Exploring the Universe With the Hubble Space Telescope

8/8/2019 Exploring the Universe With the Hubble Space Telescope

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Exploring the Universe with theHubbleSpaceTelescope

Cone Nebula In Monoceros(Anglo.Austra lian Observatory)

NI_ANational Aeronautics andSpace Administration

The Superintendent of Documents=Ptin[ing Otf_ce, Washington, {_C 20402

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Contents

Preface 6First Light 8

Observation Strategy 16Using the Hubble Space Telescope

First DaysDay-to-Day OperationsServicing in Space

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Opportunities for Discovery 45The Hubble Space Telescope: Engineering Challenge

In FocusStill LightDevelopment of the Hubble Space Telescope

Guided Tour of the Hubble Space TelescopeOptical Telescope AssemblyFocal Plane Scientific InstrumentsWide Field/Planetary CameraFaint Object CameraFaint Object SpectrographGoddard High Resolution SpectrographHigh Speed PhotometerFine Guidance SensorsSecond Generation TechnologySupport SystemsHubble Space Telescope Chronology

Acknowledgements

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rom our home on the Earth, we lookut into the distances and strive toimagine the sort of world into which weare born. Today we have reached far outinto space. Our immediate neighborhoodwe know rather intimately. But withincreasing distance our knowledge fades,and fades rapidly, until at the last dimhorizon we search among ghostly errorsof observations for landmarks that arescarcely more substantial.The search will continue. The urge isolder than history. It is not satisfied andit will not be suppressed.

Edwin R HubbleAstronomer

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e are stargazers.child pointing everyone searches the

with delight to the firststar at twilight, a rooftopamateur with telescopescanning the night sky, anastronomer dwarfed bythe mirror in a silentobservatory on some dark,remote mountain peak--

heavens. What is it thatwe seek?

Astronomy has cap-tured our imagination asperhaps no other science,for it embraces suchincomprehensible sizeand distance and time

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that we can barely ponderit. We think that the uni-verse originated some 15billion years ago, but who,in an average life of only70-odd years, can conceiveof such eternity? We esti-mate that our galaxy con-tains about two hundredbillion stars and that it isonly one of more than a

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hundred billion galaxies,so there must be sometwenty thousand billionbillion stars in the uni-verse; the mind balks atsuch incredible numbers.We who measure distancein miles and kilometersstruggle with the conceptof a single light year,almost six trillion miles,


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auchlesscosmicdis-ancescountedn millionsLndbillionsoflight years._stronomystretchesouraindsandinvitesusintoherealmofthemaginary.Whenwe looktothe_eavens,wearenotonlyLwed by the immensity ofime and space; we also

learn something. Starlightis information. In thestars and galaxies, wegain clues to the originand history of the uni-verse and we begin tounderstand the exquisitepatterns of the cosmos.All that we know aboutthe universe, we knowfrom observing.

Today, we are on theverge of exciting discover-ies as a new observatoryis prepared for use. TheHubble Space Telescope,an orbital observatorydesigned to probe the uni-verse with extraordinaryprecision and clarity, muchfarther than we have everseen, will dispel much of

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the mystery and uncer-tainty that limit ourknowledge. As we repeatthe age-old act of star-gazing, the wonder andbeauty of the universewill be revealed anew,astonishing us again as itsurprised Galileo when hefirst used a telescopealmost 400 years ago. t_

Cluster of Galaxies in Hercules i_itPea_at_o,arOse,_at_,_j7


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We are going

tofollow the trail oflight with anext remely sensi-

tive new observatory--theHubble Space Telescope. Thelargest and most sophisticatedastronomical telescope everplaced in space, it can viewmore celestial objects, much far-ther away and in much sharperdetail, than any other telescope.

This remarkable observatorywill show us the early universeand almost everything that hasappeared since. For the firsttime, we will be able to see themost distant features of the cos-mos as clearly as we can nowsee the nearer stars and galax-ies. As we use the HubbleSpace Telescope to interceptand interpret the messages car-ried in light, the prospects fordiscovery are exciting.

Astronomy is not really thestudy of stars; it is the study ofstarlight. We come to know theuniverse by capturing starlightas it passes by on its voyagethrough the cosmos, bringingit into focus and examining it.

Where has starlight been,and what can it tell us aboutthe universe? Imagine a beamof light, emitted from an earlygalaxy, perhaps 10 billion yearsago. Upon departing from thestars that produced it, the lightflows through interstellar gas,

the hot "filler" between starswithin a galaxy. During thispassage, which may last a hun-dred thousand years or longer,the light may be modified byatoms in the gas and lose someof its energy. It then exits thegalaxy and enters a vast, ratherempty territory between galax-ies. In a million years or so itmay pass near an object with astrong gravitational field and bebent from its course, or it maywander into a dense cloud ofdust left over from a star's for-mation and be absorbed, neverto be seen.

While light is in transit,cosmic history marches on.For eons, our own part of theuniverse does not yet exist.The Milky Way galaxy takesshape about three billion yearsafter the light begins its voyage.About two billion years later,the Earth condenses out of thegaseous nebula surrounding ouryoung sun. It is another billionyears before signs of life appearon our planet, and severalbillion more before humancivilization arises and becomescapable of thoughtfully observ-

ing the stars. The light almostpasses us before we becomeclever enough to recognize itssignificance.

Light that ultimately arrivesat Earth and enters a telescopeto be collected and magnifiedand guided into the eye, a cam-era, or other detector bears themarks of both its origin and itsvoyage. Its original energy andpath may be altered by condi-tions in the space environment,but light has a message and asignature. Thus, light tells usabout conditions at its source(What is the star composed of?How hot is it?) and along itsway (What makes up the gasbetween stars in galaxies? Howstrong is the embedded magnet-ic field?). Light also brings usnews of changes in the cosmos:the birth and spectacular deathof stars, galaxies in collision,and the demise of stars beingdevoured by their neighbors.

Although the speed of lightis the fastest possible way tocover distance, the distancesin the universe are so great thit takes even light a very longtime to reach us--millions andbillions of years. A light year,the distance light travels in oyear, is about 6 trillion miles.The light we receive today frothe most distant object actuallywas emitted long, long agowhen the universe was veryyoung.

Thus, detecting light fromdistant objects is looking backin time, seeing the universeit was when the light left theobjects, not as it is now.

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_ome 12 to 20 billion years ago, astronomers think a "primeval atom";xploded with a big bang sending the entire universe flying out at incredible;peeds. Eventually matter cooled and condensed into galaxies and stars. lanets formed around at least one star. Eons after life began to develop on-arth, humans appeared. If all events in the history of the universe until nowvere squeezed into 24 hours, Earth wouldn't form until late afternoon._umans would have existed for only two seconds.

(Jairne (?uintero, artist; National Geographic Society)

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Astronomy through the Ages

To study the universe, itshistory and its contents, we usewhatever we can to catch andanalyze light. The most commontool for astronomy is the eye.When we look up at the sky, oureyes receive light, form animage, and send a signal to thebrain for processing. Virtuallyeveryone can use this "instru-ment"; for millennia, it was theonly one available.

Unfortunately, the eye is lim-ited in sensitivity and can per-ceive only the brighter celestialobjects. The patient observa-tions of early astronomersyielded an atlas of the universevisible to the unaided eye. Theycharted the motions of the plan-ets, recorded the positions ofmany stars, and witnessed afew spectacular explosions ofdying stars. However, they hadno inkling of the universe wecan observe today.

About 400 years ago, Galileowedded technology and astron-omy by using a telescope, apiece of equipment to enhancehuman vision. His simplearrangement of two magnifyinglenses in a tube enabled the

first detailed look at some of theobjects in the sky and multipliedthe number of visible stars. Themoon, previously seen as a mot-tled disk of light and darker

Amateurastronomersand home-madetelescopes[Dennis dl Clco)

patches, suddenly showed a ter-rain of mountains and valleys.Four of Jupiter's moons wererevealed, and stars "so numer-ous as to be almost beyondbelief."Technology made a world ofdifference in our ability to formimages. The history of astrono-my since Galileo's time is a his-tory of technological advancesleading to discoveries. Progressin astronomy depends onimproved telescopes and detec-tors to supplement the eye.

Over the past four centuries,larger and larger mirrors havebeen constructed for reflecting

telescopes. The larger the mir-ror, the more light is collectedand focused into the image.One result is brighter images;objects that were previouslybelow the threshold of detectionare now "seen." Another resultof using larger mirrors isenhanced fineness of detail(resolution) that can he seen inthe image; images are sharper,containing more detailedinformation.For collecting light and form-ing sharper images, bigger isbetter. Thus, observatories viefor pre-eminence on the basisof mirror size, graduating frommirrors the size of coins anddinner plates to disks as largeas backyard satellite dishes.Improvements in mirror grind-ing, polishing, and reflectivecoatings also have made adifference.

Galileo,credited with first useof the telescopein 1609

tYerkes Observatory)OP,_,I,4b, L PAGE

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stronomer In observing capsuler the 200-inch Hale telescope_lorna_ Obscrvatory, Cahforn_a Inst tute of Tcchnolo_'_

In the past century, and espe-[ally during the last 50 years,dvances in detector technologyave influenced progress instronomy even more than mir-)r size. Putting a camera, rather1an an eye, at the viewing endf a telescope makes it possiblej record more light and pre-erve the image for analysis.]creasingly sensitive photo-raphic films and longer expo-ures are bringing very faintelestial objects into the rangef detectability. New electronicystems capable of recording_dividual photons are now dis-lacing film as the detectors ofhoice for imaging faint (distant)stronomical objects.

