Focal-plane wave front sensing strategies for high contrast imaging: experimental validations on SPHERE

Abstract

Direct detection and spectral characterization of extra-solar planets is one of the most exciting but also challenging areas in modern astronomy. The challenge is due to the very large contrast between the host star and the planet at very small angular separations. SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research in Europe [1]) is a secondgeneration instrument for the ESO VLT dedicated to this scientific objective. It combines an extreme adaptive optics system [2], various coronagraphic devices and a suite of focal instruments providing imaging, integral field spectroscopy and polarimetric capabilities in the visible and near-infrared spectral ranges. The limitation of such a high contrast imaging system is mainly driven by the presence of intensity residual in the scientific focal plane, caused by uncorrected quasi-static optical aberrations upstream of the coronagraphic mask. The measurement and compensation of these aberrations is mandatory in order to reach the level of contrast requested by exoplanet imaging. We present in this paper the final experimental validation of the baseline method developed in the framework of SPHERE instrument for the conpensation of NCPA. The method is based on a differential measurement with phase diversity, and a compensation with an optimised modification of reference slopes.