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R. Belikov, N. J. Kasdin, R. J. Vanderbei, Michael Carr (Princeton University)
Direct imaging of extrasolar planets, and terrestrial planets in particular, is an exciting but difficult problem requiring the development of new high contrast imaging systems. To this end, our group at Princeton has been pioneering a type of a high contrast system called the Shaped Pupil Coronagraph (SPC). The principle of the SPC is to apodize the telescope pupil with an appropriately designed binary mask (shaped pupil) that yields a high contrast point spread function (PSF) in the image plane. As compared to other coronagraphs, the features of the SPC include simplicity, broadband and off-axis performance, as well as minimal sensitivity to aberrations.
We have implemented an SPC prototype with a shaped pupil mask designed for NASA's Terrestrial Planet Finder Coronagraph (TPF-C) mission. It has been tested with laser light at wavelengths of 632nm, 594nm, and 532nm, as well as white light from 550-750nm. In all cases, we are getting a contrast of 105 at a working angle of 4 \lambda/D, gradually droping to 107 at 10 \lambda/D. The limiting factor is speckle caused by wavefront errors of the mirrors in our setup. Adaptive optics is required to correct for these and realize the full potential of the coronagraph.
However, gains in exoplanet imaging sensitivity can be achieved even in the absence of adaptive optics, by using differential imaging techniques. Such techniques entail suppressing residual speckle by taking the difference of 2 (or more) images where the planet moves with respect to the speckle pattern (or vice versa). We report an order of magnitude improvement in sensitivity when using two images at different wavelengths, and two orders of magnitude when using two rotated views of the planet. In the former case, the two images can be taken simultaneously (SDI technique), which makes it possible for use in a ground-based observatory.
This project is supported by NASA/JPL and the Michelson Science Center.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.