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to be complete down to the limit of what the instrument in the
satellite could observe (about magnitude 12),  and measured the same
things as Hipparcos.  But its precision is considerably less than that
of Hipparcos.  The magnitudes and positions are still better than
those from any other catalog.
                                                             
   The Hubble Guide Star Catalog (GSC),  version 1.3,  is used for
dimmer stars,  down to about magnitude 14 or 15.  (This version is
also known as "GSC-ACT".) use in fine-tuning the aim of the Hubble
Space Telescope.  It is the most detailed star catalog widely
available,  with about 15 million stars.  However,  it does have
some oddities.

   It was generated by scanning in survey plates from two telescopes,
the Palomar Schmidt camera in California and the U.K. telescope in
Australia.  As with the SAO,  the main intent was to get enough stars to
be confident of having some guide stars in a given field of view.  In
dense areas such as Sagittarius,  they could get away with a magnitude
13 limit and would have the required density. In vacant areas like
Virgo,  it was necessary to go to mag 15 or 16.  That's why the limiting
magnitude of the GSC (and of Guide) varies as you move in the sky.

   Objects were automatically collected and classified as stars or
non-stars,  with varying degrees of accuracy (thus the switch to turn
off "Non-Stars" in the Star Display menu).  A more complete description
of how the GSC was made can be found in the information distributed
with the GSC and copied onto the Guide CD,  in the TABLES directory.

   For purposes of aiming,  you need accurate positions,  and the GSC
positions are indeed usually accurate to a better than one arcsecond.
However,  you don't really need accurate magnitudes for aiming.  Each
plate was calibrated using stars near the center of the plate;  the fact
that stars at the edge get distorted wasn't taken into account,  and
stars at the edges of plates get inaccurate magnitudes (usually about .5
magnitude errors). Also,  some plates provide magnitudes corresponding
to what a human eye would see ("visual" magnitudes),  while others were
sensitive mostly to red light,  making red stars seem brighter than they
would look to a human.

   For finder charts,  this is usually just enough of a problem to be
slightly annoying.  All the stars in a given region will be offset in
brightness by roughly the same amount,  and you will be able to use the
chart to find your way around the sky.  However,  if you want to
estimate the magnitude of an object by comparing it to a GSC star,  the
errors may become evident.

   Another problem with the GSC has to do with completeness.  While it
has millions of magnitude 14 and 15 stars,  it omits a few of the
brighter stars.  Again,  for purposes of aiming the Hubble Space
Telescope,  this did not upset the creators of the GSC very much.  But
it can be a little disconcerting for the rest of us.

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