TPF-C Technology Plan - Exoplanet Exploration Program - NASA

Optics and Starlight Suppression Technology
Final performance of the masks will be evaluated in the HCIT at JPL. This activity will
demonstrate that hardware can be manufactured to meet optical requirements of the coronagraph
in space environment.
Pupil plane masks: A second approach for coronagraphy is to employ masks at a pupil plane
that apodize the pupil so as to suppress and diffract away unwanted star light, thereby providing
the required contrast between star and planet lights at the image. Several theoretical designs have
been proposed for such masks with varying through-put efficiencies and system complexities,
primarily at universities under JPL subcontract. Experimental demonstrations are aimed at
discriminating between technologies to allow selection of those with the best system
performance. To support and complement experimental work, teams at JPL (Hoppe et al.),
Princeton (Kasdin et al.), UC Berkeley (Neureuther et al.), Ball Aerospace (Lieber et al.), and
GSFC (Lyon) focus on incorporation of experimental results into the optical system model and
validation of those models. The goal is to develop an understanding of the limits of the scalar
diffraction theory for this application and determine whether any design modifications are
necessary to achieve the desired coronagraph performance.
Progress to Date
Focal plane gray scale masks: Significant progress has been made in the past year in fabricating
and testing focal plane masks at JPL. The first approach uses HEBS glass manufactured by
Canyon Materials, Inc., San Diego, CA, and further modified for JPL to meet TPF-C
requirements. HEBS glass will darken to different levels of absorption of visible light when
exposed to different electron beam levels. This makes it possible to create controlled optical
density profiles in the glass with each mask design. Masks with linear 1-sinc 2 patterns have been
written in such glass with a state-of-the-art electron beam exposure system at the Micro Devices
Laboratory (MDL) at JPL. Tests with such masks during the past year in the HCIT have shown
average contrast of 0.9 × 10 -9 in the dark field with 784 nm laser light 1 and more recently in the 5
× 10 -9 level with about 40 nm bandwidth around 785 nm. Figure 3-1 shows a mask and a picture
showing the dark region in the image plane when such a mask is employed. Further experiments
Figure 3-1. Mask (left) and image (right). The mask is included in the optical train of the
HCIT where the corresponding image is recorded.
1 Trauger et al., “Coronagraph contrast demonstrations with the High Contrast Imaging Testbed,” Proc.
SPIE V.5487, i.3, pp. 1330–36 (2004)
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