TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Structural, Thermal, and Spacecraft Technology
The testbed is located inside a large ~6-m wide, ~6-m high thermal vacuum chamber. The
sunshield is simulated by conically symmetric and concentric sheets of material of high specular
reflectivity with an opening angle between each sheet of ~3 degrees. The inner surface of the
inner sheet is a high emissivity surface (black) which serves an additional purpose as a visible
light baffle. If additional optical baffling is added to the design in the future, it would be included
in the testbed. The PM is simulated by an approximately 2-m circular mirror with a structural
design similar to the PM. It is not required to be figured, but may need to be polished and coated.
The simulated mirror is instrumented with temperature-measuring devices with tens of micro-
Kelvin precisions. Thermal simulations of the spacecraft, AMS, and payload instruments are
included, as are their thermal enclosure and relevant conduction paths. This package is
surrounded by cold shrouds intended to simulate as well as possible the cold heat sink of space.
Etched foil Kapton film heaters are attached to the outer surface of the outer most sheet, which
can be controlled to simulate solar heating at differing spacecraft angular orientations with
respect to the sun.
Figure 4-17 shows the modeled steady-state thermal distortion response of a 6 m × 3.5 m PM to
a 20-degree rotation of the telescope around the z-axis (the dither maneuver). The displacements
are 0.53 pm rms and 1.9 pm peak-to-valley. The corresponding total range of all temperature
changes in the PM is approximately 0.26 mK. These displacement levels are characteristic of
those that must be maintained during exposures ranging up to 24 hours. It is not clear that system
noise can be made low enough to detect temperature variations of this magnitude.
The components needed to build the Isothermal Cavity portion of the testbed (thermometry
systems good to a precision of ~0.01 mK and PID controllers good to ~0.05 mK) are felt to be in
the 4-5 TRL range based on an initial industry survey. The Microwave Anisotropy Probe (MAP)
flew sensors with precisions of 0.1-0.2 mK based on integrations times of less than one second,
Figure 4-17. Steady-state thermal distortion response of the primary mirror to a 20 degree
dither.
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