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Hello Kato-san-- I was in Nashville, TN last week and returned to find an very large number of messages concerning time- series photometry with AIP4Win. The routines in AIP4Win have been carefully written and checked, and under nearly all circumstances give good and reliable results. However, several important conditions must be met to produce good aperture photometry. 1.) Star images must be properly sampled. We have found that undersampled star images give poor results with front-side illuminated CCDs because the starlight must pass through the gate structures on the chip, and these acts as a geometrically variable filter. Undersampled images and tracking errors could give rise to a regular signal variation. 2.) Presence of a faint companion star between the diaphragm and sky annulus. If the star moves in and out of the diaphragm and annulus during a sequence, the signal will be less good and constant than would be desireable. While AIP normally deals well with background stars, in certain locations they can be troublesome. 3.) Poor seeing coupled with a small star diaphragm. This results in varying amounts of starlight falling inside the aperture, resulting in larger than desirable scatter in the photometry. 4.) Small star images having low amplitude. AIP's centroid algorithm requires at least three pixels to be above a threshold that is ~2 sigma above the sky noise. If there are fewer than three illuminated pixels, the star images are considered undersampled and will not give good results in any case. 5.) Incorrect dark-frame subtraction and flat-fielding can also create unexpected error amplitudes. We would be happy to examine the data set that is giving you problems. You could copy the raw dataset to a CDROM (I assume it is ~100MB or so of data) we would examine it to determine why you report seeing unexpectedly large errors. Please include a description of the AIP tool and the settings you are using with it. --Richard Berry
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