1. Oct 17, 2001SALT PFIS PDR - Optics1 Optics Refractive Optics - Design rationale Performance Slitmasks Gratings Polarimetric Optics Risks and Concerns Kenneth Nordsieck University of Wisconsin

2. Oct 17, 2001SALT PFIS PDR - Optics2 Design Goals wavelength range 320 – 900 nm preserve 850 nm – 1.7  future beam all transmission; spherical surfaces beam size 150 mm images 0.3 arcsec (dispersion direction) maximum use of SALT 8 arcmin FOV

3. Oct 17, 2001SALT PFIS PDR - Optics3 Detector/Camera Design Detector size –camera F > 2.2 => spatial 60 mm –spectral res elements => dispersion 90 mm –15  pixels 0.13 arcsec Camera –speed, wavelength range => NaCl –FOV 18 deg drives complexity –filters near detector for affordable size –redesigns have eliminated 4 elements and 1 group

4. Oct 17, 2001SALT PFIS PDR - Optics4 Camera Layout NaCl is center element in 2 triplets Uses symmetry principle to reduce aberrations

5. Oct 17, 2001SALT PFIS PDR - Optics5 Beam/Collimator Design Beam –Diameter 150 mm set by maximum etalon size –Pupil at VPH grating, between etalons –Shutter (modified Prontor E150) just before etalons –Beam length set by NIR beam fold mirror Collimator –Telescope F/4.2 8' (105 mm). Flat. –Entrance pupil distance 60 cm - need strong field lens –Space for waveplates after field lens –Last element after Visible NIR split: different for each beam –Redesign eliminated 2 elements (including 1 NaCl).

6. Oct 17, 2001SALT PFIS PDR - Optics6 Collimator Layout 1 NaCl triplet Focus is at final doublet

7. Oct 17, 2001SALT PFIS PDR - Optics7 Coatings Baseline: MgF 2 / SolGel –Best broad band performance –Durability concerns Collimator –needs very broad band for NIR –Fallback MgF 2 Camera –Fallback Goodman multilayer Fallback all surfaces: down 20% FPRD minimum: down 10% Fold Mirror LLNL coating

8. Oct 17, 2001SALT PFIS PDR - Optics8 Imaging Performance Imaging specification –EE50 < 0.3 arcsec enslitted energy –EE50 < 0.4 arcsec encircled energy Well within spec, allowing for manufacturing and assembly errors

9. Oct 17, 2001SALT PFIS PDR - Optics9 Volume Phase Holographic Gratings Grating parameters –Groove density  600 – 3600 l/mm) –Hologram thickness D (> 2  ) –index modulation  n (< 0.1) –tilt  (6 – 45 deg): R = 1000-6500 Approximate properties (m = 1, “Kogelnik” approximation) –blaze peak B = (  =  ) –superblaze peak ~  n D –bandwidth  D Procedure –choose to select (R) –maximize  for a given superblaze peak: –choose maximum  n = 0.1 –choose D to place superblaze peak

10. Oct 17, 2001SALT PFIS PDR - Optics10 Grating Complement 780 l/mm VPH: R = 1500, 2x wavelength coverage blue and red (on axis) (non- Kogelnik behavior) 300 l/mm conventional transmission grating: R = 500, 2x wavelength coverage to 3 arcmin off axis 4 high-density VPH’s: R = 2500 - 6500

11. Oct 17, 2001SALT PFIS PDR - Optics11 VPH Field of View FOV perpendicular to dispersion: full 8' Useful field of view parallel to dispersion is limited for VPH by blaze shift For PFIS gratings: efficiency > 50% over 4' parallel to dispersion; usable to 6'. Possible improvement: if  n > 0.1 proves feasible, can reduce D and increase bandwidth (30%?) For low dispersion programs needing largest possible FOV, may be advantageous to provide conventional 600 l/mm transmission grating On-axis Coverage

