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Re: problems with AIP4win This is from Bos' reply to my comment: === By all means forward my opinions to the VSnet discussion group (I must join). When I have some time I intend to do some more experimentation with AIP and some of the other packages. >From what you describe there seems to be a non liniarity somewhere in the relationship between the algorithm and the actual pixel values. Berry's and Burnley's book (Chapter 8.2.2 Extracting the Star Brightness page 287) give a reasonable description of what they do with the algorithm, and they admit that under certain circumstances (stars in the sky annulus) the sky back ground subtraction will be to high. Furthermore the calculation is based on the average pixel value with the top and bottom 20% removed, so if the sky annulus is contaminated by light from the variable (AIP4Win has no inner sky annulus it is set by the star aperture) the average pixel value of the sky annulus for the variable would change as it gets fainter. Anyway that is how I see it I may be wrong, but I would prefer to keep the inner sky annulus away from the star aperture Incidentally when you change the aperture sizes in Munipack the delta magnitude also changes. Rudolf Novak reports this problem with Minipack in a read me file with the software. The only explanation he can give of is a chromatic effect coursing the PSF of red stars to be different of those of blue stars. Many amateurs use SCT which have a small residual chromatic aberration but I have also found this effect with normal Cassegrains when running test data to determine the best aperture settings. The telescopes curved field would also see the PSF to change as you move away from the optical axis. Cheers Marc Bos Mt Molehill Observatory NZ
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