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Re: long term variation and instrumental drift

Michael R. wrote:
>  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.
  First, any such transformation should be done properly
somewhere in the future.  You need to be sure that you can
back out the current transformation and insert a better one.
Perhaps rerunning the pipeline is easy enough, or perhaps your
starlists already contain the specific coefficients used; sometimes
transformations are difficult to reverse.  The original concepts
for Mark IV databases were to leave the observations in raw form
and transform on-the-fly for that reason.
  Second, setting zeropoints is fine with Tycho stars; there
are enough of them to average out Bt/Vt->V,Ic transformation errors.
However, they are nowhere near good enough as "standards" and
should not be used for generating transformation coefficients.
  Third, I assume you are setting a single zeropoint for each
image.  Depending on the flatfielding, you can get quite radical
errors across a frame.  Likewise, clouds will effect parts of
a frame more than others, again skewing the results.  The only
solutions here are to break the frame into subframes and set
subframe zeropoints; fit a low-order surface to the deviations;
or avoid nonphotometric nights.
  Fourth, setting transformation coefficients per frame rather
than keeping them fixed for long periods of time is very risky.
Nonphotometric nights will skew things.  The particular set of
Tycho stars will skew things, especially in and out of the Milky
Way or other reddened regions.  Your transformation coefficients
are going to be weighted towards the fainter, higher error Tycho
stars.  The current Bt/Vt->Ic transformation is poor; I am redoing
it for tasscat, but for now, don't assume it is very good.  Note
that transformations are weighted strongly towards the reddest or
bluest stars, and those are the ones that the Bt/Vt transformations
will not work for.
  I can go on and on, sorry.  You have to be careful in both
astrometry and photometry once you want to place things on a
standard system.