Quick note on Disk 18a

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

  I've been working on a new distribution kit for my Mark IV
reduction pipeline.  I grabbed the first few images off Data Disk 18a
to test the code, and noticed something odd.
 
  If one makes a histogram of the pixel values in the raw images
on Disk 18a, one finds

       - the distribution of pixel values in I-band images shows
             a small peak every 8'th value, and a very large
             peak every 64'th value

       - the distribution of pixel values in V-band images shows
             a small peak every 2'nd or 4'th value; the amplitude of 
             the variations is smaller than those in the I-band images

  Perhaps some sticky bits in the ADC?

  I have to be careful here -- the pipeline uses an XVista program
called "sky" to calculate the sky value in each image.  That program
computes a single "sky" value for the frame by fitting a gaussian
to the distribution of pixel values.  The results of this fit may
not be well defined when there are big jumps every N'th pixel.

  It helps to combine the data into bins at least 8 values wide,
but that leaves the every-64'th-pixel peaks.  I can't make the bins
64 values wide without losing resolution -- I think.

  Anyway, just a warning to other data munchers.

  On the bright side, when I ran the pipeline on a set of 8 images
in the middle of disk 18a -- which did show evidence for clouds --
it successfully matched up every one with stars from the Tycho catalog
and found a decent astrometric solution.  Rah!  

  I discovered that I need at least 45 or so reference stars per 
field in order to achieve success (with a 3rd-order polynomial distortion
model).  I wasn't getting that many Tycho stars when I cut the catalog 
at sigma(Vt) <= 0.05 mag, so I relaxed the requirement to 

       sigma(Vt) <= 0.07 mag and sigma(Bt) <= 0.07 mag

This gives me enough reference stars in every field I've checked
so far.  

                                      Michael Richmond