The Multiple Mirror Telescope at Mt. Hopkins, Arizona--an innovative design for a large l ight -col lecting surface[MMT Observatory)

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lstronomer Annie Cannon, early_.Oth.century p_oneer In spectrallnalysis (Harvard College Obse,_alnfy,I

lO0-inch Hooker telescopeat Mount Wilson(California Insttute of Technology)

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Advances in mirror anddetector technology yield dra-matic improvements in obtain-ing information about objects inthe sky, up to a point. Thenastronomers face a barrier thatis not affected by bigger, moresensitive telescopes: the atmo-sphere. Despite centuries ofobserving, we have not yet seen

Severalofthe 11 telescopesof theinArizona

the universe clearly, because weare looking up through a hazyshroud of gases around ourplanet. As light from celestialobjects is bent, scattered, andabsorbed by passage throughour turbulent atmosphere,images are distorted, blurred,

and dimmed. From the ground,we can distinguish objects wellonly to a distance of some twobillion light years, although theuniverse probably extends tentimes farther.

Placing telescopes on highmountain peaks above partof our atmosphere improvesviewing, but the best place fora telescope is in space. Toobserve the farthest, faintest

stars and galaxies in as muchdetail as possible, we must liftour telescopes into orbit beyondthe turbulent, obscuring atmo-sphere. If it were practical, mosttelescopes would be located inspace, where viewing condi-tions are most favorable.


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The Hubble Space Telescopeduring assembly and testing(Lockheed Missi_es and Space Company)

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Astronomers have dreamedf placing a large visible-light_lescope into space, but only_cently has the technologyeen developed to make thisossible. The Space Shuttle; powerful enough to carry intorbit a large telescope compara-, ]e to some of the best tele-copes on the ground'urthermore, the Shuttle canake a maintenance crew to anrbital observatory for repairsnd servicing. For the first timethe space age, we have a

warranty for a long operationallife. Space telescopes are expen-sive; to get the most sciencefrom our investment, we mustbe able to use them for manyyears.

The Hubble Space Telescopeis the realization of that dream:a virtually perfect mirror, superi-

occurred in fairly small steps;mirror sizes have doubled ortripled, and improvements insensitivity and resolution havebeen measured up to 50 timesbetter. With a telescope ]nspace, we leap to new capabili-ties at least 10 times betterthan optical telescopes on the

or detectors, and an orbit above ground. The Hubble Space _This is the Telescope is the best optical . _he atmosphere.instrument that will reveal what telescope in the world, a pre-lies beyond our present power cious international resource for

to see. Until now, advances in satisfying out human need totelescope capability have knowthe 6smos ' ' :: _ - - -_' " *_

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Observation StrategyFor generations, humans havelooked at the same sky, askedthe same questions, and pressedagainst the same physical barri-ers to observation. How largeand how old is the universe, andhow do we measure i t? Whatare the objects we see there?How do galaxies form andevolve? What is the life cycle ofstars? Do other planets exist? Isthere life elsewhere in the uni-verse? At first, our attempts toexplain the cosmos took theform of myths and folklore,which have now given wayto scientific theories andobservations.Of all the observatoriesthat exist in the world, only theHubble Space Telescope cananswer some of the most per-plexing ast ronomical questions .The Hubble Space Telescopecan detect objects 12 to I4 bil-l ion light years distant as clearlyas a much larger telescope on

the ground can detect objectsa mere 1 to 2 billion light yearsaway. Images will be at least10 times sharper in clarity thanany others, and light emittedvery early in the life of the uni-verse, 30 to 50 times fainterthan our current threshold, willbe easily detected.

The observing strategy takesadvantage of these unique abili-ties to see objects farther awayand with greater clarity thanany other telescope. The high-est priority investigations arethose that cannot be performedfrom the ground; we do notknow exactly what theseexploratory observations willreveal. Some are focused whereour knowledge is quite limited,at objects beyond 10 billionlight years, toward the "edge"of the observable universe.Research requiring extraordi-narily precise position measure-ments, now possible for the firsttime, is scheduled early in the

TheseImagesof M101 galaxy inUrsaMajor,one10 timessharperthantheother,simulatethe improvementndetailexpectedin ImagesfromtheHubbleSpaceTelescope._c.,_o,.,anst_[_lefTechnor

Inlongertimeexposures,morelightis collectedandresultingimageshavemoredetail,Theseimagesof two galaxiesinAndromedaesultedfromobservationsastingI minute,5 minutes,30 minutes, and45 minutes_yespectively.TheHubb/eSpace Telescopewillbeable to obtaindetailedimagesoffaintobjectswith shorterexposuresthanobservatorieson theground.( Ki ll P ea k Natona! Obset_ 'ator_;

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_lescope's mission. Anotherigh priority is examination ofhe ultraviolet radiation fromelestial objects, radiation thatannot be observed from theround.The "eye" of the Hubble

pace Telescope will be turnedpon both familiar and exoticbjects, neighboring areas inpace and the farthest territo-ies. Producing images at a rate,f a bout 20 a day, we shouldLave more than 7,000 images a'ear and, over its planned5-year operation, more than00,000 pictures as enthrallingLsthose from the planetary_xplorers. The information inhese images is the sourcenaterial for discoveries.Observation is exploration.

_ecause we cannot yet venturento the far reaches of time and',pace, we have to explore by:ollecting radiation. With thetubble Space Telescope, we_ave the opportunity to see and;now more about the universe.['his observatory, perhaps morehun any other, appeals to ourmagination with the promise_fn ew knowledge, beautifulmages, and answers to thenost int riguing quest ions.

theHubbleSpaceTelescopemay_nswerquestionsaboutall classesof_stronomicalobjects,fromneighboring_lanetsandstarstothemostdistant_,alaxlesandquasars._cto_ COS[aRZO, #., and Br_arr Su_tr_an a_tFsIsI


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Edwin P. Hubble1889-1953

s a child, Edwin Powell Hubble wandered the Kentuckycountryside where he grew up, observing the habits of birdsand animals. As an adult , he scrutinized the stars and galaxies.

Although Hubble was always interested in science, he did notsettle on the profession of astronomy Immediately. In 1910, hereceived an undergraduate degree from the University of Chicago,where he also lettered inbasketball and almost becamea professional boxer. Instead,he studied law under a Rhodesscholarship at Oxford, passedthe bar, and practiced lawbriefly and halfheartedly. Hereported that he "chucked thelaw for astronomy, and I knewthat even if I were second-rateor third-rate, it was astronomythat mattered."

Hubble completed graduatestudies at the YerkesObservatory of the Universityof Chicago, where he began hisexamination of spiral nebulae.He earned his doctorate in1917 and was invited to jointhe Mount Wilson Observatoryin Pasadena, California, butHubble did not yet undertakethe studies that made himfamous. Answering the call toWorld War I, he enlisted in theinfantry, telegraphing observa-tory personnel, "Regret cannotaccept your invitation. Am offto the war."

Two years later he f inal ly tN,e_s 8ohr Library, American Insfi tute of Phys Cslbegan working with the tool that would enable him to make greatdiscoveries--the lO0-inch reflector at Mount Wilson, at the timethe largest telescope in the world. Except for four years of servicein World War II, Hubble was devoted to astronomy until his deathin 1953.

Hubble's patient, painstaking observat ions revealed a muchlarger universe than anyone had imagined. He was enchanted bydim, foggy patches called "nebulae," the Greek word for cloud.One called Andromeda was the most spectacular nebula observedduring the early decades of the century, but telescopes were notpowerful enough to see if it harbored any stars like the vast stellarpopulations of the Milky Way. Since the 18th century, scientistshad argued whether these areas were "island universes," sepa-rate galaxies, or simply nebulae in our galaxy. Was the Milky Waythe only galaxy? Was it the center of the universe?

In 1924, Hubble ended the debate when he reported stars inthe outskirts of Andromeda and found Cepheids, stars that reveal

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their distance by the way their l ight var ies. Careful observations othe Cepheids enabled him to measure the distance to Andromeda,far too many light-years away to be in our galaxy. He classified thegalaxies, grouping them by sizes and shapes, and established thatmany other nebulae were also galaxies even more distant thanAndromeda. Hubble measured the depths of space out to 500 mil-

lion light years, distances fartherthan any previous surveys.

As he continued to studygalaxies, he concluded that theywere moving away from Earth atvelocities proportional to theirdistances. This supported theconcept that the universe origi -hated in a cosmic explosion, andall the matter in the universe waexpanding from the site of an initial Big Bang. The galactic surveyresulted in Hubble's law: themore distant the galaxy fromEarth, the faster it moves away.Of course, if all the galaxiesoriginated from one explosion,residents of other galaxies wouldsee the same thing: a universe ofleeing galaxies with more distanones moving more rapidly.

Hubble found that the ratio ofthe velocity of receding galaxiesto their distance from Earth isconstant (the Hubble constant),a significant astrophysical numbest ill not calculated with certaintytoday. Unfortunately, as the.... _ distances to objects Increase,astronomers are overwhelmed

with uncertainties: distances are hard to measure. Currentestimates of the Hubble constant, and thus the rate of expansionof the universe, differ by a factor of two. More powerful telescopesare needed to make more precise measurements and determinewhether the universe will expand forever or halt and perhapsreverse.

