Coronagraphic phase diversity: a simple focal plane sensor for high-contrast imaging.

Exoplanet direct imaging is a challenging goal of today's astronomical instrumentation. Several high-contrast imaging instruments dedicated to this task are currently being integrated; they are ultimately limited by the presence of quasi-static speckles in the imaging focal plane. These speckles originate in residual quasi-static optical aberrations, which must be measured and compensated for, typically at a nanometric level. We present a novel focal plane wavefront sensor (WFS) designed for this particular application. It is an extension of the phase diversity technique to coronagraphic imaging. This sensor requires no dedicated hardware and uses only two scientific images differing from a known aberration, which can be conveniently introduced by the adaptive optics subsystem. The aberrations are therefore calibrated all the way down to the scientific camera, without any differential aberrations between the sensor and the scientific camera. We show the potential of this WFS by means of simulations, and we perform a preliminary experimental validation.