Detecting exoplanets with high contrast coronagraphy

Abstract

The first images of exoplanets are now in hand, but the imaging of even fainter planets near bright stars requires the development of very high contrast detection techniques. The two necessary aspects are precise wavefront control and efficient starlight rejection. These essential aspects were recently demonstrated at the Palomar Observatory on a 1.5 m diameter "well-corrected subaperture" on the Hale telescope. "Extreme" adaptive optics wavefront correction was achieved using fine-scale wavefront correction on the subaperture, combined with phase-retrieval to reduce non-common path errors such as faint speckles. Starlight rejection has been maximized with a novel vector vortex coronagraph, precise tip-tilt and focus control within the coronagraph, and the ``locally optimized combination of images" speckle calibration algorithm. The Palomar system provides small-angle contrast sensitivities comparable to those of much larger telescopes, allowing the imaging of e.g., the three HR8799 planets and the HD32297 disk. These results provide a first validation of the steps needed to achieve high-contrast in on-sky observations, and illustrate the promise of future ground- and space-based high-contrast instruments.

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