9 Conclusions

The Tycho Reference Catalogue is a new reference catalogue which provides an accurate, dense and stable realization of the ICRS reference system in the optical domain. TRC realizes the ICRS system at epoch J2000.0 with a median accuracy of 45 mas. Its high-precision proper motions, derived using the hundred years old Astrographic Catalogue data, ensure a reference system realization with an accuracy better than 100 mas over the time span of several decades. The quality of the TRC proper motions demonstrates the immense potential of the Astrographic Catalogue for the massive derivation of high-precision proper motions. It should be noted that TRC, although containing a million stars, still utilizes less than a quarter of the AC data.

Further development of the Tycho Reference Catalogue is planned. Reprocessing of the Tycho data, now under way at Copenhagen University Observatory (Høg et al. 1998b), will result in the Tycho-2 Catalogue containing positions and photometry of 2500000 stars, scheduled to appear in 1999. Proper motions will also be included, based on a conventional adjustment of the Astrographic Catalogue onto the ICRS/Hipparcos system, like the present versions of TRC and ACT.

A major improvement of the systematic accuracy of the TRC proper motions can be expected in another version, to be called TRC2, which will be based on the Tycho-2 Catalogue positions and photometry and a global block adjustment of the Astrographic Catalogue, undertaken at Sternberg Astronomical Institute. Reduction of the AC using plate overlap technique will enable:

• direct reduction of the Astrographic Catalogue onto the Hipparcos system, eliminating the need for an intermediate reference catalogue;
• simultaneous and correct (from the statistical point of view) determination of the significant plate-scale systematic errors based on all plates, without the partial averaging that was used to yield the zone-specific corrections under the present approach;
• detailed investigation of the magnitude and colour equations, réseau and micrometer errors, and other types of systematic errors probably present in the AC data which were neglected under the classical approach.

Acknowledgements

The TRC would not have been possible without its two main foundations, the machine-readable Astrographic Catalogue and the Tycho Catalogue. The former resulted from many years of work by the Sternberg Astronomical Institute's Astrometry Division, headed by Vilen Nesterov. The latter marks the great success of the Tycho Data Analysis Consortium under the leadership of Erik Høg. The time and effort by the project leaders, and many other individuals who contributed to the projects, to ensure the highest possible quality of the catalogues is gratefully acknowledged. The third indispensable ingredient to TRC was the primary optical reference frame provided by the Hipparcos Catalogue.

The TRC project was supported by the Russian Foundation for Basic Research, by the Danish Space Board and the Velux Foundation of 1981, by the Swedish National Space Board and by the Deutsche Forschungsgemeinschaft.

We finally acknowledge constructive comments of the referee, J. Kovalevsky, that helped to clarify the presentation.