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Empirical PSFs and Distortion in the WFC Camera

This report summarizes two upcoming ISRs, one on empirical models of the WFC PSF and the other on the distortion solution. Before we can come up with an accurate distortion solution, we must first be able to measure accurate positions for stars. With a good PSF model, positions for well-exposed stars can be measured to about 0.01 pixel in a single exposure. To make the most of such accurate positions, we need a distortion solution which is at least as accurate. Distortion in the WFC comes from three primary sources: (1) the off-axis optics of the telescope, (2) a small perturbation introduced by each filter, and (3) a small irregularity in the CCD with a pattern that repeats every 68.270 columns. My solution addresses all three sources of distortion. I will describe how to access my solution and will also give advice on how best to use it. In addition, I will evaluate how breathing and secular variations introduce small additional distortions.