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The spatial resolution of optical monitoring satellites increases continuously and it is more and more difficult to satisfy the stability constraints of the instrument. The compactness requirements induce high sensitivity to drift during storage and launching. The implementation of an active loop for the control of the performances for the telescope becomes essential, in the same way of astronomy telescopes on ground. The active loop requires disposing of informations in real time of optical distortions of the wavefront, due to mirror deformations. It is the role of the Shack-Hartmann wave front sensor studied by Sodern. It is located in the focal plane of the telescope, in edge of field of view, in order not to disturb acquisition by the main instrument. Its particular characteristic, compared to a traditional wavefront sensor is not only to work on point source as star image, but also on extended scenes, as those observed by the instrument. The exit pupil of the telescope is imaged on a micro lenses array by a relay optics. Each element of the micro lenses array generates a small image, drifted by the local wavefront slope. The processing by correlation between small images allows to measure local slope and to recover the initial wavefront deformation according to Zernike decomposition. Sodern has realized the sensor dimensioning and has studied out the comparison of various algorithms of images correlation making it possible to measure the local slopes of the wave front. Simulations, taking into account several types of detectors, enabled to compare the performances of these solutions and a choice of detector was carried out. This article describes the state of progress of the work done so far. It shows the result of the comparisons on the choice of the detector, the main features of the sensor definition and the performances obtained.
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