Ten years Tycho-2 Catalogue
 
Dear coauthors and many others,
 
The Tycho-2 Catalogue of the 2.5 million brightest stars in the sky was released on the 8th of February 2000, and we can celebrate the tenth anniversary enjoying that the catalogue has been very well received.
 
The Tycho-2 Catalogue contains positions, proper motions and two-colour magnitudes for the 2.5 million stars covering the entire sky, and the authors are E. Høg, C. Fabricius, V.V. Makarov, S. Urban, T. Corbin, G. Wycoff, U. Bastian, P. Schwekendiek, and A. Wicenec. Positions and magnitudes were based on observations from 1989 to 1993 with the ESA Hipparcos astrometric satellite. The Tycho-2 positions and 100 years of ground-based astrometry contained in more than 140 catalogues were utilized to obtain the proper motions. Publications and data are available at www.astro.ku.dk/~erik/Tycho-2.
 
2) The paper in Astronomy & Astrophysics announcing the Tycho-2 (see website) was reprinted in 2009 in a special issue of A&A, Vol. 500, p.583, with a commentary by Catherine Turon (see website), as one of the 40 most cited among 50 000 papers published in the journal during the 40 years since its foundation in 1969.
 
2) Nearly 700 citations of the A&A paper since 2000 are presently recorded at the SAO/NASA Astrophysics Data System (ADS), but many applications do not result in recorded citations, as pointed out under items #4 and 5.
 
3) The positions, proper motions and two-colour magnitudes of Tycho-2 are widely used for all kinds of astrometric and astrophysical studies. Among the hundreds of catalogues available at the VizieR in Strasbourg it is the 4th most popular after the 2MASS, the USNO-B1.0 and the Hipparcos and Tycho Catalogues, and the VizieR mirrors have recorded a number of up to 600 000 interrogations per month, according to Turon's paper. The corresponding maximum number for the Hipparcos Catalogue is 940 000.
 
4) The practical more technical applications should be particularly mentioned. For preparation of observations and control of telescope setting on the ground and for monitoring and control and of satellite attitudes and orbits, the Tycho-2 is the catalogue of choice. These applications appear as citations in the technical papers, but since most of them are either project internal or at least not refereed there are no real citations which would be counted at ADS.
 
5) A quotation related to the technical applications of item #4 is of interest. Andreas Wicenec, co-investigator on the Tycho project since 1983, and then since many years at ESO, Garching, wrote to me last year: "I'm missing one very important use case in Catherine's very nice paper: Observation preparation and support for observatory operations. I'm mentioning this because when I've seen the requirements for the implementation of a catalogue to be used for the optical pointing tests of the ALMA antennas, it was immediately clear that there is no other choice than Tycho-2. This is mainly due to the magnitude range and the proper motions (I had to add parallaxes though). For this reason we have implemented Tycho-2 servers for ALMA in all instances of the ALMA archive installations, including the many test installations around the world."
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On behalf of all participants in the Tycho project, I acknowledge again the support we have received from ESA, the Hipparcos Science Team, and all national funding authorities.
 
With best regards
Erik Høg
Niels Bohr Institute
Copenhagen University
25 Jan. 2010