Tech Note 100: Check on Mark IV photometry around IO Aur

[Stupendous Man <richmond@stupendous.cis.rit.edu>]

  Since there has been quite a bit of discussion about the quality
of Mark IV photometry in the field around IO Aur (and a new eclipsing
binary nearby), I decided to take a look at the measurements.
I examined both the "raw" Mark IV data for stars in a half-degree
region around IO Aur, straight from the database, and the same 
measurements after a round of ensemble photometry.  You can find
pictures and charts and my conclusions in Tech Note 100,

      http://stupendous.rit.edu/tass/technotes/tn0100.html

  Here is my executive summary:

    * the "raw" Mark IV measurements in the Mark IV database 
      can have systematic errors of up to 0.20 mag

    * ensemble photometry significantly improves the results in V-band, 
      reducing the systematic errors to less than 0.02 mag, but 
      doesn't help the I-band data as much in this particular region

    * given the small amplitude of the new eclipsing binary, and its 
      relatively faint V-band magnitude, it should not show a 
      signficant pattern in Mark IV V-band data ... and it does not

    * since the new star is bright in I-band, one would expect to see 
      evidence for variation in Mark IV I-band data. An ensemble solution 
      shows that it has a larger-than-typical scatter from the mean, 
      but the remaining errors in photometry are large enough that one 
      can't see the variation clearly


  Let me suggest that people view the Mark IV data in its current
form as a "chain saw".  It is blunt and suited for rough tasks
only.  It can be dangerous to those who have little experience with it,
but when wielded properly, can perform some useful tasks.
In this context, "when wielded properly" can sometimes mean "when given
some non-trivial post-processing".  It is certainly NOT the right
tool for every job, and poses no threat to table knives, butter knives,
scalpels, cross-cut saws, or scissors.

                                             Michael Richmond