[vsnet-chat 2966] Re: Tycho - Tycho 2 comparison.

[Brian Skiff <bas@lowell.edu>]

     I see Sebastian's problem now.  The case of GSC 8659-0930 is a good
example of why individual entries in on-line databases aren't always utterly
trustworthy.  The query centers around why this star is so much brighter
in Tycho-1 than in Tycho-2.  The basic data are:

Tyc-1  VT=10.15 +/- 0.14   BT=10.95 +/- 0.19
Tyc-2  VT=10.80 +/- 0.06   BT=10.99 +/- 0.05

Although for stars near the faint end of the Tycho-1 catalogue (like this one),
the magnitudes are usually too bright by ~0.2 - 0.3 mag., in this case there
is quite a large discrepancy.  However, a look at the digitized sky survey
provides a probable explanation:  the star lies less than 2' from the first-
magnitude star beta Cru.  I would be willing to bet that scattered light in
the spacecraft optics caused the Tycho data to have problems.  The reduction
for Tycho-2 was done differently from the start, and evidently there was less
of a problem with scattered light, so the magnitudes are fainter---and
probably more nearly correct.
     I show the uncertainties for each measurement above to point out again
that stars with errors > 0.05 mag. in either BT or VT should _not_ be relied
upon.  These errors are _undestimated_ by 50% according to the recent Hoeg
et al. paper, so even for the Tycho-2 data, the uncertainties are +/- 0.09 mag.
in both colors.  Might be suitable for visual use, but that's about it.

\Brian

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