A combined approach to HST wave-front distortion analysis

Stellar images taken by the Hubble Space Telescope (HST) at various focus positions have been analysed to estimate the wave-front distortion. Rather than using a single algorithm, we found that better results were obtained by combining the advantages of different algorithms, which also permitted us to avoid some of their pitfalls. Each algorithm was thoroughly tested on computer simulations and its performance was evaluated. Gershberg-Saxton iterations were used to recover the pupil geometry appropriate to each image. Error minimization algorithms were found to be more efficient to recover low-order Zemike aberration terms and were used to estimate defocus and spherical aberration. Phase retrieval techniques including a modification of the Misell algorithm were used to refine our wave-front error estimate. These algorithms are described in details in a JPL contract report [1], We summarize here the results obtained so far.