Laboratory Demonstration of Efficient XAO Coronagraphy in the Context of SPHERE

Abstract

ABSTRACTExtreme adaptive optics systems (XAO) dedicated to the search for extrasolar planets are currently being developed for 8–10 m telescopes. The high-order test bench (HOT) is a high-contrast imaging adaptive optics bench developed at the European Southern Observatory to test and optimize different techniques and technologies (e.g., wavefront sensors, coronagraphs, speckle calibration methods, image postprocessing). It reproduces realistic conditions at a telescope (e.g., Very Large Telescope, VLT), including a turbulence generator, a high-order adaptive optics system, and a near-IR coronagraph. We discuss the results of XAO coronagraphy obtained in the laboratory in the context of imminent planet-finder instruments (e.g., SPHERE, GPI, and HiCIAO). In particular, results obtained with HOT will be discussed and compared with contrast goals of the near-IR camera of SPHERE. Under a generated dynamical turbulence of 0.5″ seeing, which is efficiently corrected by the XAO system to an H-band Strehl ratio above 90%, and combined with a near-IR coronagraph, we demonstrate contrasts of 5 × 10-5 at 0.1″, and 4.5 × 10-6 at 0.5″. Contrasts were obtained using differential imaging techniques by sequentially taking either a series of two closely-spaced narrowband filter images in H band centered around 1.6 μm, or two images with orthogonal polarization states. Results obtained in the laboratory are in good agreement with the contrast goals of SPHERE, which provides confidence in the expected performance of this class of instruments.