Improving the accuracy of a Shack-Hartmann wavefront sensor on extended scenes
Résumé
In order to achieve higher resolutions, current earth-observation satellites use larger lightweight main mirrors which are usually deformed over time, impacting on image quality. In the context of active optics, we studied the problem of correcting this main mirror by performing wavefront estimation in a closed loop environment. To this end, a Shack-Hartman wavefront sensor (SHWFS) used on extended scenes could measure the incoming wavefront. The performance of the SHWFS on extended scenes depends entirely on the accuracy of the shift estimation algorithm employed, which should be fast enough to be executed on-board. In this paper we specifically deal with the problem of fast accurate shift estimation in this context. We propose a new algorithm, based on the global optical flow method, that estimates the shifts in linear time. In our experiments, our method proved to be more accurate and stable, as well as less sensitive to noise than all current state-of-the-art methods. 1. Introduction Adaptive optics was originally developed for the field of astronomy to remove image aberrations induced by wavefronts propagating through Earth's atmosphere. Its task is to correct the aberrations of an incoming wavefront by using a deformable mirror in order to compensate for the distortion. It has been shown that adaptive optics allows to reduce these aberrations thus improving the image quality [1]. A key component of an adaptive optics system is the wavefront sensing mechanism. In astronomical imaging a wavefront sensing device is frequently used in conjunction with a deformable mirror in order to correct the undesired effects of atmospheric turbulence, thus improving the quality of sensed images. A Shack-Hartmann wavefront sensor (SHWFS) is one of such devices. It uses an array of lenslets to measure the deformation of the incoming wavefront. The shift of each lenslet focal plane image is proportional to the mean slope of the wavefront in the subaperture onto this lenslet, thus allowing to obtain a discrete local approximation of the slope of the wavefront, as shown in Fig. 1a. The measured slopes are then used to approximate the actual wavefront. This deformation is usually measured using point sources such as a star. Adaptive optics can also be applied in the context of earth-observation satellites [2, 3]. In this setting, the problem of atmospheric turbulence is negligible. However, lightweight mirrors are required to increase the resolution, since mission costs are driven by the payload weight. The drawback of such mirrors is that time-varying deformations due to thermal effects and vibration
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