FLAMINGOS2 Optics

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FLAMINGOS-2 Optics

• Stephen Eikenberry
• University of Florida

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F2 Optics Outline

• Requirements and Introduction
• Nominal Imaging Performance
• Tolerancing and As-Built Performance
• Spectroscopic Performance
• f/32 MCAO-mode Performance
• Summary

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F2 Optics Requirements
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F2 Optics Brief History

• Initial design at CoDR by R. Elston and H. Epps
• Further optimization by Epps, including long
  collimator (removes large fishbowl field lens)
• Final optimization, test-plating, cooling to
  operating temperature by Telic Optics
• Tolerancing and other analyses by Telic Optics
  and S. Eikenberry

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F2 Optics Basic Summary

• Standard collimator/camera design
• 9-lens all-spherical system
• CaF2 and S-FTM16 (OHara) materials (well-known
  properties, moderate-index)
• 1st collimator lens is large (dictated by FOV
  small compared to planned KIRMOS lens)
• All surface radii achievable with standard
  fabrication techniques

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F2 Optical Layout - I
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F2 Optical Layout - II
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F2 Optical Prescription -I
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F2 Optical Prescription -II
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F2 OpticsNominalImaging Performance
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F2 Spot Diagrams
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F2 Encircled Energy - J
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F2 Encircled Energy - H
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F2 Encircled Energy - K
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F2 Distortion

• Maximum distortion lt0.17 at all field angles and
  wavelengths
• Figure shows magnified distortion map over entire
  array 1 grid block equals 100 pixel spacing and
  1 pixel distortion

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F2 Ghosts Assumptions

• All lens surfaces are 1 reflective
• Detector subtrate is 10 reflective detector
  surface is 30 reflective
• Filter is 11 at S1 and 1 at S2
• These are realistic numbers for lens/filter based
  on past experience detector/substrate numbers
  are deliberately over-estimated (making the
  resulting ghosts worst-case estimates)

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F2 Ghosts Results
(a) this ghost has no displacement from the
actual image on the detector, and thus no actual
ghosting effect
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F2 Ghosts Summary

• Largest ghost is in the filter and has zero
  displacement from the actual image on the
  detector ? no operational impact
• Largest noticeable ghost is within the sapphire
  substrate of the HAWAII-2 array and is
  unavoidable
• All other ghosts have a combined intensity per
  pixel smaller than the Sapphire Ghost
• Thus, F2 is as ghost-free as reasonably possible

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F2 Stray Light Assumptions

• Launch rays filling a hemisphere at field stop
  (telescope focal plane)
• lt50 reflectance of sandblasted and anodized lens
  barrels
• ABg model of lens surface roughness
• 50 diffuse reflectance of lens edges
• Trace rays through system to determine
  pseudo-irradiance at detector

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F2 Stray Light Results

• TBD

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F2 OpticsTolerancedImaging Performance
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F2 Tolerancing - Scope

• First need to get approximate scope of the
  error budget
• Nominal design has RMS WFE ?n 0.122 waves (at
  1.7 ?m) and has D50 16 ?m
• For ? 1, D50 ? ?
• Need D50 lt25 ?m ? we can afford a final value
  of ?f (25/16) 0.122 0.191 waves
• This gives us an expected error margin of
  ?msqrt(?f2-?n2) 0.147 waves

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F2 Tolerancing - Budget
Given the expected margin above, we now
allocate the WFE to various parameters. Overview
is below, with details on following slides.
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F2 Tols Surface Radii
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F2 Tols Element Thickness
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F2 Tols Airspace
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F2 Tols Element Tilt
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F2 Tols Element Decenter
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F2 Tols Element Wedge
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F2 Tols Group Tilt/Decenter
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F2 Tols Surface Irreg.
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F2 As-Built Estimation

• Simulate actual achieved performance using
  Monte Carlo simulation in ZEMAX
• Perform 100 realizations using above error
  budgets and Gaussian statistics
• Take 90-ile RMS WFE case to be as-built
• Thus, we have a 90 confidence level we can beat
  this performance

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F2 As-Built Performance - J
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F2 As-Built Performance - H
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F2 As-Built Performance - K
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F2 Pupil Spots
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F2 Pupil Summary

