Re: Post flat processing

[aah@nofs.navy.mil]

Mike G. posted his proposed method of correcting flat fields.  In general,
it looks good, though I have a few questions and comments.
(1) I hope that this processing will be used *only* for already acquired
    data.  The far better approach in the future is to use a flatfielding
    system that doesn't need corrections.
(2) The process only works for photometric nights.  What are you proposing
    for the remainder of the nights?
(3) Why are you dividing the declination into 4 zones?  Do you see evidence
    of slope differences between the top and the bottom of the camera?  If
    so, I would assume a quadratic or higher-order fit to the entire
    declination coverage might be a better approach.
(4) While the median linear least squares technique does a nice job of
    rejecting outliers, I'd also like to see weighting on the basis of
    Tycho sigmaV and Star sigma(mag).  Does that subroutine permit weighting?
    If not, you might consider using a more standard least squares routine
    and iterate to remove outliers.
(5) Color transforms require iteration, since you need flatfieldcorrected
    instrumental magnitudes before you can obtain proper colors.  An
    alternative solution, depending on how many stars you have, is to
    use Tycho (B-V) to preselect a set of stars with similar colors to
    use in your fits.
(6) Since this adjustment is subject to change, you need to save the
    raw magnitudes in your output list as well as the adjusted magnitude,
    unless you can document clearly how to recover the raw magnitudes
    after the correction has been applied.
(7) You may find that the correction is pretty stable night-to-night,
    depending on how the original flatfield was created, and might be
    able to use the photometric nights to correct the nonphotometric
    nights.
(8) Michael R. mentioned the problem of correcting the I-band data.  Note
    that my transformations of Tycho data yield B,V,R,I magnitudes, so
    at some level you do have I-band measures for calibration purposes.
    However, you will have to limit the color range, and the fit isn't
    nearly as good as for V, which means there will be additional scatter
    in your median-lsq fit which may require a larger number of Tycho
    stars.  Michael's suggestion of just using Landolt standards for the
    I-band correction won't work since the Landolt standards don't cover
    the entire declination zone (especially for Glenn's southern
    coverage).
(9) There is definitely the chance of systematics arising from this
    process, but there are already systematics in the data from the
    improper flatfields.  You have to weigh one against the other.
    The best procedure IMHO is to get proper flatfielding on future
    scans, use those to set up the master photometry list, and use
    the earlier scans for differential photometry of any discovered
    variable stars, etc.  Or at least give future scans higher weight
    in the solution.
Arne