more evidence for errors in photometry due to position in field

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Date: Sat, 19 Aug 2006 13:57:08 -0400
From: Michael Richmond <richmond@stupendous.cis.rit.edu>
To: tass@tass-survey.org
Cc: mwrsps@rit.edu
Subject: more evidence for errors in photometry due to position in field


  I've mentioned many times before that one problem with the
Mark IV photometry is that there are systematic errors due
to the change in the PSF across the field -- especially in the corners
and very edges of each image.  This is not news.

  There is Yet Another piece of evidence for these effects in
the latest section of Tech Note 102 (appended to previous work).
I examine areas of the sky measured by two Mark IV units,
so that stars appeared near the top edge of the field as seen
by one unit, but near the bottom edge of the field as seen
by another.  The differences in the measured magnitudes show
clear trends as a function of Declination (a stand-in for
position in the image), and as a function of magnitude --
biggest errors for faintest stars.

  I think I will try one more time to see if I can make a
map of these errors, then correct for them.

                    Michael