Re: long term variation and instrumental drift

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

  The way the pipeline does photometric calibration, I don't
see instrumental effects being very important.  The measurements
of Tycho stars in each field are used to transform the relative
magnitudes to pseudo-standard magnitudes.  Most instrumental
changes would affect the Tycho stars in the same way as the
stars of interest.

  One sort of effect that might build up over the long term
is dust on the lens -- it might cause some small color changes.
Hmm... But again, the pipeline removes first-order color terms
on an image-by-image basis, so _this_ probably wouldn't amount
to much.

  Still, I wouldn't DARE to suggest that there are no possible
long-term effects that might mimic long-term variabibility. 
I've seen too many wierd combinations of unseen circumstance
to make such a boast :-)

                             Michael R