quick notes on Data Set 20

[Stupendous Man <richmond@a188-l009.rit.edu>]

  I'm working on images from DS20; my first goal is to compare the
photometric precision and accuracy of analysis with night-sky flats
vs. analysis with light-box flats.  Some early results:

      - light-box and night-sky flats do have significantly different 
              "shapes"

      - light box flats are pretty consistent after box is rotated
              by 90 degrees; the ratio of rotated to non-rotated
              flats differed from unity by at most 0.06% in V,
              1.3% in I

      - I'm pretty sure that the filter names are swapped from their
              proper values in the FITS headers for disks 6,7,8.
              That is, files with names like

                  hvra2239820.fits      and   FILTER = 'V'

              are really I-band, and vice versa.  
              I haven't checked the values for the other disks...

  More notes later.  Just thought people might like to know about
the FILTER mixup -- if I'm wrong, please correct me!

                                          Michael