The Hubble Space Telescope will build on Hubble's research,measuring distances with greater accuracy than ever beforepossible. It is fitting that this premier space observatory is namedfor the American astronomer whose work revolutionized modernastronomy. Hubble's research proved that larger , more powerfultelescopes are needed to see more of the universe. He assistedin the design of the 200-inch Hale telescope at Mount Palomarnear San Diego and made the f irst observations with it. Whenasked what he expected to find with the new telescope, he said,"We hope to find something we hadn't expected." With the HubblSpace Telescope, this quest continues.

ORIGINAL PAGE EdwinP.Hubbleat the 4B-inSchmltelescope at PalomarObservatoryANI') WHITE PHOTOG_API -J _ie_sBoh,Li_rary,.,_ic_.l,_s._.reorPh_18


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Better "readmaps"areneededforaccuratedistancemeasurementstogalaxiesmuchfartherawaythantheseneighboringalaxies.

How do we measurethe universe?When the United States decidedto send men to the moon, scien-tists and engineers lacked anessential bit of information: theexact distance to the destina-tion. At that time, no one knewprecisely enough for navigation

purposes just how far away thenearest celestial body waslocated.

Although the distances tosome stars and galaxies arepublished as so many lightyears, these measurements inthe celestial atlas are, in reali-ty, estimates. The actual dis-

tance scale beyond the MilkyWay has never been preciselydetermined; current estimatesare so uncertain that galaxiesmay be twice as far away as wethink, or only half as far.

Distance is crucial to almostevery problem in astronomy,especially issues in cosmologyabout the origin and age of theuniverse. Astronomers need toknow how far away the varieusbright and dim objects are andhow far apart they are A pre-cise measurement of distance isalso the basis for establishingan accurate value of the Hubbleconstant, a measure of the ageand rate of expansion of theuniverse.

A high-priority project is touse the Hubble Space Telescopeto refine the distance scale ofthe universe and to calculate amore accurate value of theHubble constant With its ability

TheHubbleSpaceTelescopewill resolvesomeofthe detailthatis usuallylostovervastdistances._o_p_chat_e__s_j

Cepheidvariableare commonreferencepointsforastronomicalmeasurements.ThedifferenceinbrightnessoftheseCepheldsntheAndromedagalaxy(betweenthedashedines)isevident.(Palomar Obser_3Tor}_Cahforma Ins;_tote o!

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Moreaccuratemeasurementsofdistanceso objectsinthe Virgoclusterofgalaxies,60millionilghtyearsfromourgalaxy,willgivescientistsbetter"yardsticks"orcalculatingprecisedistancestoobjectsmuchfartheraway.{Kltt Peak Nat,onal O_se: _ator_/_l

:o measure very small angular_hifts in the positions of stars,_he telescope will survey the:listance to a special class ofstars known as Cepheidvariables.Cepheids are stars that

brighten and dim on a regularschedule; they are used as"standard candles" because[heir intensities correlate withtheir periods of variation. Thesesources of known intensity pro-vide a "yardstick" with which tomeasure distance. The apparentfaintness o[ a Cepheid trans-lates directly into its distance.

However, part ial obscurationof the light by intervening dustand gas complicates the issue.We can now make accurate dis-Lance measurements only tonearby Cepheids -- a few tensof millions of light years away.If we can measure the distanceto galaxies out to the Virgocluster, about 60 million lightyears away, then we can usethis longer yardstick to deter-mine more accurate distancesto other objects farther away.

By analogy, if at home weneeded to measure a room but

did not have a ruler or measur-ing tape, we might use a pieceof string for successive mea-surements until we could saythat the room was so manylengths of string wide by somany lengths of string long;that unit of measure could beused as a "yardstick" for gaug-ing the distances to otherobjects. The Cepheid projectwill take several years to com-plete, since measurements mustbe repeated and compared atregular intervals to refine thedistance calculations.

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What are quasars,active galaxies, andother exotic objects?Among the most mystifyingcelestial bodies are quasars,active galaxies, neutron stars,and other compact objects thatappear unusually luminous fortheir size. Because these objectsare relatively small and very faraway, we have not been able to

determine exactly what theyare. With the Hubble SpaceTelescope, however, the natureof these enigmas should berevealed.

Quasars are especially puz-zling. The most distant observ-able objects, they should bevery faint; however, a quasarthe size of a solar system canproduce hundreds of times as

much radiation as an entiregalaxy. Interestingly, a quasarmay be hardly noticeable in visi-ble light, but it blazes forth inhigher-energy radiation (ultravi-olet and X-rays). The brightnessof some quasars varies frommonth to month, even from onehour to the next. There is noexplanation for the source ofthis immense energy or its vari-

ability. Are quasars powered byblack holes or by something yetto be discovered? The quasarproblem challenges our currentunderstanding of the laws ofphysics.

The most distant quasars, afar away as 15 billion lightyears, are thought to be at thelimits of the observable uni-verse. Because looking far out


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ntospaceslookingackn:ime,weseequasarsodayas:heyappearedillionsfyears_go;heymaynotevenexist_ow.Quasarsaybeexplo-;ions that precipitate galaxies,:he violent first step in an evolu-:ionary process that gradually:esults in a normal galaxy like)ur own.

In some galaxies, the centralregion emits extraordinarilyintense and variable radiation.It is thought that quasars orblack holes hidden in theseactive galaxies may be thepower source. High-resolutionstudies of the centers of galaxiesby the Hubble Space Telescopeshould reveal this mysteriousphenomenon.

Observationsn differentradiationbandsrevealdifferentfeatures,as shownIn thesetwoimagesof theM101 galaxy,oneIn visibleight(left)andthe otherInultraviolet(right).Onlythehigher.temperaturebjectsandareasappearin ultravioletmages,whilethecooleronesdisappear,_t_peaka_io_arbse,,ato_,,,s,_e_sAGod_._S_ceP,_gh,*e._e;,a_.,oP_!

Quasar3C273 in virgoIsa mysteriousobjectthatmayradiate100 timesmorelightthan thebrightestordinarygalaxy.(K_ftPea_ NaI_onalObsef _,alor)')

Somequasarsandotherobjectsemitperplexingets ofradiation.(Adolph 5chatler, artJst_

Theexistenceofblackholes,whichmaydevourmatter fromnearbystars,is theorizedbutnotyetconfirmed.rOon DJ_onart_sl)

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How do galaxiesform and evolve?There are an estimated 100billion galaxies in the universe,and we know only one reason-ably well: our own Milky Way.As recently as the 1920's, whenEdwin P. Hubble corrected therecord, no one knew with cer-tainty that other galaxies existed.Since then, galaxies have beenidentified, labeled, and placedin a classification scheme basedon their structure or other fea-tures, but only a fraction of thetotal population of galaxies hasbeen sampled.

Galaxies are intrinsicallyinteresting evidence of the ori-gin of the universe, for it isthought that they were the firstobjects to form after the cosmic"Big Bang." They seem to bethe basic building blocks thatgive structure to the universe.Curiously, there is little evi-dence that new galaxies areforming today.

Astronomers are eager tosee the farthest (youngest)galaxies out toward 15 billionlight years, for these shouldreveal the conditions underwhich the universe began totake shape. To understand howthe universe evolved,astronomers need to study asmany galaxies of as many typesand ages as possible. Today,only the 20 nearest galaxieshave been studied in any detail;these are contemporaries of theMilky Way, similar in form andcontent. A more thorough cen-sus is required.

With the very sensitiveHubble Space Telescope,astronomers will be collectinginformation about galaxies

SpiralgalaxyM83 inHydra_,CerroTo_olo#lter American Observafory)

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during the early stages of theirformation and adding newgalaxies to the list ofthose available for study. Inaddition, they will get a moredetailed look at some puzzlingphenomena, such as activegalaxies that produce extraordi-nary energy compared to typicalgalaxies. It is thought that thenuclei of such galaxies may har-bor quasars or black holes,which are net yet well under-stood. With the improved reso-lut ion of this telescope,astronomers will be able to peerinto the crowded centers ofgalaxies to find what lies hid-den there.

Celestialmerger:galaxiesmaycollideandinteract,exchangingmatter andresultingin theformationof newstars._e,_a__.,_, a_r_)


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The Hubble Space Telescope will be used toobserve a variety of galaxies of dif ferent types,distances, and ages:Small Magellanic Cloud(Ro)at Obser_lor_)

Whirlpool Galaxy In Canes VenatlclIPalonlar Obse_ v_ {o r_ Cab fo rn _J f_sIJfule of T_chnolo_!

Cluster of galaxies InComa Berenlces{Kill P e_ N at _o r_ at Obser_'_Ior) _

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What is thelife cycle of stars?Stars are forming in our galaxyand elsewhere today. Althoughthe Milky Way is more than10 billion years old, we see starsthat are thousands of timesyounger. Occasionally, we alsosee stars in the death process,exploding as supernovas anddisappearing or shrinking intowhite dwarfs .

The rationale for observingstars is much the same as thatfor observing galaxies; scien-tists need to study many starsof many different ages andtypes to understand how theyform, exist, and die. Stellar pop-ulation studies are an importantitem on the agenda of theHubble Space Telescope.Thousands of stars will be mea-sured and categorized to obtaina large statistical sample.

