Primary mirrors for exoplanet imaging Developments at Steward Observatory Mirror Lab

1. Primary mirrors for exoplanet imagingDevelopments at Steward Observatory Mirror Lab • MMT with deformable secondary • Off-axis figuring • active thermal figure control for large, lightweight honeycomb primary mirrors

2. Codona phase apodization at the MMT at 5 mm (50% in core) Giant planets anomlously bright at 5 mm 90% Strehl with deformable secondary Flux at 2.5 l/D in circle 3.10-3 of peak Rms fluctuations in 20 sec 2.5 10-4 (9 magnitudes) (Codona ,Kenworthy and Hinz) Phase apodization at 5 mm at MMT

3. Current status of NST off-axis mirror. parent f/0.7) Mirror is 1.7 m diameter, R = 7.7 m, 1.84 m off-axis. Aspheric departure is 2.7 mm. nm surface Central 1.2 m subaperture 21 nm rms surface error Smoothed with 30 mm FWHM Gaussian 19 nm rms surface error Spurious data due to fiducial markers on test optics have been masked out. Alignment aberrations and flexible bending modes have been subtracted.

4. 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

5. 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)

6. 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

7. 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

8. 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

9. Fractional residual errors for thermally induced Zernike terms.

10. Start on lab test Hextek borosilicate honeycomb sandwich mirror 2 inch cells, 2.5 inches deep, 8 mm thick ribs

11. 3 cells have enlarged back holes and radiative cooler plates

12. Interferometric surface metrology after cooling for 11 minutes

13. Rib cooling influence function t=0 min, T=22°C t=11 min, T=22 - 2.7°C 250 nm t=27 min, T=22 - 4°C 320 nm 100 mm

14. For space use proven fused silica honeycomb technology