TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 4
Figure 4-1. Overview of the metrology beam arrangement for secondary mirror position sensing
and hexapod control.
and fiducial mounting techniques and metrology sources. The extension of the technology to
achieve the frequency stability is expected to be relatively straightforward.
Progress to Date
SIM has demonstrated the required displacement and angular sensitivity on the KITE Testbed
with SIM Milestone #4, although at much shorter timescales. Much of the additional attention to
date has focused on demonstrating the laser frequency stability to address the previous stability
requirement of ~10 -11 over a 24-hour timescale, associated with the 4 th order masks and now
considerably relaxed.
4.1.2 Precision Hexapod
Objective
The TPF Coronagraph instrument requires that the Optical Telescope Assembly deliver a
wavefront of extraordinary quality and stability. One of the key parameters contributing to the
quality and stability of the wavefront is the absolute position of the secondary mirror relative to
the primary mirror in 6 DOF. The current TPF architecture and error budget envisions a system
in which the relative position of the secondary mirror (SM) to the primary mirror is held on the
order of 25 nm. A 6-DOF laser metrology system is envisioned to provide feedback to a 6-DOF
56