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

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)

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.

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

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)

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

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

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

Fractional residual errors for thermally induced Zernike terms.

Residual Error After Correcting Zernike Shapes

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Zernike Number

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idua

l Err

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Start on lab testHextek borosilicate honeycomb sandwich mirror 2 inch cells, 2.5 inches deep, 8 mm thick ribs

3 cells have enlarged back holes and radiative cooler plates

Interferometric surface metrology after cooling for 11 minutes

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

• For space use proven fused silica honeycomb technology