[vsnet-chat 1323] Re: (fwd) U Gem H/T sequence (Schmeer)

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

     The Hipparcos/Tycho astrometry is for _equinox_ 2000 at _epoch_ 1991.25.
Even most professional astronomers do not know the difference between epoch
and equinox.  Almost always when you see "epoch" in amateur publications
(atlases, etc.) the term "equinox" is meant.
     The mag. 10 GSC stars are evidently not in the Tycho catalogue because
they are much fainter at V and B than in the redder color system of the GSC
in the north.  Tycho is nearly complete only to about V mag. 10.0, and
becomes progressively less complete as you go to fainter limits.  Note also
the large errors on the magnitudes among these fainter stars.  For instance,
the star Patrick mentions, is given as V = 11.75, but the uncertainty is
0.33 mag, so the true V magnitude has only a two-thirds chance of being
somewhere between 11.42 and 12.08---and a one-in-three chance of being even
farther off from the Tycho value.  Just as with the errors on the Hipparcos
parallaxes, you must look at the errors on the photometry in the H/T 
catalogue.  It is _not_ the be-all, end-all data source!
     Also, you may find it much more convenient to use the Strasbourg VizieR
utility to search for stars in the H/T dataset:

http://vizier.u-strasbg.fr/cgi-bin/VizieR

...or for folks in North America, there is now a mirror copy at Goddard:

http://adc.gsfc.nasa.gov/viz-bin/VizieR

...and also now at the ADAC in Tokyo:

http://z13.mtk.nao.ac.jp/vizier

In all three cases, to look at the H/T data, key-in catalogue "i/239" in
the box at item 1 on the starting page.

\Brian

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