Once more, the results of this paper reveal that at the dawn of the
age of satellite astrometry early photographic observations are still
a valuable source for measuring proper motions of fainter stars.
With a modern measuring machine and with the new reference
system established by Hipparcos the full wealth of positional information
preserved in these plates can today be unveiled (see also
Geffert et al. 1996a). The internal accuracy
of the proper motions derived in this study is at the same level as
or even better than the accuracy achieved with Hipparcos itself.
It was also shown that astrometric results of good accuracy can
be obtained for non-pointlike extragalactic objects, if the intrinsic
structure of the source is evaluated by means of a cross-correlation
method. The absolute reference frame that was set up in this way by
three sources in M81, M82 and NGC3077 proved to be absolute
at the level of 1mas/a. This was confirmed in comparison with the Lick NPM
catalogue and with the mean absolute system of the Bonn Extragalactic
Link program. We expect that in future the cross-correlation technique
will be relevant especially for the astrometric reduction of
CCD observations as obtained for example at modern transit telescopes.
Together with results from other link fields our absolute proper motions
can make an important contribution to the extragalactic link of the
Hipparcos system. However we pointed out that the Hipparcos results
may not be fully compatible with our photographic measurements because
of the influence of orbital motion.
The discussion of double stars and high proper motion stars has shown
that the combination of proper motions and photometry bears a potential
for investigating kinematic questions. This will be the subject
of a later paper.
The complete catalogue of absolute proper motions and photometry for
the field around M81 as well as a revised version of the catalogue
of proper motions in the neighbourhood of M51
is available on request.
Acknowledgements
We thank R.Catchpole from the Royal Greenwich Observatory Cambridge, V.S.Kislyuk from the Main Astronomical Observatory Kiev and G.Klare from the Landessternwarte Heidelberg, who kindly provided plate material for this project. We also thank the Astronomisches Institut Münster for generous allocation of PDS scan time and H.-J.Tucholke for scanning some of the plates. We thank the Hipparcos Science Team for the contribution of H37 data. Finally, we gratefully acknowledge financial support from the Bundesministerium für Forschung und Technologie (FKZ 50OO9101). This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France.