOPE Large ORU Protective Enclosure

LOS Line of sight

LS Light Shield

luminosity Intensity of a star’s brightness

-M-

m Meter

µm Micrometer; one millionth of a meter

mm Millimeter

MA Multiple access

magnitude, absolute How bright a star appears without any correction made for its distance

magnitude, apparent How bright a star would appear if it were viewed at a standard distance

MAMA Multi-Anode Microchannel Plate Array

MAT Multiple Access Transponder

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MCC Mission Control Center

MCP Microchannel plate

metrology Process of making extremely precise measurements of the relativepositions and orientations of the different optical and mechanicalcomponents

MFR Manipulator Foot Restraint

MHz Megahertz

MLI Multi-layer insulation

Mpc Megaparsec (one million parsecs)

MOPE Multimission ORU Protective Enclosure

MSFC Marshall Space Flight Center

MSM Mode Selection Mechanism

MSS Magnetic Sensing System

MT Magnetic torquer

MTA Metering Truss Assembly

MTS Metering Truss Structure

M Absolute visual magnitude

m Apparent visual magnitude

-N-

NASA National Aeronautics and Space Administration

NBL Neutral Buoyancy Laboratory at JSC

NASCOM NASA Communications Network

NCC Network Control Center

NCS NICMOS Cooling System

nebula Mass of luminous interstellar dust and gas, often produced after a stellarnova

NICMOS Near Infrared Camera and Multi-Object Spectrometer

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nm Nanometers

nmi Nautical miles

NOBL New Outer Blanket Layer

nova Star that suddenly becomes explosively bright

NPE NOBL Protective Enclosure

NSSC-I NASA Standard Spacecraft Computer, Model-I

-O-

occultation Eclipsing one body with another

OCE Optical Control Electronics

OCE-EK OCE Enhancement Kit

OCS Optical Control Subsystem

Orientation Position in space relative to Earth

ORU Orbital Replacement Unit

ORUC Orbital Replacement Unit Carrier

OSS Office of Space Science, NASA Headquarters

OTA Optical Telescope Assembly

-P-

PACOR Packet Processing Facility

parallax Change in the apparent relative orientations of objects when viewedfrom different positions

parsec A distance equal to 3.26 light years

PCEA Pointing Control Electronics Assembly

PCS Pointing Control Subsystem

PCU Power Control Unit

PDA Photon Detector Assembly

PDM Primary Deployment Mechanism

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PDU Power Distribution Unit

PFR Portable Foot Restraint

photon Unit of electromagnetic energy

PIP Push in-pull out (pin)

pixel Single picture element of a detection device

POCC Payload Operations Control Center

polarity Light magnetized to move along certain planes. Polarimetricobservation studies the light moving along a given plane.

primary mirror Large mirror in a reflecting telescope the size of which determines thelight-gathering power of the instrument

prism Device that breaks light into its composite wavelength spectrum

PSEA Pointing/Safemode Electronics Assembly

PSO HST Project Science Office at GSFC

-Q-

quasar Quasi-stellar object of unknown origin or composition

-R-

RAM Random-access memory

radial Perpendicular to a plane (i.e., instruments placed at a 90-degree anglefrom the optical axis of the HST)

RBM Radial Bay Module

RDA Rotary Drive Actuator

reboost To boost a satellite back into its original orbit after the orbit has decayedbecause of atmospheric drag

reflecting telescope Telescope that uses mirrors to collect and focus incoming light

refracting telescope Telescope that uses lenses to collect and focus light

resolution Ability to discriminate fine detail in data. In an image, resolution refersto the ability to distinguish two objects very close together in space. Ina spectrum, it is the ability to measure closely separated wavelengths.

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resolution, spectral Determines how well closely spaced features in the wavelengthspectrum can be detected

resolution, angular Determines how clearly an instrument forms an image

RF Radio frequency

RGA Rate Gyro Assembly

Ritchey-Chretien A modern optical design for two-mirror reflecting telescopes. It is aderivative of the Cassegrain concept in which the primary mirror hasa hyperbolic cross-section.