Studying other stars tells usmore about the nearest star, oursun, which formed relativelylate in the galaxy's life. Althoughthe Hubble Space Telescopecannot look at the sun, the

broad stellar survey will help usunderstand the processes thatgovern it. Detection of proto-stars (clouds of matter that arecondensing into stars) and starsthat form earlier in the historyof a galaxy may give insight intothe formation and early historyof our solar system.

Many faint stars will appearas bright sources to the highlysensitive instruments in theHubble Space Telescope. Thestandard deep sky atlas, thePalomar Observatory SkySurvey, includes stars as faintas 21st magnitude. While theHubble Space Telescope will notbe used for a full-sky survey, itwill be able to detect stars toat least the 28th magnitude andeasily record 24th magnitudestars, which are the faintestroutinely observable fromground observatories. It willalso be able to record faint star-light much more quickly thanlong overnight exposures fromthe ground. Hubble SpaceTelescope exposures will rangefrom tenths of a second to aboutan hour.

Nebulaereoftencalledstellarnurseries,becausenewstarsform Intheseglowingcloudsofdustandgas.(rrjF_debuta i_Sag,_ar _s,Kitt Peak Nationat Observator_

NorseheadNebulan Orlons an opaquecloudof dustthat absorbsightfromstarsbeyondt._Roya_b_e,_a_l ThePleiadesare a clusterof about3,000 starsin Taurus;heHubbleSpaceTelescopewillbeable toresolvestarsinmoredistantandverydenseclusters.

[Hanse_ Planetarium)

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A supernovaIs a stellar explosion that temporarily may outshine allother stars inits galaxy. An interesting target ofobservation is thesupernova discovered in 1987 in the nearbyLarge Magellanic Cloud._Eur_an Sout_m Observatory}

Supernova remnants,hallow shells of gas,may linger forcenturies as glowingevidence ofthedeath of a star.lSu_mova r ea l_ an_ i_ Ve!a,RoyalObservatory,_

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Do other planetarysystems exist?Given the tremendous numberof stars in the universe, it isunlikely that our solar system isthe only collection of planetarybodies. Theoretically, the forma-tion of planets about a starcould be a normal process. Thesearch for other planetary sys-tems is pertinent to understand-ing the evolution of the universe,and it is an exciting scientificadventure.

The Hubble Space Telescopeis not likely to observe planetsaround another star directly, forthey would be extremely faint,obscured by the star's brightlight. However, it may be possi-ble to find secondary evidenceof a planet from wobbles in thestar's motion that indicate thegravitational pull of an orbitingbody. Similarly, fluctuations in ayoung star's light output mightsuggest a rotating, dusty diskwhere a new planetary systemcould be forming. If companionsare suspected around a candi-date star, it is possible to createwithin the telescope an artificialeclipse of the star's light so asmaller, dimmer body may bedetected. Highly precise posi-tion measurements with theHubble Space Telescope couldthen reveal whether or not thenewly visible body were orbit-ing the central star.

Another exciting use of theHubble Space Telescope is tofocus on the planets and moonsin our own solar system, givingus a front row view of thesefascinating worlds almost

comparable to photos from theVoyager spacecraft. Whereasthe exploratory Voyagers hadonly a fleeting glance at Jupiter,Saturn, Uranus, and Neptune,the Hubble telescope can studythem whenever we desire,repeatedly and in finer detailthan observations from theground. Thus, the telescopecomplements the discovery mis-sions by providing for longer-term study. For example, whileVoyager first showed us evi-dence of volcanoes on Jupiter'smoon Io, the Hubble SpaceTelescope will tel1 us how oftenthey erupt and what chemicalsthey emit. One of the camerasin the Hubble observatory canphotograph the entire Earth-facing hemisphere of any planetin our solar system (exceptMercury, which is too close tothe sun) in a single exposure;it is possible that our first "closeup" look at Pluto will be amongthe images received whenobservatory operations begin.

Comets are important targetsof observation, because they arethought to be debris from theformation of the solar system,frozen and essent ially unchangedfor four billion years. Cometsmay tell us what chemicals, inwhat abundance, were presentwhen our solar system formed.The Hubble Space Telescope isuseful for cometary science,because it can examine a cometat many points along its trajec-tory through the solar system.Thus, astronomers need not waitfor the next close approach toEarth to collect comet data. _i.*_

TheHubbleSpaceTelescopewill enablestudiesof theplanetsin finerdetailthanis possiblefromtheground.

28

Aclose-upviewofJupiteranditsmoonsIo (left) andEuropa(right)comparablendetail to expectedHubbleSpace Telescopemages.ThisImagewasobtainedat a distanceof 20 m/Ilionkm(12.4 mi/llon mi) bythe Voyagerspacecraft._'_ASA)

Oneof thebest imagesof Jupiterfroma telescopeonthe ground_Pa_onJar Obser _ator y, Ca ilort a nst tote o Technology

The Hubble Space Telescopewill give us a front row view ofthe fascinating worlds of oursolar system.


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Voyager imageof Neptune from

82 million km(51.5 million mi)

similar indeta il to aHubble SpaceTelescope image

Pluto, never yetseen in detail!Pa omar Observator _C,]_ffornia Instit_l_' Of )ec_rlo_o_!

Uranus and Neptune observed from EarthIL c:.,bse re,orv_

A typicalground-based

image of Saturn_PaJomar Obser vato,"CaJa_ornaa_nstJtute o;

technology,

A Voyager imageofSaturn from13 million km(8 million mi),comparable in

resolution toHubble Space

Telescope imagery;N4SAI


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O!:!,3[_.!AL PAG_L0 _TL_I_I_>ii'3TOCRAPH

The Hubble Space Telescopeis an observatory unlikeany astronomical observatoryon Earth.

Light entering the telescopestrikes the primary mirror, isreflected forward to the sec-ondary mirror where it isreflected again, returning downthe telescope and through ahole in the primary mirror to thefocal plane, where the imageforms. There, detectors recordimages or analyze the incomingultraviolet, visible, or infraredlight. These focal plane instru-ments are crucial to the obser-vatory; without them, theadvantage of the perfect mirrorand vantage point above theatmosphere would be wasted.The detected light is convertedinto electronic signals andtransmitted by relay satellite toground stations. There the dataare processed by computers andreconstructed as images forastronomers to study.

The Hubble Space Telescopeis an observatory unlike anyastronomical observatory onEarth. Some of the differencesare obvious. It doesn't have the

familiar dome shape of observa-tory buildings, because it needsno protection against weather.It has peculiar features likesolar panels, to generate electri-cal power, and communicat ionsantennas, to receive and trans-mit information. The telescopeis remotely controlled by opera-tors hundreds of miles belowit and, at times, half a worldaway. It operates around theclock, day and night, seven daysa week, year after year, unaf-fected by clouds and weather.VisibiliW is always perfect.

More subtly, the HubbleSpace Telescope represents arevolutionary shift in the waymost astronomers do their work.During the deployment, opera-tional, and servicing phases ofits mission, the telescope ishandled differently than is cus-tomary for a ground-basedtelescope. With the HubbleSpace Telescope, astronomicalresearch reaches maturity in thespace age.

Control room in the SpaceTelescope Operations ControlCenter at NASA's Goddard SpaceFlight Center In MarylandfNAS4)

The HubbleSpace Telescopewill be delivered to orbit

by the Space Shuttle.Initial checkout will occurwhile the telescope Isattached to the Shuttle; then

it will bereleased foryearsof operation In space.(Gordon Rane,K a rt s t, L oc kh ee d)

Science Primary MirrorInstruments MainBaffleCentral SecondaryBaffle Mirror

Light

SecondaryMirror BafflePath oflight through the telescope to the detectors

Aperture

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Observing with theHubble Space Telescope

In the nearby media center,journalists keep vigil, theireditors and producers savingfront pages and prime time foron-the-scene reports. Across thecountry at NASA centers andaerospace firms, thousands ofpeople involved in the projectknow that today is the red letterday, the culmination of years ofeffort. Around the world,astronomers await word thatthe new observatory isdeployed, promising new vistasfor the most ancient science.

Actually, ground controllershave been "talking" to thetelescope since four hours afterlaunch, waking the varioussystems and readying them tobe turned on. For launch, the

telescope is dormant; its activa-tion in orbit is a methodicalprocedure that lasts about threedays for initial deployment andseveral months for full check-out. This verification sequencecarefully brings the telescopeto full operational status.

About 24 hours after launch,the telescope is unpluggedfrom the Shuttle power supplyand begins operating on its ownbattery power. It is then liftedfrom the bay by the robot armand held outside the Shuttleduring preparations for release.The solar arrays must bedeployed promptly to activatethe telescope's internal powersystem, and the antennas mustbe unstowed for two-way

command and data flow. Bythe second day in orbit, the tele-scope is ready to be released.

For the next several hours,the ground control teams put_he telescope through its paces,gradually bringing it up to fullpower, turning on its "house-keeping" systems for pointingand attitude control, loading thecomputer memories, and testingthe telemetry. These activitiesverify that everything is work-ing properly as the telescopeadjusts to the space environment.

The third and fourth days arededicated to testing the tele-scope's ability to move and pointaccurately, to acquire guide starsand targets. Then the alignmentsof the mirrors and scientific

instruments are checked, andcalibration tests are run. Abouttwo weeks into the mission,checkout of the cameras andother scientific instruments inthe focal plane begins.

