Dark Energy Survey: Optical Design and Issues

1
Dark Energy SurveyOptical Design and Issues

• 2.2 Deg. Field of View Corrector
• Requirements
• Performance
• Issues

Steve Kent, Fermilab, for the DES
Collaboration Dark Energy Survey BIRP, Aug 12,
2004
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2.2 Deg. Field of View Corrector

• 14 requirements total
• 0.39 to 1.1 µ (SDSS filter bandpasses)
• Scale 17.7 arcsec/mm
• Field size 450 mm diameter (2.2 degrees)
• D80
• No ADC (Atmospheric Dispersion Corrector)
• Minimize ghosting
• Space for filter, shutter
• Design choices should minimize procurement,
  fabrication schedule.

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Gladders may11 design

• Features
• Flat focal plane
• Five lenses Filter
• (including dewar window)
• All fused silica
• One aspheric surface
• Largest diameter 1.1 meters
• Flexibility spacing elements
• Low distortion (
• Good ghosting properties
• star halos
• exit pupil image

C5
Shutter
C4
Filter
Filter
C3
C2
C1
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Obligatory Spot Diagrams
g filter
r filter
i filter
z filter
Field Radius
2"
0 mm
158 mm
225 mm
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Image Quality
D80 vs. filter, field position Field Radius
g r i z (mm) 0
0.32" 0.11 0.17 0.31 45 0.35
0.19 0.21 0.34 90 0.39
0.14 0.22 0.33 135 0.40
0.21 0.25 0.33 180 0.50 0.32
0.36 0.42 226 0.59 0.37
0.41 0.47

• Requirement
• Achieved
• Global rms 0.34 (FWHM .22")

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Impact of no ADC
0.4 arcsec FWHM
g
r
i
z
LMC, SMC limits
Survey limit
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Ghosting
Uniform Illumination Central intensity of ghost
is 7 of direct recorded light.
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Design Issues

• Fused Silica
• Expensive
• Excellent transmission
• Low CTE (factor 10x less than BK7) shorter
  polishing, testing times
• Largest lens requires blank to be slumped 5
  month procurement time.
• dn/dT is 3x BK7

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More Issues

• Lateral, longitudinal chromatic optimized only
  within SDSS bandpasses.
• Coatings
• Options are being evaluated
• Assume MgF2 for inside dewar, on front surface of
  C1
• Assume solGel for all other surfaces

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Tolerance Analysis

• Preliminary analysis only
• Fabrication errors in radii, thickness,
  refractive index are compensated by repositioning
  of lenses
• Static errors
• inhomogeneity, thickness, radii, centering
  positioning, wedge tilt
• polishing errors
• primary mirror alignment
• Combined 0.33" D80

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Thermal Analysis

• Assume steel cell.
• Include dn/dT of FS, CTE of FS
• 30 deg C range
• D80 changes by .1"

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Summary

• We have a corrector design that will work and can
  be fabricated by existing vendors.
• Next steps
• Firm up requirements
• Pixel scale PSF uniformity
• Distortion Ghosting amplitude
• Hire optical engineer to
• review design, identify areas where cost and/or
  schedule savings can be achieved.
• Improve analysis of tolerancing and image size