SPHERE non-common path aberrations measurement and pre-compensation with optimized phase diversity processes: experimental results


J-F Sauvage (1), T Fusco (1,2), D. LeMignant (2), C. Petit (1), A. Sevin (3), K. Dohlen (2), C. Robert (1), L Mugnier (1)


(1) ONERA, (2) Laboratoire d’Astrophysique de Marseille, (3) LESIA


The SPHERE instrument is the 2nd generation instrument dedicated to exoplanet direct imaging and characterization. The extremely high imaging performance required by these observation mode calls for a high performance AO system. Particularly, this system has to provide a wavefront corrected from turbulent and internal defects. We present here the experimental results for the complete focal-plane calibration procedure of the SPHERE instrument internal defects. An optimized phase diversity method is applied allowing to deal with model uncertainties in the image formation (noise, residual background, amplitude fluctuation, sampling factor, défocalisation distance, object size and SH-model for reference slope modifications) The full procedure includes both Non-Common Path Aberrations (NCPA) compensation at the level of the coronagraphic mask using the eXtreme AO system itself (by the mean of modification of the filtered SH WFS reference slopes during close loop operations), but also additional measurements of IRDIS differential optical path aberrations for post processing of dual-band images.

We validate the algorithm and the pre-compensation procedure using data obtained during the first eXtreme AO bench (SAXO) integration and tests. We also applied the Diversity tool on stand-alone IRDIS data obtained at LAM during its local integration. In both cases, we demonstrate the robustness and the ultimate performance (nanometric precision of the residual quasi-static pattern) of the phase diversity approach which will allow us to obtain nanometric accuracy on the final SPHERE system.

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