1

University of Wisconsin/ MadisonSALT Prime Focus Imaging Spectrograph

Thursday, May 11, 2000

K. Nordsieck

SALT and the UW

Instrument roles: HET vs SALT

PFIS NichesUV spectroscopyHigh throughput medium resolution spectroscopyFabry-Perot spectroscopySpectropolarimetry

PFIS TechnologiesVPH gratingsSol-Gel Coatings

Optical design

Performance

Comparison: unique; superior; competitive

Programmatics: weight; cost; program

Needed input for trade study

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SALT and the UW

Southern African Large TelescopeLocation: Sutherland Observatory (32.4° S, 1800m)Telescope design: Hobby-Eberly Telescope (HET)

spherical mosaic 11m primaryconstant zenith distance (HET: 35°; SALT: 37°)target track: choose azimuth, move focal plane ±6°target availability: 2 ~1 hr traverses of 12° annulusconstruction cost ~20% of conventional 10mspecialize in spectroscopy

SALT enhancementsSpherical Aberration Corrector (SAC)

Field of View: 4 -> 8 arcminmaximum pupil: 9.2 -> 10 or 11mSAC coatings: blue limit 3800 -> 3000 D

Primary mosaic edge sensors. Imaging 0.9 arcsec (medianseeing), 0.7 arcsec (best)

SALT Cost. $24.7M constr, $4.8M instruments, $5.5M 10 yroperations = $35M

Commissioning: end of 2004

Proposed UW involvement1/6 cost/ telescope time: $6M~$2M would be "in kind" contribution of instrumentationpartner with Rutgers (Fabry-Perot), SAAO (CCD)

Figure 1: Motion of the pupil on the HET Mirror with Tracker Position.

90 ft. Mirror Alignment Tower

Control & Service Building (2,100 s.f.)

SPECTROSCOPIC SURVEY TELESCOPE

86 ft. Dia. Dome

Dome Vents

Main Instrument Room

Tracker Beam

Primary Mirror

Visitors Gallery

Telescope Structure

Telescope Pier

Dome Opening

Figure 2. The HET and its major components

3

Instrument Roles

• How do you multiplex both spectral and spatial information(3D datacube) onto a 2D detector with limited pixels?

Suggest:

HET LRS -> SALT Prime Focus Imaging Spectrograph. Thismaximizes spatial coverage at expense of simultaneouswavelength coverage. • Very high throughput low - medium spectral resolution

without cross-dispersion. • Exploit SALT UV sensitivity. • Exploit larger SALT field with multislits.

MRS (fiber). This maximizes wavelength coverage at low -medium spectral resolution. Allows multiple fibers untildetector is filled. PI institution for SALT not yet identified.

HRS (fiber) This gives up spatial coverage (one object only, ormultiple objects, single order) to use the whole detector forhigh spectral resolution. NZ may do an HRS for SALT.

PFIS Constraints• Size ~ 1 × 1.5 m• Weight ~ 200 kg• Access (once/ week?)

HET/ SALT Instruments

100

1000

10000

100000

Wavelength (Ang)

Re

solu

tion

0.5'' slit

> 15%

> 25%

> 50%

100%

3000 5000 10000 16000

HET HRSHET HRS

MRS

LRS

SALT PFISSALT PFIS possible IR beampossible IR beam

possible VPHechelettepossible VPHechelette

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PFIS Niches

• UV Spectroscopy (3100 - 4000D). Rare on large telescopesRest wavelength features

Ca H,K; OII 83727; Balmer JumpBowen Lines: OIII 83133, 3343, 3444Novae: [NeV] 83340, 3425Comets: OH 83090

Redshifted VUV featuresMgII 82800 z = 0.1 - 0.482200 Bump z = 0.4 - 0.8Ly" z = 1.6 - 2.3

• High throughput, medium resolution spectroscopyLarge telescope gain largest when not background limited"Low resolution" R < 2000: V. > 150 km/s. QSO's, S.N.'s"Medium" R < 10000: V. > 30 km/s. galaxy kinematics;

stellar envelopes

• Fabry-Perot imaging spectroscopyR = 2500 - 250,000 On 10m telescope at R = 2500, get monochromatic imaging

over 3.5 arcmin field (scan to get velocity-space datacube)

• Spectropolarimetry. Rare on large telescopesUV: most important stellar feature: Balmer JumpMedium resolution: polarimetric line profilesMulti-slit: surveys, appropriate for SALT queue scheduling

• Upgrade Path: echellette; simultaneous near IR beam

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Enabling Technologies

Volume Phase Holographic ("VPH") GratingsHigh Efficiency 1st order transmission grating - enables very

compact all transmissive system with simpler cameraStill efficient at high diffraction angle => higher resolution in

