The expected error of the connection of the HIPPARCOS system with an extragalactic reference system by means of ground-based photographic astrometry was calculated. Two different catalogues of proper motions with respect to extragalactic objects and the HIPPARCOS proper motions were simulated. An accuracy of OY14 per century using only 10 fields with 119 link stars of the Tautenburg Schmidt telescope (134/200/400) and OY07 per century using 93 fields with 336 link stars of the Kiev long focus astrograph (401550) Key words: reference system ~ HIPPARCOS-proper motions
IntroductionThe astrometric satellite HIPPARCOS provides a catalogue of star positions and proper motions of high internal accuracy. As the instrument is not capable of directly observing extragalactic objects, the link to an extragalactic quasi-inertial system must be established by additional investigations. In particular the HIPPARCOS proper motions have to be transformed into absolute proper motions. There is a known nuniber of programmes providing valuable catalogues of stellar proper motions linked to extragalactic objects (DEJCH 1952, WRIGHT 1950. Absolute proper motions of stars have been obtained, for instance, in Kiev (RYBKA 1978) by referring them to the system of background galaxies. A special programme of deriving absolute proper motions using the Tautenburg Schmidt telescope is now under way , DICK et al. 1987. The transformation of the HIPPARCOS system to the quasi-inertial system must be implemented with an accuracy corresponding at least to the accuracy of the HIPPARCOS proper motions. For this purpose FROESCHLE and KOVALEVSKY (1982) proposed the use of radio stars which would be observed by VLBI. DICK et al. (1987) estimated that it is possible to determine the rotation of the HIPPARCOS system from ground-based photographic astrometry with a suitable Schmidt telescope. We will give a more detailed analysis of the random errors to be expected by using photographic astrometry.
Initial equationsThe method of linking the HIPPARCOS system with an extragalactic reference frame has been described by FROESCHLE and KOVALEVSKY (1 982). We are especially interested in the differences of the proper motions of the two catalogues to be connected (cf. ROSER 1983):
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