PSF sharpening & post focal sensing for the VORTEX coronagraph

Small angle coronagaphy, such as the vortex phase mask, is a key scientific driver
for the coming generation of VLT-like instrument and for the future ELTs. It is
appealing for the full scientific exploitation of ground-based telescopes for the
detection and characterization of exoplanets and circumstellar discs. One key
element to small angle coronagraphy is a precise low-order wavefront control to
avoid any leak decreasing the contrast level and even possibly mimicking off-axis
signal. In this contribution, we present our early on-going effort in using post-
coronagraphic focal images to perform wavefront control with a vortex coron-
agraph. First, I will present an empirical approach similar to PSF sharpening
techniques, sometimes used in classical imaging to reduce NCPA aberrations,
to post-coronagraphic images. Then, I will discuss an alternative based on the
general weak-phase solution to retrieve the complex Efield in the Lyot plane
using three images, and how we can use this to control the phase in the pupil
plane. Finally, after presenting those two approaches and first simulations, I
will discuss the possible implementation of such techniques on our bench, the
vortex optical demonstrator for coronagraphic applications (VODCA).