TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Structural, Thermal, and Spacecraft Technology
Figure 4-15. Fine Steering Mirror / Fine Guidance Sensor testbed block diagram.
4.2.4 Sub-scale Engineering Model (EM) Sunshield and Isothermal
Enclosure
Objective
Successful observations with TPF-C require extreme stability of the wavefront during multi-hour
observations. A major source of wavefront instability is thermally induced changes in the optical
surfaces and in the structure linking these surfaces together. For example, temperature changes in
the PM in excess of 1 mK during an observation are prohibited given the previous error budget,
though this is likely to relax by an order of magnitude due to the adoption of the 8 th order mask.
In order to provide the required thermal stability, a three-fold thermal design approach is taken.
Approach
First, the PM, the SM and the metering structure between them, all of which comprise the OTA,
are decoupled from solar radiation by a multi-layered V-groove sunshield surrounding the
telescope.
Second, two isothermal enclosures, one of which blocks direct thermal inputs from the Sun and
the spacecraft and controls the temperature of the Payload Support System, one of which
radiatively bathes the back of the PM with a constant background flux and isothermalizes the PM
aft metering structure (AMS), and one of which bathes the back of the SM with a constant
background flux, are provided and controlled to the required precision.
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