The United States Naval Observatory (USNO) has officially released the
Second Edition of the USNO CCD Astrograph Catalog (UCAC2), the most
astrometrically precise star catalog ever produced by ground-based
instrumentation. UCAC2 comprises over 48 million stars with brightness
ranges from 8th to a limit of 16th magnitude and covers about 85
percent of
the entire sky from the south celestial pole to a declination of about
+40
degrees, with some areas covered to about +52 degrees. It is the latest
edition of an ongoing project that will be completed in 2005.

Astronomers measure the sky in terms of angular degrees, minutes, and
seconds of arc. The apparent size of the disc of the Full Moon is
approximately 30 arcminutes, or one-half of a degree. The apparent
size of
the disc of Mars at the upcoming opposition (the closest in recorded
history) will be just over 25 arcseconds, slightly less than one-half an
arcminute. One arcsecond is the apparent size that a U.S. penny coin
would
appear if it were viewed from a distance of about one mile (2
kilometers).

The positions of the faintest stars in the UCAC2 are known to an error
of 70
milliarcseconds (“mas”, or 70 thousandths of a second of arc), which
would
be equivalent to the width of the “I” in the word “LIBERTY” on our
penny a
mile away. The positions of the brighter stars increase in precision to
about 20 mas, better than those produced by the European Hipparcos
satellite
for its 2.5 million star Tycho-2 Catalog. In addition, data on the
colors
of the UCAC2 stars (known as “photometry”) is based in the reference
photometry of the 2-Micron All Sky Survey (2MASS), a collaborative
program
of which USNO is one of several partners.

The observations for the UCAC2 are being conducted with a modified
8-inch
(20-centimeter) aperture telescope that uses a 4096 x 4096 pixel
Charge-Coupled Device, or CCD. The telescope is now completing
observations
of the northern hemisphere sky from the USNO’s dark-sky station near
Flagstaff, AZ. Prior to its move to Flagstaff, the telescope spent
three
years observing the southern hemisphere sky from Cerro Tololo
Inter-American
Observatory in Chile, taking advantage of the clear, steady air offered
at
that site.

The raw data for the UCAC2 has been compiled from nearly 250,000
overlapping
CCD frames and contains over 4 terabytes (4 X 1012) of compressed image
data.

By measuring so many stars with such precision, it is possible to
re-examine
older photographic star catalogs to determine the “proper motions” of
the
stars on the plane of the sky. Proper motions have been obtained for
all
the stars in the catalog.

The UCAC2 is being distributed on three CD-ROMs. Information on this
catalog may be obtained from http://ad.usno.navy.mil/ucac/.

UCAC2 Facts At A Glance

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 has been constructed using the
Tycho-2 Catalog 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-2 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                91%   complete as of June 2003
southern hemisphere location:  CTIO, Chile
northern hemisphere location:  Flagstaff, Arizona
------------------------------------------------------
average density           1600    stars / square degree
catalog accuracy          20 mas  R = 10 to 14 mag
                       30..40 mas  R =  9 mag, 15 mag
                           70 mas  R = 16 mag = limit
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The following table gives details about the amount of data acquired up
to 10
June 2003. 







---------------------------------------------------------------
Project time so far       5.4 years   1998 Feb - 2003 June
Total number of frames      243,000   including rejects
Raw data                     4.0 TB   compressed FITS
Backup on                 > 1100      exabyte tapes each for 2 copies
> 6000      CDROMs (single copy)
Number of stars           > 58 million (with at least 2 images)
------------------------------------------------------