How to measure precisely the position of a star in an HST ACS image? The imager is not well sampled, so you can't just interpolate a smooth PSF model to the sub-pixel offset that is relevant. You have to build a pixel-convolved model for every sub-pixel offset. That is annoying! After figuring out how unlikely it is that our current software (not written by us) is doing this correctly, we returned to the issue of the high-order polynomial terms. In the morning, Robert da Silva (UCSC) gave a very nice talk about the observational consequences of the fact that galaxies form stars in bursts and therefore provide stochastic illumination.
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