THE TYCHO CATALOG

The principal observational characteristics of the Tycho Catalogue, an d a summary of its main astrometric and photometric properties, are pr esented. Median astrometric standard errors (in position, parallax, an d annual proper motion) are typically around 7 mas for stars brighter than V-T similar to 9 mag, and approximately 25 mas for V-T similar to 10.5 mag, at the catalogue epoch (J1991.25). The Tycho Catalogue was adjusted so that the six global rotation and spin components of the ca talogue 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 +/-0.6 mas, and with proper motion s consistent with an inertial system at the level of +/-0.25 mas/yr. T he 1 058 332 constituent stars provide a mean sky density of similar t o 25 stars deg(-2). The catalogue and associated epoch photometry anne x data are available in machine-readable form as part of the final Hip parcos mission products.