;   Assorted glossary terms for  UCAC-4.  These are used
; in  ucac4.tdf and vizier.tdf.

~Overview of the UCAC-4 catalog
   The UCAC-4 (USNO CCD Astrograph Catalogue),  released in 2012,  is a
catalogue of over 100 million stars,  covering the entire sky at
^magnitude^s from about 8 to 16.  UCAC-4 (and its predecessors,  UCAC-1,
UCAC-2,  and UCAC-3) are 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-4 has about five times as many stars as the predecessor ^GSC^
catalog,  with considerably greater accuracy and more data (such as
^proper motion^s and ^magnitude^s in other systems, as well as
cross-references to other catalogs).  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 or as detailed as that from UCAC;  it's also
much more error-prone,  with many "stars" that aren't actually stars.
But these catalogs do go considerably fainter.  A2.0 includes about
five times as many stars as UCAC-4;  B1.0 has about a billion stars.

~Running number
   UCAC-4 uses a 'running number' designation scheme.  The catalog is
divided into 900 declination zones,  each 1/5 degree high;  the first
zone runs from dec -90 (the south ^celestial pole^) to -89.8,  and so
on,  up to zone 900 at declination +89.8 to +90.

   Within each zone,  the stars are in order of ^right ascension^.  So
'4U314-159265',  for example,  would be in zone 314 (declination
-27.4 to -27.2),  and would be the 159265th star within that zone.

   In addition,  UCAC-4 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-4
(and UCAC-3) stars is preferred.

   The USNO gave an MPOS number to each star in UCAC-3 and UCAC-4.  This
number runs almost exactly in ^declination^ order,  from 3 (for the
southernmost star) to about 182 million (for the northernmost). There
are about 80 million gaps in the sequence,  due to stars that didn't
make it into the final UCAC-4.

~APASS
   The ^AAVSO^ has been working on the AAVSO Photometric All-Sky Survey
(^APASS^) to get good photometry in five photometric bands (B, V, g, r,
i) for stars of magnitude 10 to 17.  The idea is that it will cover the
sky to about the same depth as ^UCAC-3^ and ^UCAC-4^.  Details may be
found at the ^AAVSO's page on this project.//xhttp://www.aavso.org/apass^

