TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Optics and Starlight Suppression Technology
3.1.2 Technology Demonstration Mirror
Objective
The TDM is a 1.8-m mirror that will demonstrate mirror technology required for the primary
mirror (PM). The static error budget for the entire system requires sub-nm wavefront quality, and
the dynamic error budget is two orders of magnitude more stringent for TPF-C. For both static
and dynamic errors the PM is expected to be the largest error contributor. Errors on the primary
mirror must be within the capture range for the wavefront sensing and control system. Control
of the deformable mirror is then relied upon to reduce the WFE to the required sub-nm level. The
technology necessary to control these errors for the large TPF-C can be demonstrated on a 1.8m
sub-scale mirror if it is a light-weighted, off-axis optic like the PM. For both the TDM and TPF-
C static errors will be driven by the structural design, figuring and polishing techniques, coating
process, and the sensitivity of the metrology to these errors. Dynamic errors like the static errors
are driven primarily by the structural design but also by material characteristics and fabrication
processes. Timing of the TDM is very important so that technology risks are reduced before the
fabrication of the TPF-C PM. Due to the long lead time for fabrication of the PM, it must be
procured early in the formulation phase if TPF-C is to meet its launch readiness date. TPF-C
performance requires that mirror technology sufficient to achieve the required levels, as outlined
in Table 3-1, is demonstrated early in the program.
Table 3-1. TDM Performance Requirements
Parameter Range Requirement Goal
SURFACE ERROR REQUIREMENTS
Low Spatial Frequency (LSF) 10 cycles/cm 10 Å rms 5 Å rms
COATING RESIDUAL REFLECTANCE REQUIREMENTS
Mid Spatial Frequency 0.025–0.5 cycles/cm