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 (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.
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