Accessing The Catalog
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/. Tycho-2 File Formats WCSTools software uses the files catalog.dat and index.dat.
Read more about this topic: Tycho-2 Catalogue