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Re: (fwd) problems with AIP4win > When using AIP4Win, or even MIRA, on a target with a close companion (making > it difficult or impossible to exclude the companion from the background > annulus) I often wish the annulus could be 'uncoupled'. This would allow > the background measurement to be made from a nearby clear area. Does the > fact that this option isn't available (at least in the software I'm familiar > with) mean it is too dangerous to use, or just a limitation of the program? This is often dangerous, because this background selection does not compensate for the linear gradient (sometimes present, particularly if the sky is bright), but may be a choice when other selections are unavoidable. If you choose this background selection, you are recommended to take two or several sets of areas located symmetric to the object and comparison star. However, as is well-written in CCD photometry textbook, the "clearest" area is not the best selection for the background estimate. A selection of "average populated" area in the frame is a better representative, since the object's diaphragm should contain similar faint (usually invisible) background objects. If one chooses the "clearest" area, the brightness of the object is overestimated. When manually choosing the background location, please be careful to choose the same location with respect to the object between different frames. Regards, Taichi Kato
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