Positions of 118000 stars, their proper motions and parallaxes obtained from measurements with ESA's astrometric satellite Hipparcos have a high degree of internal consistency. The accuracy of mutual positions of celestial objects in the Hipparcos catalogue is on a few milliarcsecond (mas) level. However, the satellite was not able to observe extragalactic objects directly so that the Hipparcos catalogue system may in principle contain spurious rotation. Therefore, the Hipparcos system needs calibration which includes determination of orientation of the system in a certain epoch and its spin correction. There are some methods of calibration of the Hipparcos data (Lindegren & Kovalevsky 1995, Kovalevsky, Lindegren et al. 1996) based on the use of both new measuring instruments (VLBI, VLA, HST) and traditional methods of ground-based astrometry. One can determine either only the orientation at epoch of the Hipparcos system (VLA, MERLIN), or only its spin correction (using absolute proper motions of stars with respect to galaxies or photographic observations of optical counterparts of extragalactic radio sources). Some methods allow to find jointly both the orientation of the Hipparcos system in a certain epoch and its spin correction (VLBI, MERLIN, HST, use of Earth rotation parameters).
In this paper we present the results of the application of absolute proper motions of stars with respect to galaxies in selected fields of the sky, obtained within the KSZ and MEGA programmes, to determine a spin correction of the final Hipparcos catalogue system. In spite of the comparably low accuracy of these data, it is possible to link the Hipparcos system with random errors less than as it has been shown in (Yatsenko et al. 1987). The present investigation represents the Kyiv part of the Bonn-Potsdam-Kyiv project "Hipparcos extragalactic link by photographic astrometry" (Brosche et al. 1995).
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998