Checkout is a time forpatience. A full six monthshave been allocated for meticu-lous verification that all systemsand instruments are functioningproperly. This period will bepunctuated by some actualobservations, but the observa-tory's schedule will not beturned over to the scientistsuntil the checkout is complete.

Two NASA centers shareresponsibility during this trialperiod. The Marshall SpaceFl ight Center in Huntsvil le,

Astronautsworkingnthe aPtflightdeckof theSpaceShuttl6deploytheHubbleSpaceTelescope.

COL,..'._ PHOTOGRAPH


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Space Telescope in Orbit

First DaysSlowly and gently, the telescopeis lifted from its cradle in thepayload bay of the Space Shuttle.Attached to the spidery remotemanipulator arm, the 43-foottelescope clears the bay doorand is held overboard for amethodical checkout beforerelease. There is no ribbon-cut-ting ceremony, no corners toneto set in place, no speech under

a dome. Instead, opening nightactivities proceed in a brisk,businesslike fashion. Only fivepeople are present .Inside the Shuttle, an astro-naut intently watches througha window and operates themechanical arm to position thetelescope for the first stages ofits deployment. The commanderis on duty to oversee activity,while the pilot maneuvers the

Shuttle. Two other crew mem-bers stand by, prepared to go towork outside the cabin if someproblem ar ises.Meanwhile, scores of person-nel at ground control centersmonitor every electronic signaland issue commands as thetelescope is being prepared tooperate in space. The mood isvibrant ; scient ist s, engineers,managers, and VIPs concentrate

on video screens and adjusttheir headsets, listening quietlyto terse status reports. "Solararrays deployed...power-upsequence initiated..." The twosolar arrays gradually unfurl,and then the two antennas foldout. The Shuttle eases away toa respectful distance, where ithovers until the initial checkoutis complete.


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Alabama,ASA'seadcenterfordevelopmentftheHubbleSpaceelescope,illconductthegeneralpacecraftystemscheckout.heGoddardpaceFlightCenternGreenbelt,Maryland,sresponsibleorverifyingheperformancefthescientificnstruments.ctiVationandcheckoutillbeconductedPromheSpaceelescopeOperationsontrolentertGoddard.ngineeringeamsntheHuntsvilleperationsSupportenterndatsitesofthevariousontractorsespon-sibleorconstructinghetelescopeillsupportheseactivities.fterhegeneralys-temscheckout,perationalndmanagerialontrolftheHubblepaceelescopeillpassromMarshalloGoddard,whichwillthenbetheocalpointofday-to-dayperationssuchastelescopeointing,instrumentommanding,nddatalow.AsinallSpacehuttlemissions,heKennedypaceCentertCapeCanaveral,Florida,ndheJohnsonpaceCenternHouston,exas,erveascontrolentersorthelaunchandmannedpaceflighthasesoftheHubblepaceelescope'sdeploymentndsubsequentservicingissions.

HubbleSpaceTelescopenorbitItockheed)

IThe first several months in orbitare reserved for checkout andverification of the telescopebefore science operations begin.

Or_ 13:00 14:00IMEMET [),'HR 01 0000 010100ORBIT#LIGHT/SHAOOW

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Day-to-Day OperationsSeveral months after launch,when all checkout and verifica-tion exercises are completed,NASA officially hands overscience planning responsibilityfor the Hubble Space Telescopeto the Space Telescope ScienceInst itute in Bal timore, Maryland.The Institute is responsible forthe scientific use of the observa-tory, which will continue to beoperated by the Goddard SpaceFlight Center. This dual controlof the telescope's operation is anew arrangement that reflects

some of the basic differencesin the ways astronomers useobservatories on the ground andin space.The Hubble Space Telescopeis not the first telescope in space;a number of smaller instrumentshave flown since the 1960's.However, it is the first large or-bital observatory to be accessibleto the worldwide astronomicalcommunity and the first suchtelescope to be considered "per-manent." Like Mount Palomar orother world-class observatorieson the ground, the Space

Telescope will host a changingprogram of investigations andguest observers while it is inservice. What is it like to be anastronomer using the HubbleSpace Telescope?

To work in some of the bestobservatories on Earth, anastronomer may fly halfwayaround the world and then trav-el many miles overland to reacha remote mountaintop for a fewnights of observation. Normally,he or she books time at theobservatory several months inadvance, arrives on the sched-

uled date and takes up resi-dence there, hopes for severalnights of good visibility, and, ifthe weather isn't cloudy, pro-ceeds to observe whatevertargets are critical to his or herresearch. Usually no one else isscheduled to use the facility atthe same time, so the astron-omer works without inter ruption.The observatory staff assist inconfigur ing the equipment,pointing the telescope, and sup-porting the guest observer'splan. The astronomer and obser-vatory technicians may sit side


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bysidenthecontroloom,choosingargetsndadjustingthenstrumentshroughhenightnresponseoobservingconditions.heelescopesessentiallynextensionftheobserver'senses,espondingimmediatelyotheastronomer'sdesireolingeronaparticulartargetorlookelsewhere.henastronomersotobedatdawn,theirworkhasonlybegun.heharvestfdataromonenight'sobservationsayakemonths,evenyears,oanalyzendunderstand.

Inobservatoriesn theground,astronomersworkin a controlroomafewstepsawayfromthetelescope;however,heHubbleSpaceTelescopesa remotelycontrolledobservatory.{Ha(e O_ser_aror_;

Mountaintopobservatoriest MaunaKea[Canada-FranceHa_,'aii Telescop_CorpotaI_on)


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GaseousNebula in Serpens(Hale Observatories)

Right: A console Is available forastronomers who wish to bepresent when their programmedobservations occur. Some directcommanding is possible tocontrol instrument settingsand final positioning of theastronomical target in the fieldof view.(NASA)

To use the Hubble SpaceTelescope, an astronomer neednot leave home. This orbitalobservatory is almost totallypreprogrammed, and its opera-tion involves complex commandprocedures that are the busi-ness of engineers in the SpaceTelescope Operations ControlCenter. The astronomer inter-acts with the Space TelescopeScience Institute, not directlywith the observatory or itscontrol center. The Institute isresponsible for science planningand scheduling and for sciencedata processing and archiving.

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Like an observer at a groundobservatory, the Hubble SpaceTelescope guest observer mustrequest observing time well inadvance. However, because ofthe complexities of schedulingan orbital telescope, the leadtime between request andobservation is much longer, andthe astronomer must describethe observing plan in muchgreater detail. Proposals forobservations are submitted tothe Institute, evaluated by peerreview, and selected as much asa year or two before the actualobservation occurs.

During the interim, theobservation plan is refined, andall activities necessary toaccomplish it are specified. TheScience Institute then begins tocompile a master calendar,usually six months at a time, toschedule the selected observa-tions.

Because observation time inspace is costly, the governingphilosophy in scheduling theHubble Space Telescope is effi-ciency. The aim is to maximizethe time on target and minimizethe time spent reorienting thetelescope from target to target.

_'_ _ f_-_ _ _ _,__

Thus, observations of stars andgalaxies located in the samepart of the sky are generallyscheduled together. If anastronomer wants to observeobjects in different parts of thesky, these observations proba-bly will not occur successivelyin one orbit nor even in one dayor week. An astronomer's planmay be split up and scatteredthroughout the six-month periodto improve overall schedulingeffidency,

v.

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Likewise, if an ast ronomer'sprogram requires a long obser-vation, perhaps three hours on atarget, the observation may beinterrupted as the view from theorbiting telescope is blocked bythe Earth or as some otheractivity takes precedence.AIthough the telescope operatesaround the clock, not all thetime is spent in observing. Eachorbit lasts about 90 minutes,with time allocated both forhousekeeping functions and forobservations. "Housekeeping"

includes turning the telescopeto acquire a new target or avoidthe sun or moon, switchingcommunications antennas anddata transmission modes,receiving command loads anddownl inking data, calibrating,and s imi lar act ivit ies.

When the Science Institutecompletes its master observingplan, the schedule is forwardedto Goddard's Space TelescopeOperations Control Center,where the science and house-keeping plans are merged into

a detailed operations schedule.Each event is translated into aseries of commands to be sentto the onboard computers.Computer loads are transmitted("uplinked") several times a dayto keep the telescope operatingefficiently.

Normally, two of the scientificinstruments will be used simul-taneously to observe adjacenttarget regions of the sky. Forexample, while a spectrographis focused on a chosen star ornebula, one of the cameras willbe photographing a sky regionoffset slightly from the mainviewing target. During observa-tions, at least two of the threeFine Guidance Sensors will beoperating, each tracking a sepa-rate guide star to keep the tele-scope pointed steadily at theright target.

