The TYCHO Catalogue

The principal observational characteristics of the Tycho Catalogue, and a summary of its main astrometric and photometric properties, are presented. Median astrometric standard errors (in position, parallax, and annual proper motion) are typically around 7mas for stars brighter than V_T_~9mag, and approximately 25mas for V_T_~10.5mag, at the catalogue epoch (J1991.25). The Tycho Catalogue was adjusted so that the six global rotation and spin components of the catalogue reference frame coincide with those of the Hipparcos Catalogue. It therefore materialises the ICRS reference system, coinciding with its principal axes at the level of +/-0.6mas, and with proper motions consistent with an inertial system at the level of +/-0.25mas/yr. The 1058332 constituent stars provide a mean sky density of ~25 stars/deg^2^. The catalogue and associated epoch photometry annex data are available in machine-readable form as part of the final Hipparcos mission products.