TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Chapter 3
are in progress with continuous improvements in the mask material, fabrication progress,
algorithms, and testbed optics. Initial radiation tests show minimal HEBS mask performance
degradation for the expected environmental exposure over mission lifetime.
In addition to the HCIT implemented for evaluating mask performance and perfecting
methodologies for star light suppression, an interferometer system has been developed at JPL to
characterize HEBS mask material. This system, shown in Figure 3-2 incorporates 830, 785, 635,
and 532 nm wavelength laser sources and a cooled CCD camera to capture interference fringe
images. Algorithms have been developed to extract phase retardation/advance from such fringes
from the various regions of different optical densities in the HEBS mask. This information is fed
into models to validate experimental results. Reduction of error bars in measurement is an
ongoing activity. Additionally, precision spectrophotometry and spectroscopic ellipsometry are
employed to measure optical density and optical constants of the material as a function of
wavelength.
More recently Kuchner, Crepp, and Ge 2 have proposed replacement of the linear sinc 2 mask with
an eighth order mask which is predicted to yield better tolerance to pointing instabilities and low
order aberrations 3 .
Focal plane binary masks: Focal plane masks of opaque and transparent regions
lithographically formed on a metal film on glass are predicted to perform similar to the gray
scale masks described above. Patterns of 1-sinc 2 , continuous sin 2 , and discontinuous sin 2 have
Figure 3-2. Interferometric characterization of mask phase retardance.
2 M.J. Kuchner, J. Crepp, and J. Ge, “Eighth-Order Image Masks for Terrestrial Planet Finding,” Ap.J.
628, pp. 466–73 (2005).
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