Re: [TASS] Flat fielding

["Arne A. Henden" <aah@NOFS.NAVY.MIL>]

Andrew Bennett asked a number of good questions.  I wish
we had a FAQ so that new readers would have a quick way
of getting up to speed!  When I joined in, there were few
enough emails that I could read everything before inquiring,
but now with the thousands of postings that have occurred,
it is much more difficult.  Perhaps someone who has some
spare time could consider such a task.
  Andrew wrote:
 >>  Five percent standard deviation is acceptable,
>Never!
  Never say 'never'...  Remember, what we are considering
here is standard deviation of a single point from the mean,
and with all-sky photometry, not differential photometry.
With a good site, good telescope, CCD, etc. like what we
have at Flagstaff, you can usually get 0.01-0.02mag for
such errors on a good night with lots of standard stars.
TASS does not have such luxury, and because of its limitations,
I'm willing to accept that a single point can be 0.05mag in
error.  If you are not, then come up with a better method.
>  Techniques such as those described by C. Alard &
>R. H. Lupton, astro-ph/9712287, ApJ 503, 325 1998
>claim to reduce the relative error for two
>measures of the same star to close to the photon
>limit. Has anybody tried this with TASS data?
Alard and Lupton's method is for differential photometry.
After you have mean magnitudes for objects (such as from tenxcat),
then you can consider improving the determination of the
magnitude of any given object with respect to its neighbors.
There are a number of methods for performing differential
photometry that should give better results than the 0.05mag
number quoted earlier.  As far as I know, other than perhaps
Glenn using the difcal program that I wrote, no one has tried
to reprocess any variable candidate with local differential
photometry.  They should.  However, that has no bearing
on tenxcat.
>>  What *does* worry me is that the mean magnitudes and
>>colors in tenxcat are not asymptotically approaching
>>the Landolt magnitudes.
>Um. Er. They've been fed in twice and still fail to
>emerge at the far end? Statistician needed!
  I don't understand your statement.  Expand a bit, please!
Arne