Self-Coherent Camera as a focal plane phasing sensor - Overview and early comparison with the Zernike Phase Contrast Sensor

In order to achieve the high performance required for the astronomical science programs with coming Extremely Large Telescopes (ELTs), 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 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. By contrast to several phasing sensor concepts the SCC-PS does not require any a priori 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. Applications of the estimators and algorithm developed for the SCC-PS seem to be possible to other on-segment aberration measurement. Early studies are already in progress to adapt these processes.