Post-coronagraphic PSF sharpening with the vortex coronagraph

Small-angle coronagraphs are key components for future instruments being developed for the ELTs. Because they deliver high contrast capabilities at small angular separations from the host star, they enable us to address by direct imaging one of the most important science case of the ELTs, the discovery and characterization of exoplanets and proto-planetary systems. Small-angle coronagraphs based on a focal-plane mask such as the vortex phase mask are however quite sensitive to low-order aberrations. They require precise wavefront control to avoid spurious starlight leakage that would decrease the contrast level and possibly mimic the signatures of off-axis companions. Here, we present a low-order wavefront sensor technique based on an iterative process using post-coronagraphic focal images with a vortex coronagraph. Our approach is similar to a PSF sharpening technique, which is sometimes used in classical imaging to reduce non-common path aberrations (NCPA), but applied to post-coronagraphic images with the objective of minimizing the residual light of the on-axis source. We describe the laboratory implementation of this technique on the Vodca coronagraphic bench at the University of Liège, and present experimental results validating our method. Finally, we also present a weak phase solution with the vortex coronagraph and discuss its practical implementation.