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Direct imaging of Earth-like planets from space requires dedicatedobservatories, combining large segmented apertures with instruments andtechniques such as coronagraphs, wavefront sensors, and wavefront control inorder to reach the high contrast of 10^10 that is required. The complexity ofthese systems would be increased by the segmentation of the primary mirror,which allows for the larger diameters necessary to image Earth-like planets butalso introduces specific patterns in the image due to the pupil shape andsegmentation and making high-contrast imaging more challenging. Among thesedefects, the phasing errors of the primary mirror are a strong limitation tothe performance. In this paper, we focus on the wavefront sensing of segmentphasing errors for a high-contrast system, using the COronagraphic Focal planewave-Front Estimation for Exoplanet detection (COFFEE) technique. Weimplemented and tested COFFEE on the High-contrast imaging for Complex ApertureTelescopes (HiCAT) testbed, in a configuration without any coronagraph and witha classical Lyot coronagraph, to reconstruct errors applied on a 37 segmentmirror. We analysed the quality and limitations of the reconstructions. Wedemonstrate that COFFEE is able to estimate correctly the phasing errors of asegmented telescope for piston, tip, and tilt aberrations of typically 100nmRMS. We also identified the limitations of COFFEE for the reconstruction oflow-order wavefront modes, which are highly filtered by the coronagraph. Thisis illustrated using two focal plane mask sizes on HiCAT. We discuss possiblesolutions, both in the hardware system and in the COFFEE optimizer, to mitigatethese issues.

Experimental validation of coronagraphic focal-plane wavefront sensing for future segmented space telescopes