• All spots lt5 mm geometric diameter
• All have FWHM 2.0 mm, meeting goal and better
  than requirement
• Pupil shift dominated by misalignment of L1-L3
  group wrt telescope (0.7 mm RMS given tolerances
  above)
• We will use this group to align F2 with the
  telescope ? expect lt0.1 mm pupil wander from
  optical tolerances

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F2 Tolerance Summary

• We have created a tolerancing error budget with
  reasonable requirements that is expected to meet
  F2 specifications
• Monte Carlo simulations using these tolerances
  show that with a 90 confidence level we can
  meet/beat all image quality requirements over the
  entire F2 FOV at all wavelengths

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F2 OpticsSpectroscopic Performance
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F2 Spectroscopy

• 3 grisms required
• zJH (0.9-1.8 ?m) _at_ R1300
• JHK (1.25-2.5 ?m) _at_ R1300
• J, H, or K (different orders, single waveband at
  a time) _at_ R 3000
• All CaF2 prism substrates
• R3000 standard Richardson Lab grating
• R1300 custom gratings, but very close to
  standard Richardson grating properties

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F2 JH Grism
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F2 HK Grism
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F2 Spectral FOV

• On-axis, F2 will provide spectra across
  specified bandpasses
• Moving far off-axis for MOS, long/short ends of
  the bandpass will begin to vignette and/or be
  lost off the edge of the array
• Requirement is gt90 relative throughput over
  entire bandpass for 2x2-arcmin MOS FOV (of
  6x2-arcmin available for MOS)

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F2 Spectral FOV - Results

• For individual J, H, or K bandpasses, have lt10
  vignetting over entire 6x2-arcmin FOV ? exceeds
  required performance
• For JH bandpass, lt10 vignetting over ellipse
  with 2.9-arcmin major axis, and over
  2.6x2-arcmin inscribed rectangle ? exceeds
  required performance
• For HK bandpass, lt10 vignetting over ellipse
  with 2.3-arcmin major axis, and over
  2.0x2.0-arcmin inscribed rectangle ? exceeds
  required performance

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F2 Opticsf/32 MCAO-modePerformance
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F2 MCAO-mode

• F2 desired to be compatible with Gemini-S MCAO
  for AO-MOS
• MCAO beam feeds f/32 beam with flat focal surface
  into F2
• Resulting pixel scale 0.09 arcsec/pix
• Use 52-mm Lyot stop in Lyot wheel
• Detector re-focus capability, if needed

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F2 MCAO-mode J
Near-diffraction-limit on-axis, with lt25 FWHM
degradation at field edge
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F2 MCAO-mode H
lt10 FWHM degradation over entire MCAO FOV
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F2 MCAO-mode K
Negligible PSF degradation over the MCAO FOV
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F2 MCAO-mode HK Grism
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F2 OpticsSummary Items
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F2 Filters

• 120-mm diameter required (BIG cryogenic filters!)
• J, H, and K filters currently in-hand
• JH and HK block filters (for spectroscopy mode)
  quote in-hand from Barr Associates with
  reasonable cost and delivery schedule

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F2 Throughput - Assumptions

• Lenses have 2 loss (optimistic) to 3 loss
  (conservative)
• Window has 3 loss
• Filter has 12 loss
• Lyot stop gives 2 loss
• Grism has 20 loss
• All numbers above consistent with experience on
  FLAMINGOS

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F2 Throughput - Results

• Imaging Mode
• Lens losses 18-27 (conservative to optimistic)
• Total throughput of F2 is 55-64
• Requirement is 50 ? good
• Spectroscopic Mode
• Grism losses 20 additional (per F1)
• Total throughput of F2 is 35-44
• Requirement is 30 ? good

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F2 Optics Risks
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F2 Optics Summary

• F2 has a robust, thoroughly-considered optical
  design
• All-spherical lenses with normal materials,
  surface radii, and tolerances, allowing
  straightforward fabrication
• Expected performance is within all specifications
  (image quality, throughput, spectral resolution,
  etc.)
• Acceptable quotes (both price and schedule)
  in-hand from 2 vendors