Laboratory test of application of electric field conjugation image-sharpening to ground-based adaptive optics

Abstract

High contrast imaging is an ongoing theme in the domain of astronomy, both for ground-based and space-based telescopes. Achieving 106 - 107 contrasts expected with GPI and SPHERE or 1010 contrast for space projects, requires extreme wavefront correction as well as good coronagraphic systems. With the testbed located at the Laboratory of Adaptive Optics in Santa Cruz, we statically correct the wavefront to 0.5 nm in band and reach contrast of a few 107 with an Apodized Lyot Coronagraph (APLC). The Electric Field Conjugation (EFC) allows us to further improve on this performance. EFC is a formalism of the correction problem that computes the actuator commands for the deformable mirror (DM) to correct for both amplitude and phase in a pre-defined region in the final image plane. In order to take into account non-common-path errors and potential amplitude aberrations, the proper actuator commands are computed using an image plane-based DM diversity by means of reconstructing the complex electric field. Already successfully tested for space-based telescopes, we here attempt to adapt this method to ground-based observations, using the EFC high contrast solution to record new reference centroids for a Shack-Hartmann wavefront sensor, which in turn can be used to recreate the far-field image. This paper shows results of this first use of the EFC method with an APLC. We achieved 4.108 contrast on a [4-9] λ/d square region. We also show that it can be applied to ground based adaptive optics, using Shack-Hartmann wavefront sensors.