The ACT Reference Catalog

General Information


The ACT Catalog has been superseded with the release of Tycho-2

The ACT Reference Catalog was a primary source of astrometric information following the release of Tycho-1. In early 2000, this catalog was all but superseded with the release of The Tycho-2 Catalogue, which contains 2.5 million stars compared to ACT's 1.0 million. Copies of Tycho-2 on CD-ROM are still available by contacting either Erik Hoeg at or Sean Urban at With that said, this webpage and its links will continue to give information regarding the ACT Reference Catalog, even if only for historical reasons.



The ACT Reference Catalog contains 988,758 star positions and proper motions covering the entire sky for the epoch and equinox J2000.0. The proper motions are from new reductions of the Astrographic Catalogue (AC 2000; Urban et al. 1997) combined with the positions in the Tycho Catalogue (ESA 1997). Additional photometric and cross referencing data from Tycho are included. This document briefly describes these two data sets, and the catalog resulting from their combination.

The AC 2000

The AC 2000 is a positional catalog recently compiled at The U.S. Naval Observatory using the plate measures contained in the Astrographic Catalogue (AC). The AC was an international program designed to photograph and measure the positions of all stars brighter than magnitude 11.0. (Actually, some of the brightest stars are missing, since their images were grossly over-exposed on the photographic plates. These are not contained in the measured data nor their resulting catalogs {AC 2000 or ACT}.) Since observing and measuring the 22,000 plates needed to cover the sky was too labor intensive for any one observatory to undertake, the task was spread among several observatories, each photographing and measuring stars between two parallels of declination. In total, 20 observatories participated in the data gathering and the sky was sectioned into 22 zones. By the conclusion of the project, positions of 4.6 million stars had been measured, many as faint as 13th magnitude. These positions have an extremely early epoch; the average epoch of an AC plate is 1907.

To compile the AC 2000, each of the 22 zones making up the Astrographic Catalogue was reduced independently using the Astrographic Catalog Reference Stars (ACRS; Corbin & Urban 1988, Corbin & Urban 1990). The plates of each zone were analyzed for tilt, radial and tangential distortions, coma, magnitude equation and non-symmetric field distortions. Following this analysis, reductions of each plate were made to transform the image measures to equatorial coordinates. Following these reductions, the data were placed on the Hipparcos system and the magnitudes were converted to be close to those of the Tycho B system. The resulting data were then combined into the final AC 2000 catalog.

Although positional accuracies contained in the AC 2000 vary from zone to zone and star to star, most are between 150 and 300 mas at the mean epochs of observation. The average epoch for an AC plate is 1907 (although this too can vary quite a bit). The early epochs and good accuracies make the positions in the AC 2000 extremely valuable for determining proper motions when combined with modern observations.

The Tycho Catalogue

The Hipparcos satellite was a European Space Agency (ESA) funded, astrometric instrument launched in 1989 and operated until 1993. The primary Hipparcos mission was the determination of accurate positions, proper motions and parallaxes of 118,218 stars. The results of these observations form the Hipparcos Catalogue (ESA 1997). The accuracies of the Hipparcos astrometric parameters are roughly 1 mas for position, 1 mas/year for proper motion, and 1 mas for parallax.

Early in the planning stage for Hipparcos, the mission was expanded to include data from the satellite's star mapper. The results of the star mapper's observations are contained in the Tycho Catalogue (ESA 1997) of 1,058,332 objects. The accuracies of the Tycho positions and proper motions are roughly 25 mas for position and 25 mas/year for proper motion. In addition to the positional information, the Tycho Catalogue contains two-color photometry for the vast majority of its objects. The precisions of the Tycho photometric data are generally well under 0.1 magnitude. A thorough introduction of the Hipparcos and Tycho missions can be found in the above reference.

The ACT Reference Catalog

It was recognized many years ago that the data contained in the Astrographic Catalogue could be combined with modern epoch observations to compute or dramatically improve proper motions (Eichhorn 1974) especially with a catalog such as Tycho (Urban 1993, Roeser & Hoeg 1993). Tycho is a perfect catalog to combine with the AC 2000 since the Tycho positions make it the most accurate catalog of comparable size at its epoch of observation, but its proper motions degrade it to a sub-standard reference catalog in less than 10 years. The proper motions of the Tycho stars can be improved from about 25 mas/year to about 3 mas/year by recomputing them using the AC 2000 data (Urban 1997). This combination of AC 2000 and Tycho, called ACT Reference Catalog, degrades much more slowly and is a valuable astronomical dataset for applications spanning many decades.

Matching Tycho and AC 2000

In order to make the ACT Reference Catalog high quality, it was decided that some problematic ``stars'' would not be used. Positions indicated in the Tycho Catalogue as coming from close multiple stars or those of extremely low astrometric quality were discarded.

To match the two data sets, the Tycho entries were brought to the AC 2000 epochs by the application of the published Tycho proper motions. Positional coincidence at the AC epoch of 15 arcsec or less was generally considered a good match and the images were identified as coming from the same star. Tycho stars not matched at this time were re-examined using no application of proper motion. A further search for known high proper motion stars was made. Special consideration was given to multiple star systems.

Computation of proper motions

Once the identifications between the AC 2000 and Tycho were made, the proper motions were computed using the differences in position and epoch. To bring these proper motions from the mean epoch to J2000.0, the application of standard formulae were used. In the computation of the foreshortening terms, the radial velocities were taken from the Hipparcos Input Catalogue (Turon et al. 1991) and the parallaxes were taken from the Hipparcos Catalogue.


Corbin, T.E. and Urban, S.E., 1988, IAU Symposium 133 Mapping the Sky,
ed. S. Debarbat, J.A. Eddy, H. K. Eichhorn and A. R. Upgren, Kluwer, Dortrecht, p. 287

Corbin, T.E., and Urban, S.E., 1990, IAU Symposium 141 Inertial Coordinate System on the Sky,
ed. J. Lieske and V.K. Abalakin, Kluwer, Dortrecht p. 433.

Eichhorn, H., 1974, Astronomy of Star Positions, Fredrick Ungar, p. 283

ESA, 1997, The Hipparcos and Tycho Catalogues, SP-1200

Roeser, S. and Hoeg E., 1993, The Tycho Reference Catalogue: A Catalogue of Positions and Proper Motions of One Million Stars, in Workshop on Databases for Galactic Structure,
ed. A.G.D Philip, B. Hauck, and A.R. Upgren, L. Davis Press p. 137

Turon, C., Creze, M., Egret, D., et al., 1992, The Hipparcos Input Catalogue, ESA SP-1136

Urban, S.E., 1993, The U.S. Naval Observatory's Astrographic Catalogue Project,
in Workshop on Databases for Galactic Structure, ed. A.G.D Philip, B. Hauck, and A.R. Upgren, L. Davis Press p. 145

Urban, S.E., 1997, New Reductions of the Astrographic Catalogue, in Proceedings of IAU Colloquium 165,
ed. Wytrzyszczak, I.M., Lieske, J.H., and Feldman,R.A, Kluwer Academic Publishers, p. 493

Urban, S.E., Corbin, T.E., Wycoff, G.L., Martin, J.C., Jackson E.S., Zacharias, M.I., and Hall D.M., 1997,
The AC 2000: The Astrographic Catalogue on the Hipparcos System, in preparation.

Urban, S.E., Corbin, T.E., and Wycoff, G.L., 1998, The ACT Reference Catalog, in press


[ACT home page | General Information | Obtaining the data | USNO Home]


Those requiring additional information may contact Sean Urban at

This page was last updated November 27, 2001.

Disclaimer for external links