TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 3
Table 3-3. ITT Metrology Error Budget compared to TDM Requirements and Goals
Spatial Frequency
ITT Estimated
Errors (nm rms)
TDM Requirement
(nm rms)
TDM Goal
(nm rms)
Low Spatial Frequency (LSF) 2.8 10 5
Mid Spatial Frequency (MSF) 1.6 4.8 2.4
High Spatial Frequency (HSF) 0.03 1.4 0.7
Micro-roughness 0.2 1 0.5
should easily be met since Corning’s measurements show that the volumetric average CTE for
each boule varies from −3 to +2 ppb/ºC. Radial CTE measurements for each plate will be
completed in late winter of 2005.
ITT will use a center of curvature test with a 2-element Offner null lens, to be calibrated using
computer generated holography (CGH), for the low and mid-spatial frequency measurements and
a Chapman MP 2100 line scan profilometer for the high spatial frequencies and micro-roughness
measurements. All hardware for these tests is on order, and the CGH calibration concept has
been demonstrated. ITT’s metrology error budgets are compared to the TDM requirement and
goal in Table 3-3. All hardware for these tests is on order or has been received. In particular, the
MP 2100 was received in late December ’04. Calibrating the Offner with a CGH is a technique
that has been used before at ITT. This calibration technique has been developed by Jim Burge of
the University of Arizona (U of A) and the TDM CGH work is a joint effort of ITT and U of A.
Demonstration CGHs have been built and tested and have demonstrated that the ITT CGH
related errors are realistic.
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Figure 3-7. ITT's TDM concept with the core structure exposed. Also shown is the
strongback on which the mirror will be mounted.