Noise floor in Disk 16 data

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

  I've spent a little more time working on the images from 
Data Disk 16.  Arne provided some suggestions for the 
source of the "noise floor" of about 0.005 magnitudes.
I'm checking them out.

  One possibility was noise in the master dark frames.
Each one was created from only 5 dark images.  The histograms
of pixel values show a "messy" peak, with several sub-peaks.
Tom has explained this effect as due to the chip being run 
at higher-than-optimal temperatures -- see 

    http://a188-l009.rit.edu/tass/technotes/tn0065.html

  Anyway, I calculated the contribution of the noise in the
dark frames to the total uncertainty in aperture photometry.
It turns out that the noise from the dark frames is a bit 
smaller than the noise from the sky, and contributes only
about 0.0005 mag to the overall uncertainty for bright stars
in both V and I.

  Arne also mentioned that I cut off all magnitude measurements
at thousandths of a mag.  That introduces another 0.0005 mag
of noise.

  So, these first two sources don't add up to the 0.005 mag
"floor" I found in the data.  I'll keep working ....

                                       Michael Richmond