primary mirrors for exoplanet imaging developments at steward observatory mirror lab
DESCRIPTION
Primary mirrors for exoplanet imaging Developments at Steward Observatory Mirror Lab. MMT with deformable secondary Off-axis figuring active thermal figure control for large, lightweight honeycomb primary mirrors . Codona phase apodization at the MMT at 5 m m (50% in core) - PowerPoint PPT PresentationTRANSCRIPT
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Primary mirrors for exoplanet imaging
Developments at Steward Observatory Mirror Lab
• MMT with deformable secondary• Off-axis figuring• active thermal figure control for large,
lightweight honeycomb primary mirrors
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Phase apodization at 5 m at MMTCodona phase apodization at the MMT at 5 m (50% in core)
Giant planets anomlously bright at 5 m
90% Strehl with deformable secondary
Flux at 2.5 /D in circle 3.10-3 of peak
Rms fluctuations in 20 sec 2.5 10-4 (9 magnitudes)
(Codona ,Kenworthy and Hinz)
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Current status of NST off-axis mirror. parent f/0.7)
Spurious data due to fiducial markers on test optics have been masked out. Alignment aberrations and flexible bending modes have been subtracted.
nm s
urfa
ce
Smoothed with 30 mm FWHM Gaussian19 nm rms surface error
Central 1.2 m subaperture 21 nm rms surface error
Mirror is 1.7 m diameter, R = 7.7 m, 1.84 m off-axis. Aspheric departure is 2.7 mm.
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Projected 5 nm surface after ion figuring
Difference between original and smoothed maps represents residual error after ion figuring with 30 mm ion beam. 5.2 nm rms surface error
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8m off-axis
• Mirror Lab currently making first of 6 8.4 m off-axis mirrors for Giant Magellan telescope
• Goal Magellan quality (20 nm rms surface)• Blank cast, mounts being bonded now• Metrology tower being built, 3.8 m spherical
folding mirror for test
• (6.5 m vacuum test collimator nearly completed)
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Advantages of active primary over conventional relay and conjugated dm
• higher throughput and simpler – no additional optics needed
• simpler and lighter translates to lower cost, lower mass
• no cross coupling of phase into amplitude errors, which limits spectral bandwidth for very high contrast imaging. This is very important for exoplanet imaging
• no increased field aberrations from the added relay
• Allows full spacecraft system test on ground
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Principles of thermal actuation
• The neutral state of the mirror will be one in which a steady state heat flow is established.
• Corrections made by increasing or decreasing the power in the different heaters, to expand or contract the local glass as required.
• Low order modes controlled by front-to-back gradients (bimetallic strip type bending)
• High order modes by local rib expansion and contraction
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Thermal finite element modelfor 37 cell mirror
Color coded for equilibrium temperatures when the cold fingers are held isothermal, Joule heating of face and ribs
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Fractional residual errors for thermally induced Zernike terms.
Residual Error After Correcting Zernike Shapes
0%
2%
4%
6%
8%
10%
12%
14%
16%
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20%
0 10 20 30 40
Zernike Number
Res
idua
l Err
or
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Start on lab testHextek borosilicate honeycomb sandwich mirror 2 inch cells, 2.5 inches deep, 8 mm thick ribs
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3 cells have enlarged back holes and radiative cooler plates
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Interferometric surface metrology after cooling for 11 minutes
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100 mm320 nm
250 nmt=11 min, T=22 - 2.7°C
t=27 min, T=22 - 4°C
t=0 min, T=22°CRib cooling influence function
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• For space use proven fused silica honeycomb technology