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.

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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
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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.

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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)
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