Power spectral analysis (PSA) has long been used for parameter estimation from interferometric data. It is particularly useful when only a few parameters are to be obtained from an image, such as the estimate of stellar diameters, or the measurement of binary separations, intensity ratios and position angles. For example, PSA was used to measure the diameter of the debris from the explosion of SN1987a (Karovska et al. 1991) and estimate the asymmetry in the envelope (Papaliolios et al. 1989) from speckle data recorded over a three year period. Despite the very limited SNR in the data, accurate measurements were obtained from the time when the envelope was only a few milli-arcseconds in diameter, until it was over 40 mas. PSA has also been extremely useful for rapid screening of images to detect duplicity or asymmetries, where the images are complicated by a complex point spread function (as is the case for the atmospheric PSF or the HST PSF). We have applied PSA to a number of different parameter estimation problems from HST WF/PC and FOC image data. In this paper, we will describe our approach to applying PSA and a show an example of its use with WF/PC images of gravitationally lensed QSOs.