Primary mirrors for exoplanet imaging Developments at Steward Observatory Mirror Lab

1
Primary mirrors for exoplanet imagingDevelopment
s at Steward Observatory Mirror Lab

• MMT with deformable secondary
• Off-axis figuring
• active thermal figure control for large,
  lightweight honeycomb primary mirrors

2
Phase apodization at 5 mm at MMT
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)
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
Smoothed with 30 mm FWHM Gaussian 19 nm rms
surface error
Central 1.2 m subaperture 21 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
t0 min, T22C
t11 min, T22 - 2.7C
250 nm
t27 min, T22 - 4C
320 nm
100 mm
14

• For space use proven fused silica honeycomb
  technology