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Re: flatfields

Now I'm getting totally confused.  It would sure be nice to have a TASS
triplet so I can see some of these effects myself!
  Mike G. wrote:
>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.
  We've combined data from several TASS sites, with cameras pointed at
quite different declinations, and the data agree with the 0.05mag sigma.
This would be impossible if the slope you propose is true.  On the other
hand, Michael R. had trouble on his one good night with the Vermont triplet
with a slope of comparable size.  These hints at huge gradients, at least
on some of the frames, are very disturbing.

>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.
  All that you have shown is that the sky brightness, as seen after passing
through all of the optics, in your frames does not vary by more than 5 percent.
I could conceive of a situation in which you have vignetting in your lens
that is stronger (vignettes more) at lower declinations, and at the same
time your sky is getting brighter at lower declinations.  The two effects
would then compensate.  That is why I keep harping on some external, evenly
illuminated target so that you know exactly what illumination pattern the
camera is looking at.  It isn't that difficult to do some tests, and then
we know what is going on!  I don't like the idea of using sky-generated
flatfield vectors from cloudy nights or nights where the Milky Way passes
through the strip, even without considering the gradients from city lights,
moon, etc.  The problem just gets worse with the bigger fields expected
from the mark IV cameras.  We need to solve the problems now, not later.

Glenn G. wrote about photometric limitations.  It is certainly true that
the 'all-sky' measurements require photometric skies, and that transparency
variations due to clouds, dust, smoke, etc. will limit the accuracy of the
results.  Mike G. correctly pointed out that, if the sky variation is a
slowly varying function, it can be subtracted out and does not
effect the extraction process itself (the zero point of course may change
during a single frame or from frame-to-frame on non-photometric nights).
Clouds also affect the signal/noise since they dim the stars, and you
won't be able to go as deep on cloudy or moonlit nights.
  However, I observed in Indiana for many years before moving out
west, and claim that even Midwestern sites will see 10 percent or more
photometric nights.  That is why I separated the variability project into
two segments:  use the photometric nights to generate a master star list
with good mean magnitudes.  Then do differential photometry for each star on
all of the nights, using nearby companions to remove nightly transparency
variations, and setting the zero point for the ensemble with the master
star list.  The trick is in creating a good master star list, and that means
getting lots of data so that you can discard the discrepant measures.

  I still think TASS can do 1 percent photometry.  Hopefully, there are just
a couple of 5 percent effects now that can be discovered and solved so that
TASS can reach this level.  The closer you get to one percent, the harder
the problems will be to solve, but the more useful the data become.
Arne Henden                               Instruments/software/CCDs
US Naval Observatory Flagstaff Station    Cepheids/photometry/IR
P.O. Box 1149                             ftp:  
Flagstaff, AZ 86002-1149                  Voice: (520)779-5132
aah@nofs.navy.mil                         FAX:   (520)774-3626