The Tycho-2 Catalogue presented here is an astrometric reference catalogue
containing positions and proper motions as well as two-colour photometric d
ata for the 2.5 million brightest stars in the sky, The Tycho-2 positions a
nd magnitudes are based on precisely the same observations as the original
Tycho Catalogue (hereafter Tycho-l) 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. Components of double stars w
ith separations down to 0.8 arcsec are included. Proper motions precise to
about 2.5 mas/yr are given as derived from a comparison with the Astrograph
ic Catalogue and 143 other ground-based astrometric catalogues, all reduced
to the Hipparcos celestial coordinate system. Tycho-2 supersedes in most a
pplications Tycho-l, as well as the ACT and TRC catalogues based on Tycho-1
.