Restoration of Long-Exposure Full-Disk Solar Intensity Images

Abstract

We describe an algorithm for restoring full-disk solar intensity images blurred by a smooth, quasi-stationary point-spread function (PSF). For Earth-based observations, this type of data can be obtained by using exposure times that are much longer than the redistribution time of the atmosphere. Using simulated data for a wide range of observing conditions, we show that the algorithm restores data in such a way that the RMS difference between an original, unblurred image and the restored image is typically less than 1.0%. Thus, we substantially improve the photometric precision. The simulations also show that under ``reasonable'' seeing conditions (<~4"), exposure times of 5-10 s are adequate to produce smooth calibratable PSFs if the observing instrument uses a centroid-shifting tip/tilt wavefront correction. The algorithm determines the PSF for each observation directly from the recorded image and does not require separate measurements of point sources.