The Tycho-2 positions and magnitudes are based on precisely the same observations as the Tycho-1 Catalogue (ESA SP-1200, 1997) collected by the star mapper of the ESA Hipparcos satellite, but Tycho-2 is much bigger and slightly more precise, owing to a more advanced reduction technique.
Proper motions precise to about 2.5 mas/yr are given as derived from a comparison with the Astrographic Catalogue (AC) and 143 other ground-based astrometric catalogues, all reduced to the Hipparcos celestial coordinate system. For only about 100,000 stars, no proper motion could be derived. For stars brighter than Vt=9, the astrometric error is 7 milliarcseconds; for all stars, the error is 60 milliarcseconds. 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.
To enable rapid access of specific stars in the catalog, 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 catalog is available from http://archive.eso.org/ASTROM/.
Put the catalog.dat and index.dat files in a subdirectory data/ to the directory pointed to by the TY2_PATH environment parameter or the ty2cd variable in libwcs/ty2read.c.