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Imaging faint objects, such as exoplanets or disks, around nearby stars isextremely challenging because host star images are dominated by the telescopediffraction pattern. Using a coronagraph is an efficient solution for removingdiffraction but requires an incoming wavefront with good quality to maximizestarlight rejection. On the ground, the most advanced exoplanet imagers useextreme adaptive optics (ExAO) systems that are based on a deformable mirror(DM) with a large number of actuators to efficiently compensate for high-orderaberrations and provide diffraction-limited images. While several exoplanetimagers with DMs using around 1500 actuators are now routinely operating onlarge telescopes to observe gas giant planets, future systems may require atenfold increase in the number of degrees of freedom to look for rocky planets.In this paper, we explore wavefront correction with a secondary adaptive opticssystem that controls a very large number of degrees of freedom that are notcorrected by the primary ExAO system. Using Marseille Imaging Testbed for HighContrast (MITHiC), we implement a second stage of adaptive optics with ZELDA, aZernike wavefront sensor, and a spatial light modulator (SLM) to compensate forthe phase aberrations of the bench downstream residual aberrations fromadaptive optics. We demonstrate that their correction up to 137 cycles perpupil with nanometric accuracy is possible, provided there is a simpledistortion calibration of the pupil and a moderate wavefront low-passfiltering. We also use ZELDA for a fast compensation of ExAO residuals, showingits promising implementation as a second-stage correction for the observationof rocky planets around nearby stars. Finally, we present images with aclassical Lyot coronagraph on MITHiC and validate our ability to reach itstheoretical performance with our calibration.

Calibration of residual aberrations in exoplanet imagers with large numbers of degrees of freedom