1st orderRotating grating and camera "tunes" blaze peak over 2x in 8

=> highest possible efficiency with few gratingsUsed on AAT; articulated version planned for AAT, SOARPossible future echellete configuration

Sol-Gel antireflection coatingsExisting AR coatings limited to 83200 - 87000. Would like

to extend visible beam to > 8500D to cover CCD rangeSol-Gel: chemical coating producing stack of 200D pure silica

spheres, effective index of refraction 1.22.Use over single layer of MgF2 (n - 1.38), can get reflection <

1% 3200 - 8500, degrading < 2% a 1.7:Now durable enough for astronomy; to be used on Gemini

Fabry-PerotEtalons now available to 15 cm diameterRutgers experience with Queensgate etalons on telescopes

{ 17 {

{ 30 {

Fig. 1.| F our p ossible fringe structures for VPH gratings: A) Littro w transmission grating.

B) Non{Littro w transmission grating. C) Non{dispersiv e reection grating (notc h lter). D)

Dispersiv e reection grating.

MEASURED EFFICIENCYHG-T-532-19

n= 1200 l/mm

WAVELENGTH (nm)

300 500 700 900 1100

EF

FIC

IEN

CY

(h)

0.0

0.2

0.4

0.6

0.8

1.0

a = 19°

m = 1

a = 28°

m = 1

a = 28°

m = 2 Super Blazem = 2

Super Blazem = 1

VPH Gratings. From Barden, et al, 2000, PASP (June in press)

Broadband Antireflection Coatings

0.95

0.96

0.97

0.98

0.99

1.00

3000 10000 17000

Sing

leSu

rfa

ce

Tra

nsm

issi

on

MgF2/ Sol-Gel

Multilayer

MgF2

Uncoated

SALT/ PFIS

Sol-Gel / MgF2 Antireflection Coatings

0.80

0.85

0.90

0.95

1.00

3000 10000 17000

Wavelength (Ang)

Tra

nsm

issi

on

1 surface CaF2

1 surface Silica

1 surface Quartz

Total, Vis

Collimator, IR

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Optical Design

• One beam in visible 83200 - $8500. Maintain simultaneousIR beam (8500 - 1.7:) upgrade

• All- Transmission for high efficiency and compactness

• UV Crystals and fused silica only for 3200D throughput

• Images < 0.25 arcsec to allow for reduced 0.5 arcsec slit,resolutions to R = 13000

• Allow lateral color (not to be used for broadband imaging)

Detector: at least 60 mm square for reasonable camera speed. Baseline mosaic of three Marconi/ EEV: 4096 × 6144 30:pixels (0.13 arcsec pixels)Available UV/broadband, or "deep depletion" for near IRLeach Controller with 6 outputs, standard binning 2×2Standard readout: 40 kpix/s: 3.2e-, 26 secs.Rapid readout: 1 Mpix/s: 6e-, 1 sec

Camera:Challenge: very large FOV to give adequate spectral coverage6 groups, with crystal quartz field flattener

Pupil: 350 mm long to accommodate double FP etalon or VPH, plus

polarizing beam-splitterVPH must be rotated after insertion

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Polarimetry"wide-field" design compatible with spatial coverage

maximization (used at UW). removable elements:modulator: large (50 -75 mm?) rotating achromatic

waveplate after focus. Field 5-6 arcminanalyzer: polarizing beam-splitter near pupil (mosaic of 4 75

mm Calcite Wollaston prisms). Splits 4×6 arcmin fields 4arcmin perpendicular to dispersion.

CollimatorChallenge: high speed (4.5) and large FOV (8 arcmin)5 groups, with fold (or IR dichroic) before last3 filter wheels (18 filters) + waveplate slide and shutter

Slit mechanismsBaseline: HET LRS-like ~30 slitletsDesirable: variable slit widthConsider: simple slitmask "jukebox" (~100-300 slits)

Risks:22 lens elements! Alignment; testing. (But: all spherical)NaCl elements. (But: Sealed as center element in triplet; used

in SOAR)Camera Articulation. (But: Many such devices in planning)

CostMitigate, if necessary, by reduction of FOV, detector size, red

spectral range

Filter Wheels

Shutter

WaveplateSlide

Fold

Imaging (FP)Mode

Spectroscopic (VPH)Mode (max angle)