RIU Remote Interface Unit

RMGA Retrieval Mode Gyro Assembly

RMS Remote Manipulator System

ROM Read-only memory

RS Reed-Solomon

RSU Rate Sensor Unit

RWA Reaction Wheel Assembly

-S-

SA Solar Array

SAA South Atlantic Anomaly

SAC Second Axial Carrier

SAD Solar Array Drive

SADE Solar Array Drive Electronics

SADM Solar Array Drive Mechanism

SAGA Solar Array Gain Augmentation

SBA Secondary Baffle Assembly

SBC Single-Board Computer

SCP Stored Command Processor

SDAS Science Data Analysis Software

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SDM Secondary Deployment Mechanism

secondary mirror In a two-mirror reflecting telescope, the secondary mirror sits in frontof the larger primary mirror and reflects light to the point at which itwill be detected and recorded by an instrument. In simple telescopes,the secondary mirror is flat and bounces the light out the side of thetube to an eyepiece. In more complex and larger telescopes, it is convexand reflects light through a hole in the primary mirror.

Servicing Mission NASA’s plan to have the Space Shuttle retrieve the HST and haveastronauts perform repairs and upgrades to equipment in space

SI Science Instrument

SI C&DH SI Control and Data Handling (subsystem)

SIPE Science Instrument Protective Enclosure

SM Secondary Mirror

SMA Secondary Mirror Assembly

SM1 First HST Servicing Mission, December 1993

SM2 Second HST Servicing Mission, February 1997

SM3A HST Servicing Mission 3A, October 1999

SM3B HST Servicing Mission 3B, planned for December 2000

SOFA Selectable Optical Filter Assembly

SOGS Science Operations Ground System

SOPE Small ORU Protective Enclosure

spectral devices These include spectrographs, instruments that photograph the spectrumof light within a wavelength range; spectrometers, which measure theposition of spectral lines; and spectrophotometers, which determineenergy distribution in a spectrum.

spectrograph Instrument that breaks light up into its constituent wavelengths andallows quantitative measurements of intensity to be made

spectrum Wavelength range of light in an image

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spherical aberration Image defect caused by a mismatch in the shapes of the reflectingsurfaces of the primary and secondary mirrors. Light from differentannular regions on the primary mirror comes to a focus at differentdistances from the secondary mirror, and there is no one position whereall of the light is in focus.

SSAT S-band Single-Access Transmitter

SSC Science Support Center

SSE Space Support Equipment

SSM Support Systems Module

SSM-ES SSM Equipment Section

SSR Solid State Recorder

SSRF Shell/Shield Repair Fabric

STDN Space (flight) Tracking and Data Network

STINT Standard interface

STIS Space Telescope Imaging Spectrograph

STOCC Space Telescope Operations Control Center

STS Space Transportation System

STScI Space Telescope Science Institute

-T-

TA Translation Aids

TAG Two-axis gimbal

TCE Thermal Control Electronics

TCS Thermal Control Subsystem

TDRS Tracking and Data Relay Satellite

TDRSS TDRS System

TECI Thermoelectric-cooled inner (shield)

TECO Thermoelectric-cooled outer (shield)

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telemetry Data and commands sent from the spacecraft to ground stations

TLM Telemetry

-U-

UDM Umbilical disconnect mechanism

ULE Ultralow expansion

USA United States Army

USAF United States Air Force

USN United States Navy

UV ultraviolet

-V-

V Volt

V1, V2, V3 HST axes

VCS Vapor-cooled shield

VIK Voltage/Temperature Improvement Kit

-W-

W Watt

Wavelength Spectral range of light in an image

WFC Wide Field Camera

WFPC Wide Field and Planetary Camera. The camera currently in use is thesecond-generation instrument WFPC2, installed during the FirstServicing Mission in December 1993. It replaced WFPC1 and was builtwith optics to correct for the spherical aberration of the primary mirror.

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