TYCHO astrometry calibration

The Hipparcos satellite's star mapper gives photon counts in two spectral channels simultaneously. 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. A small part, mainly three weeks of continuous Tycho observations has been used to study the accuracy of the star mapper calibration.