Construction of the Tycho Reference Catalogue

The Tycho Reference Catalogue (TRC) project, as proposed in 1992 by the pre sent authors, aimed to extend the Hipparcos reference system to fainter mag nitudes. To assure rigidity of the reference system realization, the projec t relied on the Tycho Catalogue that provides highly-accurate positions of more than one million stars on the Hipparcos system. To maintain the precis ion of the reference system over longer periods of time, high-precision pro per motions must be derived from Tycho and earlier ground-based measurement s, most notably the century-old Astrographic Catalogue (AC). The recently published first version of the Tycho Reference Catalogue provi des highly-accurate positions and proper motions for 990182 stars of the Ty cho Catalogue. It realizes the ICRS/Hipparcos system in the optical domain with a median accuracy of 45 mas at epoch J2000.0. The realization is based on stars of median magnitude B-T = 11.25 mag, providing an average of 24 r eference stars per square degree. The median accuracy of the TRC proper mot ions, estimated from external comparisons as 2.4 mas/yr, ensures a referenc e system realization with an accuracy better than 100 mas over the time spa n of a few decades. Highly-accurate astrometry combined with homogeneous tw o-colour Tycho photometry makes TRC an ideal choice for a wide range of ast ronomical applications. The present paper covers the major steps of the TRC construction, including the Astrographic Catalogue reduction onto the Hipparcos system, identifica tion of the Tycho Catalogue stars in the AC, derivation of proper motions, a posteriori systematic corrections of the proper motions to account for re sidual AC systematic errors, and the TRC quality assessment. The relation o f the TRC with the recently published ACT catalogue is outlined. A future v ersion of the TRC, to he based on the new Tycho data processing and a globa l block adjustment of the Astrographic Catalogue; is briefly discussed.