First Light for UCAC

U.S. Naval Observatory, Washington DC, USA

(this text was written in February 1998)


The U.S. Naval Observatory has installed a 0.2-meter astrograph at Cerro Tololo and has started a program to determine positions of stars between 7.5 and 16.5 magnitudes from declinations -90 to +2 degrees. This is not the first time the U.S. Naval Observatory has operated a telescope in Chile. In 1849, an observatory was erected on the hill of Santa Lucia, near the center of Santiago, by Lt. James M. Gilliss and made observations until 1852. The current project will form a star catalog called the U.S. Naval Observatory CCD Astrograph Catalog - South (UCAC-S).

 The Hipparcos Catalogue defines the optical reference frame and contains positions of about 118000 stars, accurate to about 10 mas at current epochs. The Tycho Catalogue, combined with available ground-based data, such as the Astrographic Catalogue, (USNO AC2000 catalogue), which allowed the creation of the USNO ACT catalog, currently extends this system of positions on the 20-50 mas level for one million stars to 11th magnitude. Positions of even fainter stars currently are available from Schmidt plate scans, with an expected accuracy on the 200 mas level (USNO A2.0 catalog).

 The UCAC project aims at a densification of the Hipparcos system towards many millions of stars on an accuracy level of 20 mas. The UCAC will improve the majority of Tycho positions at current epochs and will replace the Guide Star Catalog (GSC) for the Southern Hemisphere. A strong link to Hipparcos stars and extragalactic sources will be part of the UCAC observing program.

In 1990 the U.S. Naval Observatory Twin Astrograph obtained a new lens, corrected for the red spectral bandpass, which replaces the "blue lens". The detector is a Kodak 4k by 4k CCD camera. The "yellow lens" remained on the telescope and is now used as the guide scope with an ST-4 autoguider on an x/y-stage at its back end. The instrumental photometric system will be between V and R. A 4-hole Hartmann screen is used for focusing. The telescope uses a Boller and Chivens mount. A Pentium PC controls the telescope and is used as an interface to the camera for data acquisition. An HP workstation will be used for the astrometric reductions.

 The astrograph and it's support equipment arrived at CTIO just before Christmas, 1997. Unpacking of the crates was started on January 4th. With excellent support of the staff at CTIO, the astrograph had first light in the Southern Hemisphere on January 10th. The first light picture is a 30 second unguided exposure of the globular cluster 47 Tuc. A fraction (512x512 pixels = 1/64) of the full frame is available as a FITS file (dist/u3596d.fts) by anonymous ftp to

After a two week testing phase the project started on January 25th with astrometric calibration and extragalactic source fields. Regular zone observations will start in mid February.  

A 2-fold, center-in-corner overlap pattern of the entire Southern Hemisphere is planned. With a field of view of just over 1 square degree, observations of about 44000 fields will be necessary. On each field 2 exposures are planned, 100 and 20 seconds respectively. The 100 sec exposure frames will reach beyond 16th magnitude. Tycho stars will be used for the astrometric and photometric reduction of all CCD frames. The 20 sec exposures will saturate at about V=7.8m and thus allow the observation of most Hipparcos stars. This then gives the option for a block adjustment solution based only on the Hipparcos Catalogue. Additional long exposures (about 300 sec) in some 200 fields of extragalactic reference frame sources will provide 20 mas accuracy for 16th magnitude stars. Together with deep CCD frames of these sources at larger telescopes (mainly the CTIO 0.9m), a strong link to the ICRS (International Celestial Reference System) can be accomplished.

These additional observations also enable a block adjustment of all data using only the ICRF sources as reference stars. Thus a solution totally independent of the Hipparcos system can be realized for comparison at an epoch about 8 years after the Hipparcos central epoch. Possible systematic errors will be controlled by additional calibration observations with the astrograph of selected fields, utilizing reversal of the telescope, rotation of the camera, and using a diffraction grating in front of the lens.

 Astrometric reductions will run in parallel to the observing program. First results are expected after the first year of the program. The survey will start at the South Celestial Pole and work its way towards the equator with epoch differences of overlapping frames made as small as possible.

All raw data files will be archived on hundreds of exabyte tapes and thousands of CD-ROMs. The astrometric catalog will be only about 1 Gbyte and can be distributed easily. With an expected density of about 350 to 6000 stars per square degree, depending on galactic latitude, the catalog will be suitable for many applications.

Though the U.S. Naval Observatory's astrograph is one of the smallest telescopes at CTIO, it will provide the important astrometric link for the larger instruments.