Overview of UCAC project

The US Naval Observatory Twin Astrograph is used with a 4k CCD
for direct imaging of the sky with guided exposures.  A single
frame covers just over 1 square degree.  The astrometric catalog
is constructed using Tycho reference stars.

As part of the project, extragalactic radio reference sources
are observed with bigger telescopes and the corresponding fields
are observed simultaneously with long exposures at the astrograph.

The final UCAC catalog will have the option to tie in
directly either to the fainter half of the Hipparcos stars 
or to some 500 extragalactic sources as an alternative 
reference frame to the Tycho stars.

The following table gives details about the data acquisition.

number of exposures         2     per field
exposure times          25 & 125  seconds, guided 
observing throughput        13    fields/hour 
overlap pattern              2    fold 
number of fields        85,158    all sky 
sky coverage           complete   by mid 2004
sky coverage                95%   complete, September 2003
southern hemisphere location:  CTIO, Chile
northern hemisphere location:  Flagstaff, Arizona
average density           1800    stars / square degree
total                 35 million  stars / hemisphere
catalog accuracy          20 mas  R = 10 to 14 mag 
                      30..40 mas  R =  9 mag, 15 mag
                          70 mas  R = 16 mag = limit

The following table gives details about the amount of data
acquired until 2003 October 22.

project time so far       5.7 years   1998 Feb - 2003 October
total number of frames      257,000   including rejects
raw data                     4.1 TB   compressed FITS
backup on                 > 1100      exabyte tapes (single copy)
                          > 6000      CD--ROMs (single copy)
detection output             200      bytes/object 
number of stars         > 60 million  (with at least 2 images)