TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Optics and Starlight Suppression Technology
1060 nm) and suitability for large area application on the primary and secondary mirrors. The
objective of this task is to design a coating material and application process and verify through a
combination of analysis and test that it will meet TPF-C requirements.
Approach
The coatings have several challenging requirements TPF-C must achieve but that are not
normally placed on similar coatings for ground or space use. The system amplitude and phase
uniformity must be very high, of order 10 -4 , over the relevant spatial frequencies. This may be
correctible to some degree with the deformable mirror downstream, but the overall uniformity
requirements of each component will still be very high; this is not normally a consideration for
instruments in which only the total, integrated reflectivity is important. System level
polarization must be minimal, preferably small enough that we can dispense with large
polarizing beam splitters that would be required to work with each polarization component
independently. The thickness of the metallic coating is required to be uniform to within a few
percent on each optic, including over the 8-m primary, in order to maintain the figure
specifications. Each of these characteristics must be stable from coating application through
observatory lifetime.
Particularly challenging are the coatings for the primary and secondary mirrors. For these the
angle of incidence varies over the aperture from about 1 to 12 degrees, causing non-uniform
phase and amplitude and small but significant polarization. Figure 3-9 illustrates these variations
for a single element using a candidate, protected-Ag coating at 650 nm. The overall variation in
phase (~0.006 λ) can be removed using a deformable mirror corrector, however this still leaves a
residual error (due to the two polarization components) of about 5 × 10 -4 λ. The form of this
polarization error (a smooth, low order polynomial can represent the effective figure error) will
reduce contrast most significantly near the inner working angle. For the multi-element TPF-C
design, the total residual polarization will likely be larger than this single surface result; designs
which produce compensation will be considered. Ongoing design work will determine the
specific contrast sensitivity of such candidate coatings when applied throughout the entire TPF-C
optical path. Using prescribed coating designs for specific mask and wavefront corrector
Figure 3-9. Variation of phase (left panel) and amplitude (right panel) with angle of incidence
for a coating of Ag+SiO 2 (124 nm) at 650 nm for both s- and p-polarizations.
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