;   Assorted glossary terms for UCAC-3.  These are used in ucac3.tdf
; and vizier.tdf.

~Overview of the UCAC-3 catalog
   The UCAC-3 (USNO CCD Astrograph Catalogue),  released in 2009,  is a
catalogue of over 100 million stars,  covering the entire sky at
^magnitude^s from about 8 to 16.  It's based largely on CCD observations
made from 1998 to 2004.  Data from many other catalogs were also used
for error detection,  determination of ^proper motion^s,  and to provide
additional ^photometry^.

   It is currently the most accurate full-sky star catalog in its
magnitude range.  (For fainter stars,  one will usually have to use
the USNO ^A2.0^ or B1.0 catalogues.  The data from these is not as
precise as that from UCAC.  But they do go fainter,  covering more stars.)

   UCAC-3 was preceded by UCAC-2,  which covered the sky south of
^declination^ +50 or so,  and by UCAC-1,  which covered the sky
south of declination -15 or so.

~Running number
   UCAC-3 uses a 'running number' designation scheme.  The catalog is
divided into 360 declination zones,  each 1/2 degree high;  the first
zone runs from dec -90 (the south ^celestial pole^) to -89.5,  and so
on,  up to zone 360 at declination +89.5 to +90.

   Within each zone,  the stars are in order of ^right ascension^.  So
'3U314-159265',  for example,  would be in zone 314 (declination
+66.5 to +67),  and would be the 159265th star within that zone.

   In addition,  UCAC-3 stars have an ^MPOS number^.  This seems to
be deprecated;  use of the zone/running number designation scheme is
preferred.

~SuperCosmos
   The UCAC-3 catalog was cross-referenced to the SuperCosmos (SC)
catalog.  UCAC-3 was made using CCD imagery;  SC was generated by
scanning in archival plates.  Thus,  SC was able to provide positions
from up to several decades ago,  which helped improve the ^proper motion^
determinations;  and some ^magnitude^ data,  in three ^photometric band^s,
was also made available.

   Be warned that the SuperCosmos magnitudes are probably not all that
good.  For stars as bright as those in UCAC-3,  most are saturated.
But it's probably better than having no magnitudes at all.

~MPOS number
   The MPOS number appears to be of interest only inside the US Naval
Observatory.  Use of the zone/^running number^ designation for UCAC-3
stars is preferred.

   The USNO gave an MPOS number to each star in UCAC-3.  This number
runs almost exactly in ^declination^ order,  from 3 (for the southernmost
star) to just under 140 million (for the northernmost).  There are
about 40 million gaps in the sequence,  due to stars that didn't make
it into the final UCAC-3.

   In addition,  there are about 50 000 stars with high ^proper motion^
added to the catalog at a later stage,  which have MPOS numbers from
140 000 000 to 140 051 297.

