Optics

1
Optics
Kenneth Nordsieck University of Wisconsin

• Refractive Optics - Design rationale
• Performance
• Slitmasks
• Gratings
• Polarimetric Optics
• Risks and Concerns

2
Design Goals

• wavelength range 320 900 nm
• preserve 850 nm 1.7m future beam
• all transmission spherical surfaces
• beam size 150 mm
• images 0.3 arcsec (dispersion direction)
• maximum use of SALT 8 arcmin FOV

3
Detector/Camera Design

• Detector size
• camera F gt 2.2 gt spatial 60 mm
• spectral res elements gt dispersion 90 mm
• 15m pixels 0.13 arcsec
• Camera
• speed, wavelength range gt NaCl
• FOV 18 deg drives complexity
• filters near detector for affordable size
• redesigns have eliminated 4 elements and 1 group

4
Camera Layout

• NaCl is center element in 2 triplets
• Uses symmetry principle to reduce aberrations

5
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
Collimator Layout

• 1 NaCl triplet
• Focus is at final doublet

7
Coatings

• Baseline MgF2/ SolGel
• Best broad band performance
• Durability concerns
• Collimator
• needs very broad band for NIR
• Fallback MgF2
• Camera
• Fallback Goodman multilayer
• Fallback all surfaces down 20
• FPRD minimum down 10
• Fold Mirror LLNL coating

8
Imaging Performance

• Imaging specification
• EE50 lt 0.3 arcsec enslitted energy
• EE50 lt 0.4 arcsec encircled energy

• Well within spec, allowing for manufacturing and
  assembly errors

9
Volume Phase Holographic Gratings

• Grating parameters
• Groove density n (600 3600 l/mm)
• Hologram thickness D (gt 2m)
• index modulation Dn (lt 0.1)
• tilt a (6 45 deg) R 1000-6500
• Approximate properties (m 1, Kogelnik
  approximation)
• blaze peak lB l (ba)
• superblaze peak Dn D
• bandwidth Db 1/nD
• 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

10
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 VPHs R 2500 - 6500

11
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 gt 50 over 4'
  parallel to dispersion usable to 6'.
• Possible improvement if Dn gt 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

12
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 gt20 (relative to full
  unobscured centered 11m aperture)

13
Polarimetric Optics

• SALT Telescope instrumental polarization
• Primary Al
• SAC LLNL
• lt 0.1 4 ' dia FOV
• 0.2 at 8' dia
• Field effect gt track effect
• FPRD correctable to lt 0.04

14
Polarimetry - Beamsplitter

• Wollaston Beamsplitter in collimated beam after
  grating
• Split /- 45 deg polarizations 5 deg gt 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
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
Waveplate efficiency

• Pancharatnam modified for off-axis performance
• Overall polarimetric efficiency reduced, but
  still gt 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
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, lt 1 hour to
  mill
• Magazine
• removable to be loaded off-telescope
• 30 carbon-fiber slitmasks 10 aluminized
  longslits
• bar-code identifier

18
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
Procurement Status

• Refractive optics
• Lick Obs D. Hilyard 266,830 14 months
• Janos
• Crystran C. Wallace Quote in work
• Specac A. Afran Quote in work
• Polarimetric optics - beamsplitter
• Karl Lambrecht V. Vats 44,100 30 wks
• Waveplates
• Karl Lambrecht V. Vats 120,000 8-10 mo 4x4
  mosaic
• Bernhard Halle Will quote on 1/2 wave 2x2 mosaic

20
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