Return to Home Page
Re: problems with AIP4win Again this is my reply: === > 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. If this is the actual implementation, your analysis would be right, if there is a comtaminating object inside the annulus. I such cases, we usually use PSF fitting rather than complicating aperture photometry, but the "unusual" behavior of AIP4WIN even in isolated targets will probably require additional explanation. > 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. Yes, this is particularly true if PSF fitting with a constant PSF (rather than aperture photometry) is involved. We usually correct this small variation within the field by an empirically determined method. Under the constant instrumental setup, this correction is found to be very stable. However, this effect will not explain the systematically fainter magnitudes for faint objects, though... Anyway, we must more precisely learn how a specific software package works, and must know the behavior before completely trusting the data. Regards, Taichi Kato
Return to the Daisaku
vsnet-adm@kusastro.kyoto-u.ac.jp
