Performance improvements of Shack-Hartmann sensors with keystone design lenslet arrays

Shack-Hartmann based wavefront sensors, used to compensate atmospheric turbulence, appear to be less sensitive than curvature based wavefront sensors by more than a magnitude. Besides their read noise free APD detectors and different measurement principle, the sensitivity of curvature sensors may benefit from the keystone shaped lenslet geometry which optimally balances aperture coverage and illumination of individual subapertures. This paper describes the implementation of a keystone shaped lenslet array for the ALFA Shack-Hartmann based AO system. We compare the novel design with hexagonal shaped lenslets under different operating conditions such as selected modal basis set and number of compensated modes theoretically and in practice.