[vsnet-chat 3542] Re: GSC 6780-00497

[Brian Skiff <bas@lowell.edu>]

     The GSC and USNO-A catalogue magnitudes are rigged for point sources,
so when a galaxy appears, as in this case, the magnitudes come out wildly
much too bright.  Try it for some other galaxies around mag. 13-14 (or even
fainter) to see that this is the usual thing.
     Sebastian flags the GSC magnitude as "9.55p".  Note that in the south
the GSC magnitudes away from the galactic plane are from blue-green sensitive
plates, and are thus roughly "photo-blue", but in the north they are mostly
close to "photo-red", although all done "photographically".  Please do not   
interpret "photographic" magnitudes to mean anything other than their having
been measured on plates---they could be on any color system.  Takamizawa's
magnitudes are recorded in vsnet as "photographic", when they are in fact
yellow-light (roughly V) magnitudes.  Also, USNO-A magnitudes are not B and R,
as Sebastian flags them, but only "bluish" and "reddish", and certainly are
not on the standard B and R systems.  Finally, beecause of big systematic
errors in USNO-A (even on stars), I would not interpret colors very literally.
     These catalogues, as handy as they are, are loaded with pitfalls if they
are taken at face value, and this can lead to erroneous interpretations.
Be careful!

\Brian

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