Public Affairs Office
U.S. Naval Observatory
Washington, D.C.
Information Contacts:
Geoff Chester
USNO Public Affairs Office
Dr. Norbert Zacharias (UCAC2)
USNO Astrometry Department
FOR IMMEDIATE RELEASE: August 8, 2003
USNO Knows Where the Stars Are
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 10**12) 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
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
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)
Images:
The UCAC 8-inch (20-cm) Astrograph
|
Sky coverage of the UCAC2
|
| 
