The self-coherent camera as a focal plane phasing sensor

Exoplanets imaging requires very high angular resolution that will be reached with the forthcoming generation of extremely large telescopes. In order to achieve the high performance required for the astronomical science programs, the errors due to segment misalignment must be reduced to tens of nm. Therefore the development of new co-phasing techniques is of critical importance for ground-based telescopes, and to a large extent for future space-based missions. We propose a new co-phasing method directly exploiting the scientific image delivered by the self coherent camera (SCC) by adequately combining segment misalignment estimators (piston and tip/tilt) and image processing. The extension of the SCC concept towards a co-phasing sensor is presented and its parameter space and performance for phasing a segmented telescope are studied by means of intensive numerical simulations. The self-coherent camera phasing sensor (SCC-PS) is shown to be capable of estimating accurately and simultaneously piston and tip/tilt misalignments and to correct them in close-loop operation in a few iterations. The final residual RMS values over the pupil obtained with the SCC-PS are compared to similar simulations of another co-phasing sensor and we show that the SCC-PS gives the same or even better results by requiring less iterations. By contrast to several phasing sensor concepts the SCC-PS does not require any a priori knowledge on the signal at the segment boundaries, or a dedicated optical path. The SCC-PS is a non-invasive concept that works directly on the scientific image of the instrument, either in a coronagrahic or a non-coronagraphic observing mode. The primary results obtained in this study are very promising and demonstrate that the SCC-PS is a serious candidate for segment co-phasing at the instrument level or at the telescope level for both ground- and space-based applications.