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together as "Ax.0".

   The Ax.0 catalogs list about a half billion objects.  Objects   
were found by scanning in wide-field photographic plates,  some of
them dating back to 1950.  Usually,  two plates,  one red-sensitive
and one blue-sensitive,  were used.  This means the catalog can give
an indication of the object's color (not always a very accurate one,
though) and that many spurious objects could be safely ignored:  if
an object was found on only one plate,  it was assumed to be an
error,  or a flaw in the plate,  or a passing asteroid or satellite.

   In this catalog,  and in B1.0 and GSC-2.3,  the magnitude data is
of mediocre accuracy.  USNO was very interested in finding positions
to within a fraction of an arcsecond,  and succeeded in this goal;
but good magnitude measurement wasn't a priority.  The documentation
for A1.0 states bluntly that "the photometry is as bad as it can be
while still being able to claim that the numbers mean something."

   The USNO B1.0 catalog was the successor to A2.0.  It contains     
about a billion objects,  with slightly better accuracy in both
positions and magnitudes.  In this case,  up to five plates were
scanned for each part of the sky.

   GSC-2.3 is the successor to the GSC-1.1 and GSC-ACT catalogs.
It resembles B1.0,  but was assembled by the Space Telescope
Science Institute (STScI),  in some cases from different sources,
and using different methods.  So it may catch some objects missed
by B1.0,  while missing a few that B1.0 includes.

   The UCAC-3 catalog is the third version of what will probably   
become the standard catalog of choice for stars down to about mag
15 or 16.  Its positional accuracy is quite good.  The version provided
with Guide is necessarily a compressed one,  with some of the data
omitted (such as infrared magnitudes copied from 2MASS and so on).
If you use the "Get UCAC-3 data from Internet" option,  then Guide
will be able to gather full data for that part of the sky.

   You may wonder why you would do this,  since Guide is already
showing you data from UCAC-3 already,  built into the DVD.  The
reason is that not all data,  to full precision,  is provided on
the Guide DVD.  Certain fields that don't matter for star charting
are omitted.  If you download UCAC-3 data,  then click on the
downloaded stars and ask for "more info",  you'll see much more
information than before (and also much more than could have fit
on the DVD,  which is why Guide doesn't show all that data already).

   CMC-14 goes about a magnitude deeper than UCAC-3,  with
slightly greater astrometric and photometric accuracy.  But it
only covers most of a band between declinations -30 and +50,
and has no proper motion data.

17c:  INSTALLING TO THE HARD DRIVE     

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