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Zacharias, N., Rafferty, T. J., & Zacharias, M. I. 2000, in ASP Conf. Ser., Vol. 216, Astronomical Data Analysis Software and Systems IX, eds. N. Manset, C. Veillet, D. Crabtree (San Francisco: ASP), 427

The UCAC Astrometric Survey

N. Zacharias, T. J. Rafferty, M. I. Zacharias1
U. S. Naval Observatory, Washington DC


The USNO CCD Astrograph Catalog (UCAC) project is a high precision, astrometric survey. Observations started in January 1998 at Cerro Tololo (CTIO). A preliminary catalog of 30 million stars has been prepared, covering most of the area between declinations $-$90 and $-$15. It has a 10 fold improvement in accuracy over the Guide Star Catalogue (GSC) and a larger density, reaching to 16th magnitude. The catalog is supplemented by preliminary proper motions.

1. The Project

The USNO CCD Astrograph Catalog (UCAC) project is a high precision, astrometric survey for the 7$-$16 magnitude range (in the 610/60 nm band), aiming at an accuracy of 20 mas for $9-14^{m}$ stars (Gauss et al. 1996, Zacharias et al. 1997). The observations are made with a 0.2m, 5-element astrograph (Table 1) equipped with a 4k by 4k CCD camera (Table 2). The observing strategy gives a mean density of about 2000 stars per square degree, higher than that of the Guide Star Catalog (GSC), with an improvement in positional accuracy of about a factor of 10. The ACT, Hipparcos and Tycho Catalogues are currently used as reference. The final system orientation will be based on extragalactic ICRF sources, tied to the UCAC data via CTIO 0.9-meter telescope observations.

Table 1. Telescope

clear aperture 206 mm
focal length 2057 mm
plate scale 100 "/mm
number of lens elements 5  
spectral bandpass of lens 550-710 nm
Airy disc diameter (610nm) 15 $\mu m $
usable flat field of view $\approx 9$ degree

After one month of setup and testing the southern part of the survey started in February 1998 at Cerro Tololo Inter-American Observatory (CTIO). The project is scheduled for 3 years to completely cover the area from the South Celestial Pole to about +20 degrees declination. Another 2 years will be required to complete the entire sky from a Northern Hemisphere site. Through September 1999 over 30,000 fields have been taken in a 2-fold overlap pattern, covering over 70$\%$ of the Southern Hemisphere. For details on the procedures and software see (Zacharias & Zacharias 1999).

Table 2. Camera

number of pixels 4096 x 4096  
field of view 61 x 61 arcmin
pixel size 9.0 $\mu m $
pixel scale 0.9 "/pixel
spectral bandpass used 579-642 nm
filter replaces window $\lambda$ /4 optical quality
readout 14 bit
readout noise 13 $e^{-}$
full well capacity 85,000 $e^{-}$
operating temperature $\approx -18$ C
limiting magnitude $\approx 16.0$ 2 min.

2. The Catalog

A preliminary astrometric catalog, UCAC1, is being produced, containing positions of 30 million stars. First results are presented in Figure 1. These data will be supplemented by proper motions using the USNO A2.0 catalog. These low precision proper motions ( $\sigma_{\mu} \approx$ 10 mas/yr) will be improved by about a factor of 2 in the near future, when the NPM and SPM data are included. A comparison of UCAC data with other catalogs (Figure 2) shows the large improvement in accuracy it the high star density region. A public release of UCAC1 is expected by the end of 1999; see for availability. Some properties of the catalog are summarized in Table 3.

Figure 1: The UCAC1 positional precision.

Table 3. UCAC1: first preliminary catalog

overlap pattern fields 2-fold center in corner
exposures per field 1 short 1 long
exposure times 20 ... 150 seconds, guided
coverage $\delta \ \le \ -26...-10^{\circ} = f(\alpha)$
epoch of observations Feb 1998 $\ge$ Sep 1999
average density 2000 stars / square degree
total 30 million stars
catalog accuracy 20 mas, R = $9.5^{m}$ ... $14^{m}$
  70 mas, R = $16^{m}$
proper motions $\approx$ 10 mas/yr
number of CCD frames $\approx$ 70,000  
raw data so far 1.5 TB compressed FITS
backup on 700 exabyte tapes
  2,400 CD-ROMs
detection output 7 GB compressed
astrometric catalog 1 GB

3. Applications

In late 1998, UCAC data were used to tie the Hubble Deep Field South (HDF-S) to the ICRF (Hipparcos) system (Zacharias et al. 1998). The UCAC data will provide a dense net of high accuracy star positions at a recent epoch to be used for proper motion determinations. It also contains a huge amount of double star

Figure 2: Comparison of positional catalogs.

observations (separations $\ge$ 3"). The UCAC will assist the general astronomical community for a variety of applications, including Schmidt plate reductions, galactic studies, and minor planet observations. It will provide a reference frame for large telescope observations from direct imaging to fiber spectroscopy, to identify objects and to map out geometrical distortions of instruments. The UCAC1 catalog gives a first glimpse of the future final version. The UCAC data will be required for an input catalog of the FAME space mission and is likely to support other future space missions like SIM.


The authors wish to thank the entire UCAC team and the CTIO staff, in particular the observers.


Gauss, F. S., Zacharias, N., Rafferty, T. J., Germain, M. E., Holdenried, E. R., Pohlman, J. W., & Zacharias, M. I. 1996, BAAS 28, 1282

Zacharias, N., Germain, M. E., & Rafferty, T. J. 1997, in: Proceedings Hipparcos Venice 97 (ESA publication SP-402), 177

Zacharias, N., Corbin, T. E., Zacharias, M. I., Rafferty, T. J., Seidelmann, P. K., & Gauss, F. S. 1998, BAAS, 30, 1367

Zacharias, N. & Zacharias, M. I. 1999, in ASP Conf. Ser., Vol. 172, Astronomical Data Analysis Software and Systems VIII, ed. D. M. Mehringer, R. L. Plante, & D. A. Roberts (San Francisco: ASP), 345


... Zacharias1
Universities Space Research Association, Washington DC

© Copyright 2000 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: Data Pipelines and Quality Control
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