Estimation and correction of wavefront aberrations using the self-coherent camera: laboratory results

Abstract

Context. Direct imaging of exoplanets requires very high contrast levels, which are obtained using coronagraphs. But residual quasi-static aberrations create speckles in the focal plane downstream of the coronagraph which mask the planet. This problem appears in ground-based instruments as well as in space-based telescopes. Aims. An active correction of these wavefront errors using a deformable mirror upstream of the coronagraph is manda-tory, but conventional adaptive optics are limited by differential path aberrations. Dedicated techniques have to be implemented to measure phase and amplitude errors directly in the science focal plane. Methods. First, we propose a method for estimating phase and amplitude aberrations upstream of a coronagraph from the speckle complex field in the downstream focal plane. Then, we present the self-coherent camera, which uses the coherence of light to spatially encode the focal plane speckles and retrieve the associated complex field. This enable us to estimate and compensate in a closed loop for the aberrations upstream of the coronagraph. We conducted numerical simulations as well as laboratory tests using a four-quadrant phase mask and a 32x32 actuator deformable mirror. Results. We demonstrated in the laboratory our capability to achieve a stable closed loop and compensate for phase and amplitude quasi-static aberrations. We determined the best-suited parameter values to implement our technique. Contrasts better than 10−6 between 2 and 12 λ/D and even 3.10−7 (RMS) between 7 and 11 λ/D were reached in