[vsnet-chat 448] Re: SS Cyg: comparison between sequences

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

     According to the printed volume 1 of the documentation, standard V was
calculated from VT and (B-V)T, but also Cousins V-I, when it was available,
since the V-I color is a more nearly linear indicator of star temperature.
They provide a rough transformation for little-reddened star of ordinary
color that they claim adds about 0.015 uncertainty to the nominal e_VT.
So to first order, V_err = sqrt(V_T_err^2 + 0.015^2).  Looking at the 
Strasbourg VizieR display of the Tycho data, e_BT seems to be calculated
independently, so presumably e_(B-V)T is a reliable indicator of the
uncertainty.  However, again reading the volume 1 documentation, for faint
stars, particularly red faint stars, the B sensor often registered only
noise.  Thus they trimmed out the worst of the data to get some kind of B
magnitude.  The net result is that for fainter red stars, the (B-V)T is
much too blue, since in effect the noisier "red" colors were omitted from
the mean (or thinking of it another way, the B mags. are systematically too
bright for these cases).

\Brian

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