Correction of quasi-static wavefront errors for ELT with two sequential DMs

Detection and spectral characterization of extrasolar planets with Extremely Large Telescopes (ELT) will require the ability to image very faint objects with a wide spectral bandwidth. Such a goal implies the development of new exquisite wavefront control techniques in order to calibrate and correct both the amplitude and phase components of the residual quasi-static halo that remains after the AO loop. Here we focus on the correction stage at time scales orders of magnitude slower than the AO refresh rate. We report laboratory results that exhibit four important features of instrument responses necessary for exo-planet characterization: post coronagraphic, high contrast, broadband, symmetric Point Spread Functions. This experiment was conducted in the Princeton University High Contrast Imaging Laboratory, equipped with a shaped pupil coronagraph and two sequential Boston Micromachines Kilo-DMs. In this paper we first review the theory and algorithms underlying one and two DM wavefront controllers and show why a second DM ultimately leads to improved broadband performances. We then present an experimental validation of such a two DM amplitude and phase controller