[vsnet-chat 1417] Re: Star x near Nova Sco 1998

[bas@lowell.Lowell.Edu (Brian Skiff)]

     Chart-makers and CCD observers should note caerfully that "VT" and BT"
_are_not_ on the standard V and B systems, but are instrumental magnitudes
from the Tycho spacecraft.  The magnitudes and B-V colors are corrected for
and appear elsewhere in the dataset for any star.  The estimated 
uncertainties are also given explicitly for every star, and so the errors
can be evaluated at the faint levels.
     Thus for Morel's star 'x' near Nova Sco, VT=10.287, but standard V=10.09
+/- 0.11.  The Tycho data also show (standard) B-V=1.68 +/- 0.50 (!) for this
star.  Thus it appears that while the V magnitude might be okay, the color
is highly uncertain, and possibly just plain wrong.
     Once again, use the corrected V and B-V from the Tycho data, _not_ the
"raw" VT and BT.  Also, check the errors given for every star and avoid ones
with large errors.  If commercial star-chart software packages such as Guide
are giving VT and BT, then complain to the writers of it and ask that only
the corrected V and B-V be shown, since VT/BT are misleading data.

\Brian

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