PSF

Overview

The point-spread function (PSF) of the telescope modulates the scene that the telescope is able to deliver to the observer. For objects that are much larger than this fundamental resolution element, an intimate knowledge of the PSF is not necessary to do high-precision science. However, many astronomical studies can be pursued only when we have an accurate understanding of a detector’s point-spread function (PSF). For example, astrometry and photometry of point sources, bright and faint, cannot be done with high precision without PSF-fitting. In a similar vein, weak lensing and other studies of objects that are close to the resolution limit can be very dependent on the fidelity of the PSF model.

Unfortunately, even though accurate PSF models are critical to many astronomical studies, there are many reasons that very few published papers make use of good PSF models. For one thing, it is hard to construct good PSF models. The WFC3/UVIS detectors are mildly undersampled, which means that an accurate PSF can only be constructed from a dithered set of data, and one must take exquisite care to accurately represent the sub-sampled nature of the PSF. In addition, the PSF changes with position across the field, both due to variations in optical distortion and variations in the thickness of the detector (related to charge diffusion). Furthermore, the PSF also changes over time due to secular and breathing-related changes in instrument focus. All of these issues make it difficult to have perfect knowledge of the PSF in an image a priori.

Even when accurate PSF models are available, it is hard to use them to do science. PSF models are most accurate in the individual flat-fielded frames (the _flt images), since the pixel values in these images are the only true and direct constraints that we have on the astronomical scene. Even so, because of the undersampling and detector artifacts, a single exposure is not able to contain all the information the telescope can collect about the scene. It is necessary to dither the scene by whole pixels and fractional pixels in order to fully constrain the astronomical scene that has been delivered to the detector. Unfortunately, the large distortion that is present in HST’s detectors makes it difficult to interrelate the _flt pixels in different dithers. For this reason, many users make use of the Drizzle software suite, which is designed to combine the individual distorted and undersampled exposures into a single composite image that has better sampling and no distortion. This resampling process can be done in a way that preserves flux, but it is very hard to perform this operation without introducing irregularities in the sampling or introducing correlations among the output pixels. All this means that it is hard to do high-accuracy PSF analysis on the drizzle product, so all the PSFs provided here are in the _flt/_flc frame.

Also see WFC3/UVIS MAST PSF Search page