Re: Flat fields are not the problem

[Glenn Gombert <gleng@infinet.com>]

        Michael has done some *excellent work*as usual in helping to advance
the cause of TASS data reduction. I guses that I won't *worry* too much
about flat-fileds any more.

        I am more than willing to help with whatever analysis tasks /
experiments that would help us characterize these errors (and devise
solutions-whatever they might be). It seems that a combination of camera
setup and measurements under various conditions might be in order.

Thanks,
Glenn G. 

At 11:16 AM 8/22/97 -0400, you wrote:
>To all,
>
>There has been a lot of discussion lately about getting proper flat
>fields for TASS cameras.  I now think our photometric problems are NOT
>flat field related and will not be cured with better flats.  I have come
>to this conclusion based on the following.
>
>To date both Tom and I have been using sky backgrounds to generate flat
>vectors for use in the star program.  These flat vectors are typically
>slowly undulating and look just like what you would expect if a two
>dimension flat with dust doughnuts and some vignetting were squashed to
>one dimension.  What you may not know is that these flats typically show
>a 5 percent total range, peak to trough.  If these flats were total
>fiction, and I don't believe they are, they could contribute at most a
>0.015 mag SIGMA error.  Since we have a lower error limit of 0.050 mag
>sigma the problem MUST lie elsewhere.  Last night I believe I found the
>source of the problem.
>
>I was reprocessing several of my earlier images against a catalog of
>good Tycho stars.  Good in this case means sigvt and sigbt < 0.050 and
>no other Tycho star within 10 pixels.  This catalog should then give
>good astrometric and photometric solutions.  For the images I was
>processing this resulted in approximately 20 good Tycho stars per image. 
>
>I then compared the magnitudes of the Tycho stars and the instrumental
>magnitudes and found they differed by about 0.140 mag sigma, much worse
>than I expected.  I then graphed the mag difference as a function of RA,
>result - a random scatter plot.  I then graphed the difference as a
>function of Dec, voila - a straight line!  The error was a very strong
>function of declination!  A straight line fit showed a slope of 0.160
>mag per degree.  That's right, a difference of about 0.450 mags from one
>edge to the other!  This same slope was repeated in subsequent images on
>the same night.
>
>I then used the computed slope to correct the instrumental magnitudes
>and achieved a difference between the Tycho magnitudes and the corrected
>magnitudes of 0.060 mag sigma.  Since the Tycho stars in the comparison
>were good to only 0.050 mag sigma this is a good match.
>
>What could cause this error slope?  Could it be a hidden flat fielding
>problem?  In this case, again no and here's why.  If the problem were
>flat related then the slope seen above would be the same no matter what
>size aperture was used to measure the magnitude.  In fact this is not
>true.  The slope for both the PSF based aperture and the 5x5 circular
>aperture were both about 0.160.  For larger apertures the slope slowly
>decreased until it was 0.095 for the 11x11 circular aperture.  This
>indicates that the PSF was quite different from one side of the chip to
>the other.  The larger apertures were able to partially compensate but
>not entirely.
>
>What does all this mean for our future processing?  It means that if we
>want to break the 0.050 mag sigma limit we must adjust the instrumental
>magnitudes by a low order fit against a catalog like the good Tycho star
>list I used above.  We probably don't have to worry about getting
>non-sky flats since the fit above would adjust for any low order errors
>in the flat itself.
>
>Comments?
>
>Mike G.
>
>
Glenn Gombert <gleng@infinet.com>