Envelope

CCDHousing

BeamsplitterSlide

PFISSide View

DispersorMechanisms

silica

silic

a

silic

a

silic

a

silic

a

silicasilica

silic

a

silic

a

silica

silic

a

CaF2

CaF

2

CaF

2

CaF

2

CaF2

CaF

2

CaF

2

CaF2

CaF

2

NaCl

NaC

l

NaCl

NaC

l

FK

5

Quartz

Articulation

Focal Plane

PFISDispersorMechanisms

Etalon 1 Etalon 2

VPHMagazine

VPHSelector/Rotator

Bottom View;Camera shown inImaging Mode

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Performance/ Capability

Images50% enclosed energy < 30: (2 pix; 0.25 arcsec)

VPHpeak grating efficiency: 90%spectrograph/ detector efficiency: 68% peak; 34% at 3200D R = 800 - 6500 (0.9 arcsec slit) with 4 gratings.R -> 12,000 with 2×reduced throughput (0.45 arcsec slit).

Still critically sampled using unbinned readoutIn 3000 sec S/N = 100 (R=500), 10 (R=12000) at V=22

Fabry-Perotefficiency ~80% single etalon; 50% dual etalonR = 2500:

Absorption lines: few km/s kinematics to R=18 over 2.9'H" emission: 0.1 Rayleigh/ pixel; 0.01 Rayleigh averaged

over field R = 10000, FOV 1.5'; R = 100,000, FOV 30 arcsec

PolarimetryElement transmission: 85% Modes

0.1% spectropolarimetry: V=17 (R=500); 13 (R=12000)0.3% V=19.5 15.5long-slit spectropolarimetry on diffuse objects0.1% slitless imaging spectropolarimetry (R=20) to V=19

using prism chromaticismFabry-Perot imaging polarimetry (atomic scattering)

SALT PFIS VPH Grating Resolution

11m pupil, no prisms

0

1000

2000

3000

4000

5000

6000

7000

0.3 0.4 0.5 0.6 0.7 0.8

0.9Wavelength (microns)

R

780 l/mm

1600

2220

3220

0.9Slit:

0

2000

4000

6000

8000

10000

12000

14000

0.3 0.4 0.5 0.6 0.7 0.8

Wavelength (microns)

R

0.45Slit:

10

18

26

33

39

43

Artic

ula

tion

Angle

(deg)

coverage

coverage

PFIS Spot: Wavelength vs Field

0.9

arc

sec

3400 4500 5800 8500

Imaging

0

0

2'

2'

4'

4'

'

3'

3'

6100 6400 6600 6900

R = 6250

PFIS Spot Sizes50%

Enclo

sed

Energ

yD

iam

ete

r(m

icro

ns)

20

20

40

40

30 microns = 2 pixels = 0.25 arcsec

30 microns = 2 pixels = 0.25 arcsec

Imaging

5800

8500

4500

3400

6100

6400

6600

6900

R = 6250

Field Angle (arcmin)

0 42

SALT PFIS System Efficiency

0.0

0.2

0.4

0.6

0.8

1.0

3000 4000 5000 6000 7000 8000 9000

Wavelength (Ang)

Effic

ienc

y

atmosphere

telescope

spectrograph/detectortotal on sky

HET/ LRS

1

10

100

1000

10000

14 16 18 20 22 24 26 28

V Magnitude

S/N

20 SSP

500

1000

2500

6250

12500

Signal/ Noise at 3000 sec

1

10

100

1000

10000

14 16 18 20 22 24 26 28

U Magnitude

S/N

54 SSP

500

1000

2500

6250

12500

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Comparison with other Large Telescope Slit Spectrographs

(8-10m telescopes; instruments operational or underconstruction)

UV spectroscopy Superior in SouthResolution SuperiorField of View Superior in SouthVisible spectroscopy CompetitiveFabry-Perot spectroscopy UniqueSpectroPolarimetry

UV UniqueResolution Much SuperiorField of View Superior

Slit Spectrographs on larger telescopes5/2/2000

khn inputs

Facility Telescope AAT AAT SOAR MMT MMT VLT VLT Gemini Gemini HET Keck SALTHemisphere S S S N N S S N S N N SSpectrograph LDSS++ ATLAS Goodman Foltz SPOL FORS1 FORS2 GMOS GMOS LRS LRIS-B PFISFocus Cass Cass Cass Folded Cass PF/SAC Cass PF/SACStatus Oper Prop Prop Oper Oper Oper Constr Constr Constr Oper Constr

