[vsnet-chat 2964] Re: (fwd) Re: (fwd) Re: Tycho,Tycho-2,Hipparcos magnitudes

[Brian Skiff <bas@lowell.edu>]

     I have had another look at Sebastiao's messages, and I think we are in
basic agreement.  The examples he gave, besides showing odd colors from
Tycho-1, also are a good demonstration of the modest scale error in Tycho-1.
This results in the fainter stars being listed too bright.  This happened
because the processing usually omitted weak or non-detections, so the data
points that remained averaged out too bright (and too blue for red stars,
since the blue channel suffered more strongly from this).  The plots for
Tycho-2 in the previous post show that these problems have been largely
corrected in the newer data.  One must still watch for large uncertainties,
and that the uncertainties that are listed are _underestimated_ by 50 percent
(i.e. they are half again larger than claimed).  Data for stars where the
errors are given as > 0.05 mag. in either BT or VT should not be relied
upon.

\Brian

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