[vsnet-chat 4484] Re: Apogee AP-7p

[Brian Skiff <bas@lowell.edu>]

     It is hard to know exactly what is going on with the Tycho-2 data
(avoid Tycho-1 entirely!), and with Seiichi Yoshida's data and reduction
process without both measuring the stars involved to good accuracy and
to know the reduction details.  It could well be that the Tycho photometry
is poor.  The nominal uncertainties for both stars is about 0.05 mag. in each
color.  The Tycho-2 paper shows that for stars fainter than about mag. 10,
the tabulated errors are _underestimated_ by half.  So, for example, an
uncertainty of 0.05 _really_ means +/- 0.075 mag. error; this is also a
"mean error of the mean", not always a reliable indicator of the true 
external error (and in my experience with the astronomical literature, mean
errors are always a flattering estimate of the true errors by a factor of
two or three!).
     As Arne Henden has mentioned, the best way to evaluate the various
CCD systems is to take special images of the Landolt standard-star fields,
where the data are of high reliability.  If the configuration is stable,
then this need be done on only a few nights to determine the basic color
terms and rough zero-point offsets.  This avoids problematic Tycho data
entirely, so that all the errors involved lie strictly with instruments
or reductions and not with the stars being observed.

\Brian

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