Tycho Astrometry From Half Of The Mission

The Hipparcos satellite's star mapper gives photon counts in two spectral channels simultaneously, close to Johnson B and V. The transit times and the signal amplitudes for each star across two groups of four slits are derived and used for astrometry and photometry, respectively, and this constitutes the Tycho project. The present paper describes the Tycho astrometric data processing, leading from the transit times to a geometric calibration of the slit system and, finally, to the astrometric parameters of the Tycho stars.Some 17 months of Tycho observations, i.e. about half of the Hipparcos-Tycho mission, have been used to study the accuracy of the star mapper calibration and to produce working catalogues of Tycho positions of a million stars. It is concluded that a satisfactory accuracy has been achieved. For example, the external errors of a preliminary catalogue have been determined by comparison of 98 000 stars common with a preliminary, but much more accurate Hipparcos catalogue. External systematic errors of positions are found to be typically less than 10 milliarcsecond (mas) and the median value of accidental errors in position is 10 mas.