TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Optics and Starlight Suppression Technology
3.1.5 Transmissive Optics
Objective
The minimum mission design incorporated two features that make use of large (greater than
10-cm clear aperture) transmissive optics. The first is a polarizer assembly consisting of a
polarizing beam splitter (PBS) and two additional cubes required to achieve the required
extinction while compensating for the chromatic dispersion. The polarizer assembly serves to
give two separate paths for two orthogonal polarizations, each with its own set of deformable
mirrors and downstream optics. The second feature is a Michelson assembly, chosen over the
Mach-Zehnder due to its compactness, which accommodates wavefront control via the two DMs
in each path. The total path in the glass for each sub-system is on the order of 25 cm.
The objective of this effort is to identify and, if necessary, address materials issues involved in
the fabrication of large transmissive optics to the required tolerances, such as the uniformity of
the glass, stress birefringence, scatter and depolarization, and the chromatic variation of these.
Approach
The preferred approach to addressing this technology risk is to develop design options that
bypass entirely or reduce the dependence on transmissive optics. Several trades are still open in
the coronagraph design and may result in the elimination of the two separate polarization paths
or modification of the interferometric configuration. There are two main options. The first one
is to control polarization through special coatings on the mirrors along with design changes that
will allow elimination of the second polarization beam path. The second one is the introduction
of a non-interferometric DM arrangement, thus removing another large transmissive optic.
Polarization control through mirror coating is an ongoing effort. Concepts for polarizationinsensitive
occulting masks are also being considered.
However, if it is decided that a configuration similar to that of the minimum design mission is
needed, the technology development will be planned and initiated in Phase A.
Progress to Date
Progress to date has consisted solely of initial investigation into the state-of-the-art; detailed
requirements have not yet been developed for transmissive optical elements.
Homogeneity
Published data for fused silica 15 show homogeneity values in the range of 0.5 ppm. These were
assessed with blanks ~200 mm in diameter and ~50 mm thick. Wavefront error of less than
~0.007 waves rms is achievable with blank selection. Homogeneity is determined from the p-v
net wavefront error upon transmission for any given blank. It therefore leaves unanswered the
question of spatial distribution, which would need further study. For other optical glasses, the
Schott glass catalog shows values of 1 ppm as achievable depending on the type of glass and
15 J. L. Ladison et al., “Achieving low wavefront specifications for DUV lithography; impact of residual
stress in HPFS® fused silica,” Proc. SPIE 4346, 1416-1423 (2001).
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