Vers Un Nouveau Catalogue General Astrolabes

Abstract

In the context of the future Hipparcos astrometric satellite which will give a high precision catalogue, the ground based astrometry will have new possibilities of research, particularly in the field of the stellar and the dynamical reference systems. The modern astrolabes should give, as new photoelectric meridian circles, an important contribution.

Since 1956, the Danjon astrolabe was used to determine the Earth rotation parameters. Very rapidly it was demonstrated (S. Débarbat, J. Kovalevsky, 1963) that this instrument was able to give very good planetary positions, relatively to the FK4. New studies shown, later, that it was also possible to observe the Sun (F. Laclare 1975, N. Leister 1979, F. Chollet 1981). The major contribution of the astrolabe was the observation of stellar catalogues (Billaud 1978) and now, the determination of the variations of the solar diameter.

In these conditions, it is now possible to use polyvalent astrolabes, able to give good positions of stars, planets and Sun. The Hipparcos catalogue, connected to the extragalactic reference system by VLBI, will constitute a very precise “interpolation system”. The main problem will be to save this high precision during a long time. Without other Hipparcos satellite or other method, this problem can be solved only by the ground based astrometric observations. At the same time, some other fields of research can be initiated if the astrometric observations are directly connected to Hipparcos catalogue, such as the local effects, the refraction, the motions of the vertical, which will improve in the future the ground based astrometric results.

Concerning the astrolabes, some studies are made in France to improve the instrument and the methods of reduction. At the same time, the astrolabe community has to use the same procedures of observation and reduction to be able to connect the results coming from several different stations.