If an astronomer desires tobe present during his or herobservation, there is a consoleat the Science Institute andanother in the Control Center,where monitors display imagesor other data as the observationoccurs. Some limited real-timecommanding for target acquisi-tion or filter changing isperformed at these stations,if the observation program has

been set up to allow it, butspontaneous control is not pos-sible. For some investigations,the observer may make fineadjustments to the telescope'spointing, using a light pen andcursor to center within an aper-ture a small target in a widerfield. Whether or not the guestobserver is present at theInstitute, a staff astronomer andconsole operator are on dutyat all times to monitor theobservations. This is where real

M81 galaxyin UrsaMajor_KJItPea_ Nafiona_ Observak_ry)

Thefieldsof viewofthe RneGuidanceSensorsformthree "pickles,"heresuperimposednthe HorseheadNebulainOrlon.Toacquireanobjectforobser-vation,thetelescopemustfirstlocateappropriateguidestarswithtwoof thethree FineGuidanceSensors.Candidateguidestarsareshownasyellowcrosses.Theguidestar catalogincludesalmost19 millionobjects.(Sp_ceTelescope Scier_cel_sfJtu[eJ

GiantellipticalgalaxyM87 in Virgo_Kdf Pea_ NatJo#afObserva_ot,

4O


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excitement is felt asastronomers have a first look atwhat the telescope sees. Theinitial images and data are soonprocessed and refined bysophisticated computer pro-grams on the ground.In a much larger room at the

Space Telescope OperationsControl Center, 30 miles away,consoles are manned around theclock as operators stay mconstant contact with the obser-vatory, sending up commands,

GJobulaxtaycJustexnPegasus,NIJ5(KJtl Pea_ _Vat_onalObser_ato_))

monitoring status, and receivingdownlinked data. This is thenerve center. Behind the scenesat the Goddard and MarshallSpace Flight Centers and atthe industry sites where thetelescope and its parts weredeveloped, support teams stayon call to respond to any opera-tional problems that may occur.

Engineering and scientificdata from the observatory, aswell as uplinked operationalcommands, are t ransmi ttedthrough the Tracking and DataRelay Satellite System (TDRSS)and its companion groundstation at White Sands, NewMexico. Up to 24 hours ofcommands can be stored in theonboard computers. Data canbe broadcast from the telescopeto ground stations immediatelyor stored on tape and down-linked later.

The observer on the groundcan examine "raw" images andother data within a few minutesfor quick-look analysis, Within24 hours, Goddard formats the

data for delivery to the Institute.The Science Institute is respon-sible [or data processing; itcal ibrates , edi ts, distributes,and maintains data for thescientific community. The guestobserver may analyze the datawith software provided by theInstitute or with his or her ownprograms, and the data may beanalyzed at the Institute or atthe observer's home institution.Eventually, observers aroundthe world will be networkedto the Institute, enabling themto have keyboard access to thedata archives there.

Competition is keen forobservatory privileges on theground and in space, there beingfar more astronomers than largetelescopes. The line to use theHubble Space Telescope isalready long, and for every oneobserver whose proposal isaccepted there may be ten oth-ers waiting.

Most modern observator iesare cooperatively used asnational and international

research centers. The HubbleSpace Telescope will be operatedin this tradition, as a resourcefor ast ronomers worldwide.Some observing time is allocat-ed for amateur astronomers, aswell. Data will be maintained inthe Institute's archives and,when the primary investigators'research is completed, madeavailable to all astronomers, notonly those whose observationsare accepted but also those whoare waiting.

The perplexing problemsin astronomy today cannot beresolved by a single instrument,a lone observer, or even oneobservatory. The new break-throughs in theory and under-standing are likely to come as aresult of complementary investi-gations, involving several differ-ent instruments, observers, andobservatories on the ground andin space. The Hubble SpaceTelescope program is organizedto encourage collaboration anddiscovery,

M33 galaxyinTriangutum!Kitl Peak ,_,_r_on_lObser_3[or_j

Data fromthe HubbleSpaceTelescopewill berelayedbysatelliteandgroundstationsto theGoddardSpaceRight Centerforprocessingnd thento theSpaceTelescopeSciencenstitute InBaltimore,wherethearchiveswillbe maintained.1_4s_

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Servicing in SpaceTwo astronauts are absorbed ina task they have rehearsed formonths: servicing the HubbleSpace Telescope. Like membersof a surgical team, each antici-pates the action of the other andoffers the proper tool or equip-ment as it is needed. They workquickly and with the confidencethat comes from practice.

The Hubble Space Telescoperises like a tower from its repairplatform in the Shuttle. Theobservatory is temporar ilyclosed down, its solar arraysand antennas retracted and theaperture door closed. The imme-diate purpose of this servicingmission is to replace severalbatteries and sensor units thatare reaching the end of theirl imited lifetimes.

Like an observatory on Earth,the Hubble Space Telescopecan be repaired. As equipmentwears out or becomes obsolete,it will be replaced. Alreadyanother observatory, the SolarMaximum mission, has beensuccessfully repaired in space toextend its operation for severalmore years. The ability to ser-vice observatories in orbit is animportant new way to extendtheir useful lifetimes. Servicingin space protects our substan-tial investment in technology toanswer the highest priorityquestions in modern astronomy.Routine service calls at the

Hubble Space Telescope arescheduled at approximatelythree-year intervals to replacebatteries and other limited-lifeitems. These items, calledOrbital Replacement Units,include components in the guid-ance and control systems and inthe command and data handlingsystem, a computer, solar arrays,and the scienti fic instruments.Altogether. some 70 items in theHubble Space Telescope can bereplaced in orbit. The particularset of replaceable items varies

Astronauts test servicing procedures with flight hardwareon the ground to be certain that tools, fasteners, and otherequipment fit and operate properly for servicing in orbit.More than 80 tools, including common screwdrivers andwre nc hes , a re a va ilable.(Lockheed)

Like an observatory on Earth,the Hubble Space Telescopecan be repaired. As equipmentwears out or becomes obsoleteit will be replaced.

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from mission to mission,depending on the condition ofthe observatory.

A repair mission begins withdetailed planning and trainingon the ground, followed by arendezvous in space. TheShuttle crew maneuvers to thetelescope, captures it using therobot arm, brings the telescopeaboard, and attaches it to amaintenance platform in thepayload bay. There the tele-scope remains for several dayswhile astronauts inspect it andexchange equipment.The replacement units are

readily accessible behind doorsin the telescope shroud.Ranging in size from a shoebexto a telephone booth, most ofthe items can be removed orinstalled with the aid of wrench-es and screwdrivers. Other crewaids include portable lights andfoot restraints, small platformsthat give the astronaut a stablefoot-hold for the business of dis-connecting electrical cables, dis-engaging latches and bolts, andreplacing various pieces ofequipment.While attached to the main-

tenance platform, the repairedtelescope undergoes statusmonitoring and checkout exer-cises, controlled by commandsfrom the ground. If an orbitboost is necessary, the crewtakes the Shuttle to a higher

altitude for redeployment. Onceagain, the robot arm is used tolift and guide the telescope outof the payload bay. The solararrays and antennas are extend-ed again, the power is turnedback on, and the telescope isreleased to resume operations.

If any problems arise duringthe redeployment, the Shuttlecrew can retrieve the telescopeand manually override a faultymechanism. If the problem can-

not be solved, the Hubble SpaceTelescope may be brought home.If all occurs according to plan,the telescope will remain in orbit,doing its work until the nextscheduled servicing mission.

The Hubble Space Telescopeis the first observatory designedfor extensive maintenance andrefurbishment in orbit. A long-lived mission is scientifically nec-essary for both discovery anddetailed follow-up study.

Many parts of theHubble Space Telescope,suchas the batteries, are designedto be removedandreplaced. (Lockheed)Large units, such as the Fine Guidance Sensors andscientific instruments, also can be removed forservicing or replacement, iLockheed)

Servicingproceduresare rehearsedunderwaterosimulatethe weightlessenvironmentofspace.

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Opportunitiesfor DiscoveryCuriosityand technology:these are the prerequisites formodern astronomyand astrophysics. Wequestion the cos-mos, but most of the answers lie beyondthe range of ourobservational tools. Many discoveriesawait the develop-ment of new technologyfor more precise measurements.

TheHubbleSpaceTelescopeIs the firstina new familyofGreatObservatoriesestgnedto explorethe universeacrossthe electromagneticpectrum._N_SN

RosetteNebutanMonoceros(CaliformanslitutefTechnoto_)

just the past 40 years,the pace of discovery hasdramatically acceleratedas astronomers havetaken advantage of rapid

developments in technology.As a result, we now know thatthe universe contains not onlythe familiar planets, stars, andgalaxies observed for centuriesbut also a rich variety of objectsdiscovered quite recently.Quasars, pulsars, radio galaxies,X-ray stars and galaxies, infra-red stars and galaxies, andgamma ray bursts have beenidentified in our lifetimes withtechnology that was not previ-ously available. Some antici-pated objects, such as blackholes, have not been observeddirect ly; their discoveryrequires techniques not yet inuse today. Other objects not yetimagined await revelation bynew instruments.

The Hubble Space Telescopeis an instrument for discoveryand prolonged study. Despitedecades, even centuries, ofcuriosity, only now has thetechnology become available tobuild such a sensitive observa-tory and put it above theatmosphere. Advances in mir-ror, detector, materials, andspace transportation technolo-gies together make the HubbleSpace Telescope possible in ourlifetime. We will witness thenew astronomy.

In the past 15 to 20 yearswhile the Hubble SpaceTelescope was being designedand developed, progress intechnology has marched on.Already, new detectors arebecoming available, and theprogram includes developmentof "second generation" scien-tific instruments. After 5 to 6years in orbit with its presentinstrument complement, theobservatory may be refurbishedin space. Some of the scientific

instruments may be removedand replaced with newstate-of-the-art detectors toextend our range of observationagain. Just as the 40-year-oldtelescopes at Mount Palomarremain premier observatoriesthrough periodic upgradingwith new technology, so theperformance of the HubbleSpace Telescope will keep pacewith technological advances.

