TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Structural, Thermal, and Spacecraft Technology
4.1.3 Precision Structural Stability Characterization
Objective
The TPF Coronagraph will have to rely on extreme structural and thermal stability to achieve its
performance goals. The objective of this work is to determine, with the high degree of precision
necessary, the material and sub-component characteristics necessary for accurate modeling of the
thermal and dynamic stability of the TPF-C telescope performance. Most importantly, the
thermo-mechanical linear and non-linear characteristics and their scatter need to be tested and
identified to levels consistent with the error budget.
At present, the error budget for the TPF-C concept at 4λ/D with an 8-m PM requires that the
rigid-body position of the PM relative to the SM be better than 25 nm over the 10-m separation,
and that the RMS PM surface figure over the first 15 Zernicke modes be better than about 300
pm during the course of an observation, which entails a 36 degree dither and approximately 2
hours of data collection at station. This activity will assure that material properties and models of
critical sub-components are characterized to levels of precision commensurate with the mission
requirements, which in many cases is at or beyond the current state-of-the-art.
Approach
This activity will develop several experimental test facilities contributing to improved
knowledge of precision stable structures, as described below. Note that since the facilities
described herein are for the purpose of material characterization and model validation only they
are not associated with any TRLs.
Precision Dilatometer Facility
We take advantage of the (Cryogenic) Precision Dilatometer Facility (PDF) developed at JPL for
JWST to characterize the thermal strains, material variability, and long term dimensional
stability of relevant precision optical materials, at any temperature between 305 K and 20 K. The
facility is shown in Figure 4-3, along with an example of measured data in Figure 4-4.
This facility is now being calibrated, and recent data shows that the error in the instantaneous
coefficient of thermal expansion is approximately 2 ppb/°C, at least an order of magnitude better
than other existing test facilities in the United States typically used for this kind of measurement.
Preliminary sensitivity analyses on mirror CTE variations for the TPF-C Minimum Mission
configuration have shown figure requirements were achieved with variations of 5 to 15 ppb/°C,
representative of ULE fabrication capabilities. This implies that the PDF has sufficient precision
to characterize material CTE to tolerances required for TPF-C analyses. Examples of materials to
be tested include ULE and Zerodur for optical mirrors, PMN for the deformable mirrors, and
titanium and various metals for mechanical components or flexures. Other mechanical and
thermal properties will also be gathered from the literature or tested when necessary. It will be
important to also capture the accuracy with which this data has been measured, so as to
propagate the measurement uncertainties within the analytical predictions. This implies that all
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