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Re: Software problem in time-series photometry > 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. If every software package uses the same algorithm as in daofind, centroiding would not be a serious problem. Otherwise, it would be a problem depending on the actual algorithm employed. It might be possible that some software package may be more strongly affected by the background scatter or systematic trend -- though it's only a speculation. > 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. In this case, the centroiding error will grow with an increasing function with time. This is not what is observed. What is rather observed is that "once the object is measured systematically fainter, the effect last for a certain time in the subsequent images". > From what I've heard said so far, I can't imagine that AIP is handling > any of these situations incorrectly. When I noticed the observers on the potential problems on their data, I see similar replies quite often... You may not imagine, but queer things actually happen. If all algorithms (including that in daofind) you mentioned are properly incorporated in software packages, such queer phenomenon would not happen, or would be easily recognized, if any, by further analysis, since we know how daofind behaves and its limitation. Since it is not apparently the present case, something what you don't imaging may have been incorporated. Regards, Taichi Kato
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