The Hubble Space Telescopeis a key element in a broad cam-paign to observe the universeacross the electromagneticspectrum. This observatory willnot be used in isolation butwill be complemented by othertelescopes sensitive to differentwavelength bands. All matterradiates energy at differentwavelengths, depending on thetemperature of the matter. Atelescope is, in a way, a remotethermometer detecting radiationwithin a specified wavelength(temperature) range. Becausethe spectrum from radio wavesto gamma rays is so broad, nosingle telescope can detect alltypes of radiation or all objects.Thus, NASA's strategy for

astrophysics research is tolaunch a family of telescopes,the "Great Observatories," eachtuned to a different channel ofthe spectrum. The Hubble SpaceTelescope is the flagship, to befollowed by the Gamma RayObservatory, and later theAdvanced X-ray AstrophysicsFacility and the Space InfraredTelescope Facility. These largeobservatories in space will oper-ate for the next few decades.Together, they will give us acomprehensive look at the uni-verse in all its guises.

What we now know, thoughexciting, is probably less inter-esting than what we are aboutto learn. Where is technologyleading astronomy? What newquestions may be asked in the

next generation? We can hardlyguess today, but they will beguided by the Hubble SpaceTelescope, which will surelyintroduce us to objects and phe-nomena that we have not yetimagined. The really intriguingquestions are the ones we don'teven know to ask.

There is no doubt that ourknowledge of the universe isgoing to change, perhaps asdramatically as it did whenGali leo's telescopes confi rmedCopernican theory that theEarth was not, after all, the cen-ter of the cosmos and that theMilky Way contained far morestars than anyone had ever sus-pected. The repercussions ofthese discoveries were feltthroughout western civi lizat ion,not only in science but also inart and literature, philosophyand theology. We may enter the21st century with radicallyaltered perceptions of theuniverse, its origin, extent, con-tents, fate, and our place in it.

We are intrepid explorers,observing the distant stars andgalaxies because our technologywill not yet take us there. Untilwe can roam freely through theuniverse, we continue toexplore it by capturing its light.Through the Hubble SpaceTelescope, many of those specksof light will become discernibleobjects--stars, galaxies, quasars,maybe planets and exotic thingsthat we have never seen before.

This book is but a prelude;the next edition will present theuniverse newly revealed by theHubble Space Telescope. Westargazers are eager to see!

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Engineering Challenge

Theprimary mirrorIs coated witha layer of reflective aluminum[2.5 millionthsof an inch), protectedby a 1-millionth.inchlayerof magn_slum fluoride.It is thesmoothest,most uniformlycoated large mirrorevermade. (Perkfn EImer Corp.)

In FocusTelescopes collect light from adistant stellar source and con-centrate the light onto a spot,preferably as small a spot aspossible. As light travels awayfrom its source, it spreads out inan expanding sphere, just as aballoon covers a larger and larg-er area as it is inflated. A tele-

scope can be thought of as a"light bucket"; the primary mir-ror or lens is the collecting sur-face. The larger the mirror orlens, the more expensive anddifficult it is to make.

The aim of mirror design is toreflect all the collected lightthrough the optical system andconcentrate it onto a very small

spot. For a single star, the spotis smaller than the period at theend of this sentence. The tele-scope also forms images of plan-ets, galaxies, nebulae, and otherextended sources that cover alarger area of the focal plane.The larger the mirror that col-lects light, the sharper theimage of the distant object that

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Primary MirrorSciencestrurnents lmage Forms

ere

CentralBaffle

SecondaryMirror BaffleSecondaryMain MirrorBaffle 3m

Aperture

13m (43 Ft.)

4rn (14 Ft.)

Layout of the Optical TelescopeAssembly

is the light source, if the mirroris very smooth. The mirrorshould be very smooth becauseany surface ripples or bumpsdegrade the image in the focalplane. The more bits of light(photons) that arrive at onespot, the brighter the image.The smaller the spot of concen-trated light, the sharper theimage and the fainter the sourcethat car be detected

An optically perfect mirrorconcentrates light to the small-est spot possible, limited onlyby the wave nature of lightitself and not by engineeringimperfection. Early in the plan-ning, NASA and the scientificcommunity determined that theSpace Telescope should bediffraction limited; that is, thedeviation from perfection wouldbe imposed by light, not byhardware. At the time, the tech-nology did not exist to achievethis degree of optical perfection

for a large mirror. The mirrorforeseen for the Hubhle SpaceTelescope was beyond the stateof the art.Besides being large and

extremely smooth, the mirrorhad to be lightweight. Theweight-to-orbit capability of theSpace Shuttle or any other launchvehicle is limited, so everypound is critical. The 200-inchmirror at Mount Palomar weighs14.5 tons; the 94.5-inch HubbleSpace Telescope primary mirrorhad to weigh less than 1 ton.Engineers cleverly el iminatedabout 75% of the glass thatwould have been required in asolid mirror of this size bydesigning the Hubble mirror asa honeycomb structure. If it hadbeen fabricated as a solid, theHubble mirror would haveweighed about 3 tons.

The Hubble mirror also hadto be thermally stable, invulner-able to expansion and contrac-

The requirements for opticalquality, light weight, thermalstability, and rigid mounting werea stimulus for creative solutions.

Theprimary mirror surface mustbe virtuallyperfect. In these com-puterizedmaps of themirror, theidealsurface is white. Meticulouspolishingsmoothedaway high(blue) andlow (red) variationsIn

tion as the telescope repeatedly the surface. (NASA)passes from dayside heat to the _ _--_-_*_._..... _ .,_ _ _frigid orbital mght and back into

It had to be mounted ._-_._l_Lf__ -."%unlight.rigidly enough to protect itfrom damage during launch _'_ f ..f" - _vibrations but not so #_ _ "t__ _rigidly to warp or deform [ W _ _ '- :\m!"_it. The telescope built on J___ "t _][Earthhadtobedesigned|. | ;!tO operate in the much ! _ :__" _ :1different environment ofspace. This combinationof requirements for optical _1_.'_ "._. ..... _,_'_! Iquality, light weight, therstability, and rigid mounticould have been an engineer's "__ .._'*"mghtmare. Instead, ]t proved tobe the stimulus for creativesolutions.

Another requirement compli-cated the fabrication. Thescientific community wanted aversatile large space telescopethat operated not only with visi-ble light but also with ultravio-let. Because each portion of thespectrum demands a differenttype of mirror coating, thedesign of the Hubble SpaceTelescope required engineeringtrade-offs and compromises. Forexample, aluminum is the best

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A special computer-controlled machinewas designed for the grinding andpolishing process. Flnal polishing wasdone by hand,(Per_in.Elme[ Corp,)

The Innerhoneycombstructure wasvisible before themi rror wascoated.(Perkin.EtmerCorp.

Developmentof the HubbleSpace Telescopeis a story ofinnovativeengineeringand technology.coating for visible light astrono-my, reflecting up to 99.5% of thelight, but it alone reflects almostno ul traviolet radiation.Aluminum oxidizes rapidly andmust be protected by an over-coating in order to reflectultraviolet. Furthermore, theovercoating must be just theright thickness to be transpar-ent to both ultraviolet and visi-ble light.Given the different scientificobjectives for the observatory,

the solution was an aluminumlayer with a magnesium fluorideovercoating to protect the mirrorsurface and optimize perfor-mance in the ultraviolet range.A slight overcoat of magnesiumfluoride, much thinner than thissheet of paper, protects againstoxidation and increases ultravio-let reflectivity to about 75%, buti t decreases visible ref lectivityto 85%. Though decreased fromideal, performance in the visiblerange is still quite satisfactory.

The coating process was par-ticularly demanding becauseboth coatings had to beextremely uniform for properreflection, and they had to beapplied in a high vacuum withina matter of seconds to avoidoxidation. Furthermore, thethinness of the magnesium fluo-ride overcoat had to be preciselycontrolled to achieve the desiredultraviolet reflectivity. A specialchamber and equipment wereprepared for the extremely deli-cate coating process, and tech-nicians rehearsed the 10-secondprocedure for a year.

Development of the HubbleSpace Telescope optical systemadvanced the state of the art inseveral disciplines. The mirrorand optical system of the HubbleSpace Telescope are the bestthat exist at their size. The 2.4meter (94.5 inch) primary mirroris an efficient reflecting surfacefor visible light and is less effec-tive but acceptable for ultravio-let. Jt Js polished so smooth thatthe surface varies less than amillionth of an inch. The opticalpath i s optimal for concent ratinglight from a single star at thefocal plane.

Achieving this optical qualityrequired new techniques ofmirror fabrication, grinding andpolishing, cleaning and coating,as well as highly precise analy-sis techniques to verify theuniformity and quality. To meetthe weight, thermal, and mount-ing specifications, new materi-als and designs had to bedevised, tested, and used. Eachelement is a story of innovativeengineer ing and technology.

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While it awaited launch, theHubble Space Telescope waskept meticulously clean. Themirror is extremely sensitive tocontamination. Like a sunscreenthat blocks ultraviolet rays, acoating of oil a few moleculesthick on the magnesium fluoridewould destroy its ultravioletreflectivity. Grains of dust onthe mirror would scatter light,producing a background hazethat would limit the telescope'sability to detect faint objects.Components were "baked out"in a vacuum to get rid of vaporsthat would "outgas" and con-taminate the optics in orbit.

