[vsnet-chat 2974] Re: Tycho - Tycho 2 comparison.

[aah@nofs.navy.mil]

Brian mentioned:
>     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.
  Someone might not know why these errors are in question.  After
all, 0.05mag means a pretty good detection, right?  The problem
is that these are means, taken from ~100 individual measurements from
the satellite.  0.05mag error means that each individual measure had
0.5mag Poisson errors, or S/N=2!  They did some *very* fancy corrections
to account for non-detections and the like, but the bottom line is that
everything in Tycho fainter than V=10 comes from detections that I would
have thrown out in my own personal reductions.  Amazing results,
considering.  But as Brian says, you use these measures at your own risk.
Arne

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