WAVE-FRONT ESTIMATION AT LOW-LIGHT LEVELS

A simple technique for reconstructing a wavefront from the measurement s of a Shack-Hartmann sensor is described. The optimal estimator used is analysed for the effect of errors in the turbulence model at differ ent light levels. The method is then analysed in situations where the size of the wavefront sensor is restricted by the number of available photons and a guide star is not available. The algorithm uses a priori statistics of the turbulence derived by observing a nearby bright sta r which can be outside the isoplanatic angle. This differs from previo usly published estimators since the a priori information is measured r ather than derived from a theoretical model of the turbulence.