Extraordinary precautionswere taken to keep the tele-scope clean during assemblyand checkout before flight. Thechamber where it was kept wasthousands of times cleaner thana typical laboratory. Tempera-ture, humidity, and air flowwere precisely controlled, andthe air was thoroughly filtered.Technicians abided by a strictdress code and cleansing proto-col to avoid bringing anycontamination into the area.All materials, tools, cameras,even checklists used during thepreflight assembly and testinghad to be cleaner than clean.

Designing, developing, andassembling the optical systemwas a challenge faced on theground, where people andmachines could he used to solvethe engineering problems asthey arose. The telescopeoperates in space, however, farbeyond the immediate reach ofhelping hands. The pointing con-trol system was another compli-cated engineering assignment .

e HubbleSpace Telescopewasassembledd tested in a protected environment. Here9optical telescope and focal plane structure..visiblebefore theouter shell was installed.":kheed)

-m

.:

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Stil l LightEveryone who has tried photog-raphy knows the difficulty ofholding a camera still and thedisappointment of blurredimages if the camera moves.The problem of jitter is magni-fied in telescopes, which requirelong exposures, sometimeshours, on the same object toproduce images bright enoughto detect. As the Earth turns,the star or galaxy beingobserved seems to move acrossthe sky, so telescopes on theground have to move slightlyduring an exposure to compen-sate for the Earth's rotation.

A telescope must be able to lockonto a target of observation andhold the image still within thefield of view, even as the instru-ment i tself moves ever sosmoothly to track the object.Finding the astronomical

target, locking onto it, and hold-ing the image still is not an easytask on the ground, but thedifficulty is compounded in anorbital telescope. On Earth, weuse the bedrock of a mountain-top to steady a telescope, butwhat steadies a telescope inspace? The Hubble SpaceTelescope is stabilized by ahighly sophisticated dialogue

The Hubble Space Telescope isthe most precisely pointed machineever devised for astronomy.

between complex computer pro-grams and control mechanisms.

The Hubble Space Telescopeis the most precisely pointedmachine ever devised forastronomy. Its requirements forpointing stability and pointingaccuracy are expressed in terms

of multiple-zero decimals. Thetelescope must be able to main-tain lock on a target for 24 hourswithout deviating more than7/1000ths [0.007) of an arc second(2 millionths of a degree), aboutthe width of a hair seen at a distance of a mile. Like the optical


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system,hepointingontrolsystemtretchedhestateofthearttoachievehisdegreefaccuracy.Thepointingontrolystemcompriseseveralifferentle-mentsllworkingogetheromaneuverheelescope,uideittowardbservationargets,andholdt steady.aneuvers,orslews,regovernedyreac-tionwheels,hichgeneratecontrol torques that turn thetelescope, and rate gyros, whichgive the computer informationabout the telescope's orienta-tion or attitude. The telescopecan be maneuvered a quarter-turn (90 degrees) with therequired accuracy and stabilityin about 18 minutes, movingslightly slower than the minutehand of a clock. The rate gyrosreport 40 times per second, andthey are sensitive enough todetect position changes as smallas .00025 arc second (7 hundred-millionths of a degree). Thus,they are also crucial in finepointing during target acquisi-t ions and observations.

During routine operat ions,three types of position indicatorsprovid_ the information neededto point the telescope: thefixed head star trackers whichare independent of the maintelescope optics and cA3 locatebright stars to a precision ofabout 1 arc minute (1/60thdegree); the rate gyroscopesthat provide precise angularmeasurements for short4ermstability; and the fine guidancesensors which derive theirsignals from stars imaged nearthe edge of the telescope's fieldof view. The pointing controlsystem uses four of six rategyros and two fine guidancesensors to provide roll, pitch,and yaw information and keepthe observatory locked on acelest ial object .

Telescope motions are con-trolled by varying the speed ofspinning reaction wheels. Thereaction wheels receivecommands from an onhoard

computer to generate torque,the force that produces rotation,and point the telescope to agiven position with an accuracyof 0.01 arc second or less. Thesignal for these wheels isderived from a variety ofsources--a coarse sun sensor,fixed head star trackers, gyro-scopes, and interferometric startrackers--all processed by theflight computer. Four magnetictorquer bars on the exterior ofthe telescope, also controlled bycomputer, react against Earth'smagnetic field to limit reactionwheel speed.Target acquisition is gov-

erned primarily by the fine guid-ance sensors, which search forguide stars that will bring thetarget of observation into theinstruments' field of view. Threefine guidance sensors have ahorseshoe-shaped field of viewat the perimeter of the focalplane. The master "referencebook" for telescope pointingis the guide star catalog, a com-pendium of almost 19 millionrecognizable objects whosepositions are known to about1 arc second (3 ten-thousandthsof a degree). The telescope usesguide stars rather than theactual observation targetsfor pointing. For any selectedtarget of observation, the fineguidance sensors seek outguide stars that will bring thetarget precisely into the centerof the selected detector's fieldof view. Each observation targetis keyed to multiple guide stars,depending on the telescope'sorientation, where it is in itsorbit, and which detector isbeing used.To acquire a target, two ofthe three fine guidance sensors

must lock onto guide stars. Theprocess of a typical targetacquisition begins with a slewof the telescope to a predeter-mined position. The fixed headstar trackers are used to deter-mine the rough position of thetelescope. Then the coordinatesof a candidate guide star are fed

into each of two fine guidancesensors. One sensor begins toseek the first guide star, search-ing in a spiral pattern until theproper star is found. Upon find-ing a star in the correct bright-ness range, it stops and theother sensor searches for anoth-er guide star. When the secondstar is found, the relative posi-tions provide the final confirma-tion of position. If the positionis not confirmed, the searchresumes. The acquisition cycletypically takes about 20 minutes;in some cases, alternate meth-ods may be used to reduce theacquisi tion time.

Once the guide stars areacquired, their light passes toa prism interferometer that pro-vides a tracking signal. The basicstability reference is the set ofrate gyros with positions thatare updated every second bythe fine guidance sensor signal.

The telescope locks onto theguide stars that _3ring theselected target into the field ofview of the selected instrument.Each instrument has a methodof automatically centering thescience object in its aperture orsending down a real-time signalwhich will allow the observer tomake a final decision on theexact pointing. In addition, theWide Field/Planetary Cameracan be used to make a quickimage from which the positionof the science target withrespect to the guide stars canbe determined, either in realtime or later in the observingsequence. When the telescopelocks on a target, the resultingstabili ty enables high-resolution(fine detail) visible and near-ultraviolet images.Three elements have to beheld completely stable, with nojit ter or disturbance whatsoever:the focal plane and both the pri-mary and secondary mirrors.The biggest challenge for engi-neers and designers of thepointing and control systemwas to identify and control allline-of-sight disturbances that

Focal planeand Fine GuidanceSensorfieldsof view ("pickles")superimposedon the AndromedaGalaxy,M31(Spac e Teles coPe Science Institute)

(NASA)

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Oneof the two delicate wingsof thesolararray (NASA)

ORIGINAL PAGE ISOF POOR QUALITY

Fine GuidanceSensorprism(NASA)

might compromise the tele-scope' s str ingent requi rementsfor pointing accuracy and stabil-ity. Some sources of error are inthe equipment and can be con-trolled; for example, noise orvibrations from onboard elec-tronics or thermal flexing canintroduce random errors into theattitude and pointing signals.If the telescope jerks during amaneuver, the solar arrays willrespond with a low-frequencywave that takes a long time todisappear. Gyroscopes "drift"and need frequent correction.

Over a longer time, a majortemperature change broughtabout by telescope maneuversmay cause a long-term changein structural dimensions, whichcan have a significant degradingeffect on precise observations.Therefore, all parts sensitive totemperature changes had to bemade thermally stable; the trussstructure, focal plane, and sec-ondary mirror support were thusmade of graphite-epoxy, a mate-rial that barely deforms as thetemperature changes. Manyelements also have thermostatsand heaters to maintain activethermal cont rol.

Other sources of error areenvironmental and must be com-pensated for, if not controlled.For example, the telescope isaffected by a slight aerodynamicforce and gravitational torque asit moves around Earth. Torquesthat act on the spacecraft mustbe nullified by the momentummanagement system to hold thetelescope steady.

To attain the specified point-ing stability and accuracy, sever-al technological advances wererequiredl particularly in thedevelopment of vibrat ion-freeequipment. The bearings in thereaction wheels, for example,had to be the best ever manu-factured for perfect balance,and isolators were added in thereaction wheel assemblies todampen any vibration. Likewise,the onboard tape recorders hadto operate very smoothly andquietly.

The most demandingachievement in the poiand control system wasand fabrication of theGuidance Sensors, bransystems capable of maextremely precise measto a milliarcsecond. Mglass prisms for the ineters was an especiallyand difficult effort. Thehalves of each prism atogether without glue,or fasteners. They hadsmooth, so fiat, and sothey bonded by sheerattraction. When heldpolisher's hand, the glbe deformed merely bheat. The prisms wereimpossible to make torequired perfection, anrepeated trials beforeprisms for flight (twowere produced. The FGuidance Sensors willstate of the art for theable future.

The onboard computbrain for pointing andthe telescope. The comcontains the informationto operate the telescopeseveral days withoutfrom the ground, althoureally a series of commsent to the telescope24 hours. These instrucenable it to ca