Comparison of coronagraphs for high-contrast imaging in the context of extremely large telescopes

Abstract

Aims. We compare coronagraph concepts and investigate their behavior and suitability for planet-finder projects with Extremely Large Telescopes (ELTs, 30-42 meter class telescopes).Methods. For this task, we analyzed the impact of major error sources that occur in a coronagraphic telescope (central obscuration, secondary support, low-order segment aberrations, segment reflectivity variations, pointing errors) for phase, amplitude, and interferometric type coronagraphs. This analysis was performed at two different levels of the detection process: under residual phase left uncorrected by an eXtreme Adaptive Optics system (XAO) for a wide range of Strehl ratios and after a general and simple model of speckle calibration, assuming common phase aberrations between the XAO and the coronagraph (static phase aberrations of the instrument) and non-common phase aberrations downstream of the coronagraph (differential aberrations provided by the calibration unit).Results. We derive critical parameters cope by each concept in order of importance. We show three coronagraph categories as function of the accessible angular separation and proposed optimal one in each case. Most of the time amplitude concepts appear more favorable, and the Apodized Pupil Lyot Coronagraph specifically gathers the adequate characteristics to be a baseline design for ELTs.