TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Optics and Starlight Suppression Technology
wavelengths in the bandpass and very low polarization elsewhere. Figure 3-10 shows the
performance of this coating design and a similar design based on the original protected-Ag
designs of Hass et al. 21 Sensitivity analysis of these low polarizing designs is underway to
confirm that their use will allow TPF-C to achieve its required contrast level. Further work by
Balasubramanian has also shown that modifications of these basic designs may allow for some
polarization compensation, further improving the overall system performance. Integration of
these designs with the capabilities of the DM based corrector is also underway to assure that the
necessary bandpass for TPF-C and overall system performance can be achieved with such
coatings.
3.1.7 Scatterometer
Objective
The scatterometer testbed provides laboratory testing for model validation of stray and scattered
light effects. These effects must be better understood on TPF-C than on prior observatories if the
desired contrast is to be achieved. The coronagraph contrast is degraded by diffraction effects,
optical surface scatter (e.g., PSD modeling), contamination (particulate and hydrocarbon), and
stray light multiple scattering off of baffles, stops, etc.
Stray and scattered light are not easily separable. One description attributes stray light to such
elements as optical baffles and baffle design (specular effects, bidirectional reflectance
distribution function modeling, etc.), while scattered light is composed of diffraction, nonspecular,
near- and wide-angle scatter, and contamination. All of these must be modeled and well
understood to ensure the required contrast. This testbed provides checks on how the physics of
these effects are handled in various engineering codes, judges impacts to the project, and informs
the selection of a code for use during baffle design.
Approach
The scatterometer testbed at GSFC 22 will allow testing of precision laboratory optics against
modeling codes. Capabilities within the testbed allow surface testing to levels comparable to the
TDM tests across the relevant range of spatial frequencies and coupling of this metrology data to
deep scattered light measurements. We will test the modeling of the physics of scatter from
surface roughness and contamination. In the past, near-angle scatter measurements have been
21 G. Hass, J.B. Heaney, H. Herzig, J.F. Osantowsky, and J.J. Triolo, “Reflectance and durability of AG
mirrors coated with thin layers of Al203 plus reactively deposited silicon oxide,” Appl. Opt. 14, 2639-
2644 (1975) and G. Hass, J.B. Heaney, and J.J. Triolo, “Evaporated AG coated with double layers of
Al203 and silicon oxide to produce surface films with low solar absorptivity and high thermal
emissivity,” Opt. Comm.8, 183-185 (1973).
22 I. Kuznetsov, D. Content, R. Boucarut, and T. Madison, “Design, performance, and reliability of a
high-angular-resolution wide-angular-range large-aperture fully-automated UV scatterometer,” Proc.
SPIE Int. Soc. Opt. Eng. 4485 p. 417-428 (2002).
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