Survey type star catalogue | Named after Tycho Brahe | |
The Tycho-2 Catalogue is an astronomical catalogue of more than 2.5 million of the brightest stars.
Contents
Catalogue
The astrometric reference catalogue contain positions, proper motions, and two-color photometric data for the 2,539,913 of the brightest stars in the Milky Way, of which about 9000 are visible to the naked eye. Components of double stars with separations down to 0.8 arcseconds are included. The catalogue is 99% complete to magnitudes of V~11.0 and 90% complete to V~11.5. (, Table 1)
The Tycho-2 positions and magnitudes are based on the observations collected by the star mapper of the European Space Agency's Hipparcos satellite. They are the same observations used to compile the Tycho-1 Catalogue (ESA SP-1200, 1997). However, Tycho-2 is much larger and a bit more precise, because a more advanced reduction technique was used.
The U.S. Naval Observatory (USNO) first compiled the ACT (Astrographic Catalog/Tycho) Reference Catalog, containing nearly one million stars, by combining the Astrographic Catalogue (AC 2000) with the Tycho-1 Catalogue; the large epoch span between the two catalogues improved the accuracy of proper motions by about an order of magnitude. Tycho-2 now supersedes the ACT.
Proper motions precise to about 2.5 milliarcseconds per year are given as derived from a comparison with the Astrographic Catalogue (AC 2000) and 143 other ground-based astrometric catalogues, all reduced to the Hipparcos celestial coordinate system. There were only about 100,000 stars for which proper motion could not be derived. For stars brighter than Vt=9, the astrometric error is 7 milliarcseconds. The overall error for all stars is 60 milliarcseconds. The observational period was from 1989.85 to 1993.21 and the mean satellite observation epoch is 1991.5.
Photometric accuracy for stars brighter than Vt=9 is 0.013 magnitude; for all stars it is 0.10 magnitude.
Accessing the catalogue
To enable rapid access of specific stars in the catalogue, WCSTools software numbers each star using its Guide Star region number (0001-9537) and a five-digit star number within each region, separated by a decimal point. sty2 lists Tycho-2 stars by number or sky region. imty2 lists the Tycho-2 stars within an IRAF or FITS image using the world coordinate system defined in its header.
A Perl program for extracting data from the catalogue is available from http://archive.eso.org/ASTROM/. Tycho-2 File Formats WCSTools software uses the files catalog.dat and index.dat.