Optics Kenneth Nordsieck University of Wisconsin Refractive Optics

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

Design Goals • • • wavelength range 320 – 900 nm preserve 850 nm – 1. 7 m future beam all transmission; spherical surfaces beam size 150 mm images 0. 3 arcsec (dispersion direction) maximum use of SALT 8 arcmin FOV Oct 17, 2001 SALT PFIS PDR - Optics 2

Detector/Camera Design • Detector size – camera F > 2. 2 => spatial 60 mm – spectral res elements => dispersion 90 mm – 15 m pixels 0. 13 arcsec • Camera – – speed, wavelength range => Na. Cl FOV 18 deg drives complexity filters near detector for affordable size redesigns have eliminated 4 elements and 1 group Oct 17, 2001 SALT PFIS PDR - Optics 3

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

Beam/Collimator Design • Beam – – Diameter 150 mm set by maximum etalon size Pupil at VPH grating, between etalons Shutter (modified Prontor E 150) 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 Na. Cl). Oct 17, 2001 SALT PFIS PDR - Optics 5

Collimator Layout • 1 Na. Cl triplet • Focus is at final doublet Oct 17, 2001 SALT PFIS PDR - Optics 6

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

Imaging Performance • Imaging specification – EE 50 < 0. 3 arcsec enslitted energy – EE 50 < 0. 4 arcsec encircled energy • Well within spec, allowing for manufacturing and assembly errors Oct 17, 2001 SALT PFIS PDR - Optics 8

Volume Phase Holographic Gratings • Grating parameters – – Groove density n (600 – 3600 l/mm) Hologram thickness D (> 2 m) index modulation Dn (< 0. 1) tilt a (6 – 45 deg): R = 1000 -6500 • Approximate properties (m = 1, “Kogelnik” approximation) – blaze peak l. B = l (b=a) – superblaze peak ~ Dn D – bandwidth Db ~ 1/n. D • Procedure – choose n to select l(R) – maximize Db for a given superblaze peak: – choose maximum Dn = 0. 1 – choose D to place superblaze peak Oct 17, 2001 SALT PFIS PDR - Optics 9

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

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 Dn > 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 Oct 17, 2001 SALT PFIS PDR - Optics On-axis Coverage 11

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 11 m aperture) Oct 17, 2001 SALT PFIS PDR - Optics 12

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% Oct 17, 2001 SALT PFIS PDR - Optics 13

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 Oct 17, 2001 SALT PFIS PDR - Optics 14

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 Oct 17, 2001 SALT PFIS PDR - Optics 15

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 Oct 17, 2001 SALT PFIS PDR - Optics 16

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 Oct 17, 2001 SALT PFIS PDR - Optics 17

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) Oct 17, 2001 SALT PFIS PDR - Optics 18

Procurement Status • Refractive optics – – Lick Obs D. Hilyard Janos Crystran C. Wallace Specac A. Afran $266, 830 14 months Quote in work • Polarimetric optics - beamsplitter – Karl Lambrecht V. Vats $44, 100 30 wks • Waveplates – Karl Lambrecht – Bernhard Halle Oct 17, 2001 V. Vats $120, 000 8 -10 mo 4 x 4 mosaic Will quote on 1/2 wave 2 x 2 mosaic SALT PFIS PDR - Optics 19

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/FORS 1 – if necessary mitigate cost with descope of FOV to 4 arcmin; use largest Halle monolithic waveplate (60 mm). Put cost cap $160 K total on waveplates and trade among FOV and configurations Oct 17, 2001 SALT PFIS PDR - Optics 20