Telescope Telescope Aperture (m) 3.9 3.9 4.2 6.5 6.5 8 8 8 8 9.2 10 11Focal ratio 8 8 16 9 9 13.4 13.4 16 16 4.5 15 4.5telescope scale (arcsec/mm) 6.6 6.6 3.1 3.5 3.5 1.9 1.9 1.6 1.6 5.0 1.4 4.2FOV (arcmin) 12 24.0 5.0 3.0 0.3 6.8 6.8 5.5 5.5 4.0 8.0 8.0FOV (mm) 109 218 98 51 5 212 212 205 205 48 349 115Slit (arcsec) 1.5 1.5 0.5 1 1 1 1 0.5 0.5 1 0.8 0.9Slit (microns) 227 227 163 284 284 520 520 520 520 201 582 216

Optics beam (mm) 70 150 75 150 90 90 100 100 150 148 150camera focal length (mm) 140 330 375 249 279 279 480 480 210 296 330camera speed 2 2.2 5 1.66 3.1 3.1 4.8 4.8 1.4 2 2.2camera FOV (deg) 25.6 14.6 10.4 10.7 14.0 14.0 13.0 13.0 13.0 14.6 18.1spectrograph scale (arcsec/mm 26.4 24.0 9.8 19.1 8.3 8.3 5.4 5.4 16.0 10.3 8.5coatings SG/MgF2 SG/MgF2 SG/MgF2 SG/MgF2collimator groups 4 4 3 parabola 1 3 3 2 parabola 5camera groups 6 5 4 schmidt Nikon 4 4 maksutov 5 6

Detector pixel (microns) 15 15 15 15 24 24 15 15 15 15 15arcsec/ pix 0.40 0.36 0.15 0.29 0.20 0.20 0.08 0.08 0.24 0.15 0.13spatial pixels 2048 4096 2048 1024 800 2048 2048 4608 4608 1024 4096 4096spatial coverage 13.5 24.6 5.0 4.9 6.8 6.8 6.2 6.2 4.1 10.6 8.7spectral pixels 4096 4096 4096 3072 1200 2048 2048 6144 6144 3072 4096 6144median res elements 1083 985 1207 881 409 409 990 990 738 792 873median sampling 3.8 4.2 3.4 3.5 5.0 5.0 6.2 6.2 4.2 5.2 7.0

Performance Low Wavelength 5000 3600 3200 3200 3800 3300 3300 3600 3600 3600 3100 3200High Wavelength 10000 10000 8500 8000 9000 11000 11000 11000 11000 10000 6000 8500Low Resolution 500 1000 600 770 500 200 200 670 670 600 500 1000High Resolution (1st order) 500 10000 10000 3500 1000 1800 2700 4400 4400 1500 2100 6500Peak Efficiency 0.50 0.70 0.65 0.24 0.36 0.36 0.41 0.43 0.59 0.44 .6? 0.683600 Efficiency 0.00 0.00 0.50 0.13 0.00 0.13 0.12 0.06 0.15 0.10 0.50 0.50Grating (VPH) VPH VPH Reflection Reflection grism/(VPH) Reflection Reflection grism/(VPHgrism VPHMultislits slitmask slitmask 19 slitmask 13 slitmask 30?Fabry-Perot Yes YesPol No Maybe No No Yes Yes No No No No No? Yes

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Programmatics

WeightLight-weighted structure based on hollow Invar tube truss

(from sounding rocket program)200 kg seems tight; 250 more likely

Cost/ Program

PFIS ROM BudgetDevelopment through Delivery

person-year

Mar 1998$US

Staff Staff Staff StaffPhase A Phase B Phase C-1 Phase C-2 Staff Cost Cap Cost Total Cost

SAAO CCD 0.5 1 2 1 225 300 525

Rutgers Dispersor mech 0.5 0.5 1 1.5 350 400 750

UW Program Manager 0.5 0.5 0.5 0.5InstrumentScientist

1 1 1 1

Systems 0.5 0.5 0.5 0.5Mech 1 1 1 1Elec 0.25 0.25 0.25 0.25S/W 0.25 0.25 0.25 0.25UW Tot 3.5 3.5 3.5 3.5 1400 500 1900

Total 3175

SchedulePhase A: 6/00 - 6/01 Design Trade Study. PDR Phase B: 6/01 - 6/02 Detailed Design. CDRPhase C: 6/02 - 6/04 Construction. Delivery ReviewPhase D: 6/04 - Commissioning and Operation

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Trade Study - Inputs Needed

Goal: trade off cost and technical risk vs instrumentspecifications, eg

Field of View (now 8 arcmin)

Imaging specs (now 0.25 arcsec)

Detector size (now 900 spectral resolution elements)

Near IR capability (now to 8500D)

Slit geometry (30 slitlets? slit masks?)

First light etalons (now R=2500 and 10000)

Inputs:Questionnaire to consortium asking for priorities

Request for supporting scientific program writeups