Increasing the raw contrast of VLT/SPHERE with the dark hole technique

Abstract

Context. Direct imaging of exoplanets takes advantage of state-of-the-art adaptive optics (AO) systems, coronagraphy, and postprocessing techniques. Coronagraphs attenuate starlight to mitigate the unfavorable flux ratio between an exoplanet and its host star. AO systems provide diffraction-limited images of point sources and minimize optical aberrations that would cause starlight to leak through coronagraphs. Post-processing techniques then estimate and remove residual stellar speckles due to hardware limitations, such as noncommon path aberrations (NCPAs) and diffraction from telescope obscurations, and identify potential companions. Aims. We aim to demonstrate an efficient method to minimize the speckle intensity due to NCPAs and the underlying stellar diffraction pattern during an observing night on the Spectro-Polarimetric High-contrast Expolanet REsearch (SPHERE) instrument at the Very Large Telescope (VLT) instrument without any hardware modifications. Methods. We implement an iterative dark-hole (DH) algorithm to remove stellar speckles on-sky before a science observation. It uses a pair-wise probing estimator and a controller based on electric field conjugation, originally developed for space-based application. This work presents the first such on-sky minimization of speckles with a DH technique on SPHERE. Results. We show the standard deviation of the normalized intensity in the raw images is reduced by a factor of up to five in the corrected region with respect to the current calibration strategy under median conditions for VLT. This level of contrast performance obtained with only 1 min of exposure time reaches median performances on SPHERE that use post-processing methods requiring ~1h-long sequences of observations. The resulting raw contrast improvement provides access to potentially fainter and lower-mass exoplanets closer to their host stars. We also present an alternative a posteriori calibration method that takes advantage of the starlight coherence and improves the post-processed contrast levels rms by a factor of about three with respect to the raw images. Conclusions. This on-sky demonstration represents a decisive milestone for the future design, development, and observing strategy of the next generation of ground-based exoplanet imagers for 10-m to 40-m telescopes.