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Buglog #344
- Thursday, August 01 1996 @ 03:00 PM GMT
- Contributed by: admin
- Views: 1,196
MODULE: imcombine, combine
SYSTEM: V2.10.2-V2.10.4
SYSTEM: V2.10.2-V2.10.4
NUMBER: 344 MODULE: imcombine, combine SYSTEM: V2.10.2-V2.10.4 DATE: Thu Aug 1 09:00:22 MST 1996 FROM: valdes BUG: When there are exactly three images, mclip=yes, and scales, zero levels, or weights are computed and are not all the same there is a bug that will confuse the scales and weights for about 1/6 of the pixels. This becomes more significant as the scales/zeros/weights differ more from each other. This affects computation of the weighted average (combine=average) and the sigma clipping algorithms (ccdclip, crrej, sigclip, avsigclip, and pclip). In addition, even if the scales/zeros are all the same but the gains and read noise values are different for the ccdclip or crrej algorithms the gains and read noise values will be misapplied in 1/6 of the cases. The bug is that when mclip=yes the pixels are sorted and the idenification of which image each pixel came from is kept. When there are exactly three images an explict sort is done but in one of the six possible orders of the 3 pixels the identification mapping to the image is wrong for two of the pixels. Thus the wrong scale, zero, weight, gain, or read noise will be used. If they are equal there is no problem but if they are not equal the weighted average and the scaling of the sigmas in the clipping rejection algorithms will be wrong. If the values are close there is still little error. The effects of this are probably negligible in most cases unless the scales or CCD parameters of the images are grossly different. The only workaround is to set mclip=no. STATUS: Fixed for the next release expected to be V2.11.
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