Re: experiments

[Michael Gutzwiller <deepsky@fuse.net>]

Glenn Gombert wrote:
> 
> Michael,
> 
>         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.


Mike G.