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Re: photometry software problem > This shows some of the problems associated with working at low signal/noise > or effectively high sky background. The concept of "average populated" > area does not hold, since the stellar measuring aperture is always smaller > than the sky measuring aperture/annulus. Therefore, it is less likely that > there will be faint contaminating sources. This is not correct. The probablity of a contamination per pixel is the same as in the aperture and background sky. This is one of the renowned "pitfalls" of novice observers. The systematic effect from sky background measurement is not restricted to the stellar contamination. Non-uniformity in the background (from various reasons) also plays a role. I have seen a number of novice observers who try to find "the darkest region" in the same frame to measure the background. This always led to systematically brighter object magnitudes. In most extreme cases, some people even try to "find the location", with a cursory movement of the background aperture, which eventually gives a positive detection of the object. This is clearly wrong. > There are a hundred million objects that are bright and uncrowded that > a typical amateur can observe; why beat your head against the tough problems? Because there is a necessity. If every square kilometer on earth is covered with a 1 meter telescope dedicated to variable star observing, we probably don't need to tackle with faint objects ;-). However, observer will encounter almost the same thing when one tries to measure a 22 mag star with a 1 meter telescope. A better solution is not to avoid problem, but to find the best way to measure such objects close to the limit of the instrument. A very good historical example would be AL Com. This object is near 21 mag at quiescence. Several observers tried (even with Palomer 5 meter) to find the periodicity in quiescence. This history can be easily tracked with the ADS, and you will see how the reduction technique affected the results, even with the same KPNO telescope. There *are* some people with best measuring techinque, and some are unfortunately not: this is even true for professional astronomers. There must be a feasible way for everyone, unless people intentionally want to avoid learning it. By the way, this is not the original intention of this thread. Measurements with S/N < 3 will be a challenging case to everyone, but the discussed problem with commercial software package frequently occur at much higher (S/N about 10 or even more) level. This is likely from a software problem, and needs to be fixed. Regards, Taichi Kato
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