A ``Fast and Furious''' solution to the low-wind effect for SPHERE at the VLT


We present a potential non-invasive solution to sensing the so-called low-wind effect (LWE) seen in the SPHERE instrument at the VLT, based on the ‘‘Fast and Furious (F&F) sequential phase diversity wavefront reconstruction algorithm. This uses non- coronagraphic focal-plane images available from the near-infra- red Differential Tip-Tilt Sensor (DTTS), with the closed-loop correction cycle itself providing the necessary phase diversity between frames required to reconstruct the full wavefront phase. Crucially, this means F&F does not need to apply large artificial phase probes as required by standard phase diversity algorithms, allowing it to operate in a real-time ( 10 Hz) correction mode without impacting science observations. In this paper we present the results of realistic closed-loop AO simulations designed to emulate SPHERE/DTTS observations of the LWE. With this we demonstrate that the F&F algorithm is capable of effective removal of the characteristic point-spread function (PSF) aberrations of strongly LWE-affected images within a few closed-loop iterations, with the final wavefront quality limited only by the corrective order of the deformable mirror. The ultimate goal of this project is to provide an independent, real-time and focal-plane wavefront sensor for SPHERE which is capable of detecting and directly compensating the LWE as it arises, thus improving coronagraph performance under the best 15-20 % of observing conditions where the effect is most pronounced.