next previous
Up: The Potsdam contribution

1. Introduction

The ESA satellite Hipparcos has measured positions, proper motions and parallaxes of more than 100000 stars in a homogeneous system with a very high internal accuracy. Nevertheless, the Hipparcos reference frame needed a link with regard to an inertial system. There are different methods to link the Hipparcos reference frame to extragalactic objects (Lindegren & Kovalevsky 1995). One possibility is to use photographic astrometry with deep Schmidt plates for this link.

In 1986 the Potsdam group started its programme to link the Hipparcos proper motion system to an extragalactic reference system using the Tautenburg Schmidt telescope. First investigations showed that Tautenburg plates provide proper motions with respect to extragalactic objects with an accuracy of about 6 mas/yr for bright stars with tex2html_wrap_inline1052 (Schilbach 1984; Scholz 1991). A first rough estimation showed that a theoretical accuracy of 1.2 mas/yr for the transformation of the Hipparcos proper motion system can be achieved with six well-distributed link fields (Dick et al. 1987). Numerical simulations carried out with 10 fields and 119 link stars gave an accuracy of 1.4 mas/yr (Yatsenko et al. 1987). Later reports on the status of the Potsdam programme for the Hipparcos extragalactic link (Brosche et al. 1992, 1995) confirmed the more optimistic expectation of accuracies better than 1.0 mas/yr. Special efforts were made in order to increase the number of link fields.

A check of the Tautenburg plate archive showed that there are more than 70 fields with at least 4 plates taken between 1960 and 1970 which could be used as first epoch plates for the determination of proper motions (Scholz & Hirte 1991). Altogether, 26 fields (see Fig. 1 (click here)) included in different Potsdam proper motion programmes were selected for the Hipparcos link.

  figure215
Figure 1: Distribution of the link fields in equatorial coordinates. Filled squares: MEGA fields, stars: globular cluster fields, open circles: fields with dwarf spheroidal satellites of the Galaxy (excluded from final solution), asterisks: other fields


next previous
Up: The Potsdam contribution

Copyright by the European Southern Observatory (ESO)
web@ed-phys.fr