The astrometric reference catalogue contain positions, proper motions, and two-color photometric data for 2,539,913 of the brightest stars in the Milky Way.

(,[1] 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.

imty2 lists the Tycho-2 stars within an IRAF or FITS image using the world coordinate system defined in its header.

The resulting files will differ from the files on the CD-ROM in that they will end lines with only a linefeed, while the CD-ROM version for which WCSTools software was written terminates lines with a carriage return and a linefeed.

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.