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Glenn Gombert wrote:
> Do you think thay most if not *all* of the photometric errors are
> due to the FWHM of the psf varying across the image frame or are there other
> effects involved too??
> It soulds like flat-fileds don't contribue much to the large errors
> that are present when compared to Tycho reference stars...
> Glenn G.
After looking at some of the data I don't know if the difference is due
to the varying PSF or not. I looked at data from some of the SMSP A
fields. With dates ranging from Dec 1996 to May 1997 I see an
increasing slope from about 0.060 mags per degree in December till it
reaches 0.200 mags per degree in the May data. Then I took the camera
apart somewhat in July to get better focus ranges on the I filtered
cameras. Now the data I've been taking in August shows a reduced slope
of about 0.070 mags per degree.
This all makes it sound like "drooping" is the source of the problem but
I have not seen a good correlation of the slope with a corresponding
slope in the FWHM over the frame. I will continue to look at my data
more closely to see if the PSF changes in other ways that might give me
the slope I'm seeing.