Next: A New Phase Up: Better HST Point-Spread Functions: Previous: Introduction . The PSF for an optical system is determined by the amplitude and phase of the (approximately) spherical wavefront as it converges on the point of focus. ... Phase retrieval is the process of trying to recover the wavefront error (and possibly the amplitude as well) given a measurement of the PSF. Phase-retrieval methods have been used since the discovery of the aberration in the HST primary mirror (Burrows et al. ...