Calibration of the static aberrations in an MCAO system

The ESO Multi-conjugate Adaptive optics Demonstrator (MAD) is a prototype intended to be tested at the VLT Nasmyth focus. With its development raises the problem of calibration of an AO system composed of several correcting devices and wave front sensors. One part of this process is the calibration of the static aberrations of the system, always present in spite of the best efforts made during the design, the manufacturing of the optics and their alignment. In this paper we present a study to find an optimized way to correct for the static aberrations in the scientific FoV of an MCAO system. Thanks to images from the camera, the WF error in the FoV is computed, the contribution of several altitudes reconstructed, and finally projected on the deformable mirrors in order to compensate for the measured aberrations. This technique, inspired from the calibration of the static aberration of the system NAOS-CONICA, allows bringing the best quality to the scientific instrument fed by an MCAO system, by taking the most of the presence of correcting devices in the optical path.