Transcript Document

Optics
Kenneth Nordsieck
University of Wisconsin
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Refractive Optics - Design rationale
Performance
Slitmasks
Gratings
Polarimetric Optics
Risks and Concerns
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Design Goals
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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
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Detector/Camera Design
• Detector size
– camera F > 2.2 => spatial 60 mm
– spectral res elements => dispersion 90 mm
– 15m pixels 0.13 arcsec
• Camera
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speed, wavelength range => NaCl
FOV 18 deg drives complexity
filters near detector for affordable size
redesigns have eliminated 4 elements and 1 group
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Camera Layout
• NaCl is center element in 2 triplets
• Uses symmetry principle to reduce aberrations
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Beam/Collimator Design
• Beam
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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
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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).
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Collimator Layout
• 1 NaCl triplet
• Focus is at final doublet
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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
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Imaging Performance
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Imaging specification
– EE50 < 0.3 arcsec
enslitted energy
– EE50 < 0.4 arcsec
encircled energy
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Well within spec,
allowing for
manufacturing and
assembly errors
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Volume Phase Holographic
Gratings
• Grating parameters
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Groove density n (600 – 3600 l/mm)
Hologram thickness D (> 2m)
index modulation Dn (< 0.1)
tilt a (6 – 45 deg): R = 1000-6500
• Approximate properties (m = 1,
“Kogelnik” approximation)
– blaze peak lB = l (b=a)
– 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
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Grating Complement
• 780 l/mm VPH: R =
1500, 2x wavelength
coverage blue and red
(on axis) (nonKogelnik 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
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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
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On-axis
Coverage
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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)
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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%
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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
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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
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Waveplate efficiency
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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
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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
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Slitviewing Optics
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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)
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Procurement Status
• Refractive optics
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Lick Obs D. Hilyard
Janos
Crystran C. Wallace
Specac A. Afran
$266,830
14 months
Quote in work
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 4x4 mosaic
Will quote on 1/2 wave 2x2 mosaic
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Risk Status
• Refractive optics - Low to medium
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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
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