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I don't understand why this would be better. If you are doing photometry on a star too dim to centroid, you are in big trouble. As users of daofind know, you can find statistical stars at levels that are hardly visible by the eye. I can't imagine that centroiding is the problem here. The opposite problem seems much more likely to me: if you use only the C star to find your position, and the frame rotates under you due to bad polar alignment or alt/az-type issues. I'm not sure if AIP re-centroids these stars or not, but I assume it must, which would make this a non-issue as well. The aperture moving completely off the star, though, is certainly possible, and has happened to most of us. The crime there is not noticing. Sometimes you can't trust those programs to find the star for you, especially if it wiggles around a lot. In this case it usually does not slowly drift off the star, though, if gets way off very fast. From what I've heard said so far, I can't imagine that AIP is handling any of these situations incorrectly. Michael koppelman On Tuesday, July 8, 2003, at 05:56 AM, Berto Monard wrote: >> The centering error on faint stars that Taichi-san mentions >> in AIP4WIN can be avoided altogether by doing photometry as >> C-Offset where the comparison xtar (presumably bright >> enough to be reliably centered on all images and the >> variable and check are not centered except on the first >> image of the set and the apertures are rigidly placed in >> the same location relative to the comparison star. > > Thanks, Lew! Is everyone using this option? (In precise > photometry, > I regard this option as a default -- we use multiple stars to > compensate > a small-order deviation from a constant offset. With an Alt-Az mount, > one also need to incorporate image rotation). Use this option whenever > applicable. If everyone uses this option and still gets unrealistic > depressions in the light curve or other unusual features, we should > search > for a different explanation -- in any case, this would provide a > critical > test for the software.
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