Laser Guide Stars for Extremely Large Telescopes: Efficient Shack-Hartmann Wavefront Sensor Design using Weighted center-of-gravity algorithm

摘要

Over the last few years increasing consideration has been given to the studyof Laser Guide Stars (LGS) for the measurement of the disturbance introduced bythe atmosphere in optical and near-infrared astronomical observations from theground. A possible method for the generation of a LGS is the excitation of theSodium layer in the upper atmosphere at approximately 90 km of altitude. Sincethe Sodium layer is approximately 10 km thick, the artificial reference sourcelooks elongated, especially when observed from the edge of a large aperture.The spot elongation strongly limits the performance of the most commonwavefront sensors. The centroiding accuracy in a Shack-Hartmann wavefrontsensor, for instance, decreases proportionally to the elongation (in a photonnoise dominated regime). To compensate for this effect a straightforwardsolution is to increase the laser power, i.e. to increase the number ofdetected photons per subaperture. The scope of the work presented in this paperis twofold: an analysis of the performance of the Weighted Center of Gravityalgorithm for centroiding with elongated spots and the determination of therequired number of photons to achieve a certain average wavefront error overthe telescope aperture.