TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 4
giving confidence that flight temperature sensors in the precision range needed are likely
reasonable. Some level of flight qualification testing is likely to be needed on these testbed
components during Phase A in order to reduce the risk of a problem being encountered during
the later flight/protoflight unit qualification tests.
The V-groove sunshield will be fabricated from plastic (mylar or Kapton) film and overcoated
with vapor-deposted pure aluminum or gold, such as has been used in flight thermal blankets for
many years. The outer surface will most likely be a silver-Teflon second-surface mirror, which
also has many years of spaceflight history. V-groove shielding is being implemented on the
European Space Agency’s Planck observatory, scheduled for launch into the L2 orbital
environment. In this case, however, the individual shields are made from rigid aluminum
honeycomb material, with the face sheets somewhat polished and coated with vapor-deposited
aluminum. The closest technological neighbor to the TPF-C sunshield is that being developed
for JWST. The deployment scheme for JWST has been developed and successfully tested at half
scale. The primary differences between the JWST and TPF-C sunshields lie in their deployed
shapes and number of layers. The TPF-C has more layers and is deployed into a basically
conical shape. The deployment challenges are considerable and are the subject of study on
several fronts.
4.2.5 Sub-scale EM Primary Mirror Assembly
Objective
This activity constitutes an extension of the testbed described above (Sub-scale EM Sunshield
and Isothermal Enclosure). The object is to test for wavefront stability using representative optics
in the Sunshield/Isothermal enclosure testbed and change the simulated solar illumination to
approximate the dither maneuver. In this case the flat mirror is replaced by a high-quality (λ/100,
TBD) spherical, ~f/1 mirror with construction similar to that of the flight TPF-C PM.
Approach
For this test the testbed in Figure 4-16 above is augmented by an interferometer placed at the
mirror center-of-curvature with provision for isolation from the ambient vibration background,
and by a laser metrology truss to measure displacements. This is done to eliminate vibration as a
significant source of noise in the optical signal. The changes are shown in Figure 4-18. Again, all
conduction paths must be simulated, including mirror actuators if these are included in the TPF-
C design. Two stages of vibration isolation are provided to permit injection of a vibration signal
equivalent to reaction wheel noise into the simulated science payload. Care is taken to minimize
the heat entering the cold cavity from the interferometer in order to reduce thermally induced
noise in the reflected wavefront. The interference pattern developed by the interferometer will
measure the departure of the mirror surface from its ideal shape. The change in the pattern
induced by changing the spatial distribution of the heat input to the simulated sunshield will be a
measure of the effectiveness of the thermal design.
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