12. Oct 17, 2001SALT PFIS PDR - Optics12 Overall Optics Efficiency For VPH, R = 4000 Spectrograph/ Detector efficiency near 60% 400 - 800 nm FPRD Minimum ~ 40%, including CCD, coatings, VPH conservatism (about 10% each) On Sky efficiency >20% (relative to full unobscured centered 11m aperture)

13. Oct 17, 2001SALT PFIS PDR - Optics13 Polarimetric Optics SALT Telescope instrumental polarization –Primary: Al –SAC: LLNL < 0.1% 4 ' dia FOV ~0.2% at 8' dia Field effect > track effect FPRD: correctable to < 0.04%

14. Oct 17, 2001SALT PFIS PDR - Optics14 Polarimetry - Beamsplitter Wollaston Beamsplitter in collimated beam after grating Split +/- 45 deg polarizations ~ 5 deg => 4 arcmin at detector into two half-fields “O” and “E” Mosaic of 9 calcite prisms in framework Chromatic dependence of splitting: 20 arcsec from 320 – 900 nm

15. Oct 17, 2001SALT PFIS PDR - Optics15 Polarimetry - Waveplates Pancharatnam superachromatic waveplates: stack of 6 very thin retarders In collimator after field lens (to minimize diameter) ½ and ¼ waves from 320 – 1.7 microns very large SALT etendue (aperture x FOV) limits performance of waveplates in UV – reduced efficiency; sensitivity to pupil

16. Oct 17, 2001SALT PFIS PDR - Optics16 Waveplate efficiency Pancharatnam modified for off-axis performance Overall polarimetric efficiency reduced, but still > 98% (halfwave), 94% (quarterwave) Pupil shape sensitivity not significant for halfwave Quarterwave more sensitive to pupil effects, due to manufacturing limits on element thickness

17. Oct 17, 2001SALT PFIS PDR - Optics17 Slitmasks Adopt Gemini GMOS slitmask technique –observation unique carbon-fiber slitmasks, cut by dedicated commercial laser milling machine off-site –inventory of ~60 invar frame mask holders –estimate ~$1.50/ carbon fiber mask, < 1 hour to mill Magazine –removable to be loaded off-telescope –30 carbon-fiber slitmasks; 10 aluminized longslits –bar-code identifier

18. Oct 17, 2001SALT PFIS PDR - Optics18 Slitviewing Optics Estimate ~50% of programs are longslit: tilted aluminized slitjaws would provide direct view of slit for acquisition/ guiding Longslits handled/ stored by same mechanism as carbon-fiber slitmasks (double-thick) Proposed slitviewing optics using all spherical mirrors (Offner relay), feeding existing acquisition camera Not yet approved by project (difficult integration with prime Focus Platform)

19. Oct 17, 2001SALT PFIS PDR - Optics19 Procurement Status Refractive optics –Lick ObsD. Hilyard$266,83014 months –Janos –CrystranC. WallaceQuote in work –SpecacA. AfranQuote in work Polarimetric optics - beamsplitter –Karl LambrechtV. Vats$44,10030 wks Waveplates –Karl LambrechtV. Vats$120,0008-10 mo4x4 mosaic –Bernhard HalleWill quote on 1/2 wave2x2 mosaic

20. Oct 17, 2001SALT PFIS PDR - Optics20 Risk Status Refractive optics - Low to medium –complexity reduced –number of groups comparable with similar optics –quotations being obtained from 4 vendors –cost and schedule comparable to Concept Proposal Polarimetric optics - medium –quotations being obtained from 2 vendors –beamsplitter cost similar to Concept Proposal –waveplates. Lambrecht likely to drop out. Halle experience with similar waveplates on VLT/FORS1 –if necessary mitigate cost with descope of FOV to 4 arcmin; use largest Halle monolithic waveplate (60 mm). Put cost cap $160K total on waveplates and trade among FOV and configurations