TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Structural, Thermal, and Spacecraft Technology
Laser truss
Interferometer
Metering truss
Figure 4-18. Sunshield/isothermal cavity performance testbed evolved to test the optical
stability of a quarter-scale mirror representative of the TPF-C primary mirror.
Surface changes of 0.2–0.4 nm can be tolerated during an exposure, up to 24 hours and including
a dither maneuver. The test will therefore be looking for thermally induced surface deformations
of this size at low spatial frequencies. Relevant heritage for measurement accuracies of this level
is only currently being established. Researchers at the National Institute of Standards and
Technology have measured the surface of a 30-cm flat mirror at room temperature with an error
of 0.2 nm (not yet published). Blake et al. 33 measured a 17-cm flat mirror at 30 K with an error
of 1 nm. The test optics in these cases were comparatively small, allowing variables in the test to
be tightly controlled. Measurements on a larger mirror (~1.4 m), the Advanced Mirror System
Demonstrator (AMSD), at cryogenic temperatures, have error bars of 14 nm. 34 The test proposed
here on a 2-m sphere represents a challenge to current measurement capabilities. We propose to
largely eliminate noise in our measurements due to the support structure between the test mirror
and the wavefront sensing device by employing a laser truss to control a hexapod support of the
wavefront sensor. This technology is proposed to control the position of the secondary mirror in
the TPF-C telescope. A subscale demonstration is discussed in Section 4.2.1. An independent
laser system will measure the effectiveness of the truss. In addition tight thermal control of the
wavefront sensor assembly and frequency control of the light source will reduce noise from these
sources. Detector noise will be minimized. This test does not require a very accurate figure; λ/20
will suffice since the objective of the test is to track changes in the surface shape. However, good
33 Blake, P. et al., “NIRSpec Mirrors Cryogenic Testing Final Report,” Goddard Space Flight Center,
2005.
34 Reardon, P.J et al., “Advanced Mirror System Demonstrator Cryogenic Error Test Budget,” SPIE
Proceedings 4850, 2003.
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