Despite the small number of common stars in the VL1, VL2 and VL3 catalogues, the results confirm the opportunity and validity of the global reduction to compute absolute declinations. The comparison of our results with the dynamical ones confirms the astrolabe as a potential instrument for ground-based astrometry.
Table 3 (click here) gives the equator corrections obtained by Leister (1989), Penna et al. (1996), Clauzet et al. (1990), Poppe (1994), Poppe et al. (1996), and also the results obtained in this paper. These values are in agreement with one another. This is due the fact that the results obtained with the Sun uses the same set of stars.
Thus, the astrolabe can be a very important instrument for the definition of the fundamental reference equator. The results confirm also the position adopted by Fricke (1982) in order to maintain the equator of the FK4 system in the FK5 system.
A special observational programme made at two zenith distances (VL4 & VL5), including more than one hundred common stars, is underway at the OAM. We hope that this programme will establish, once and for all, the usefulness of equal altitude observations in deriving absolute declinations. Thus, the astrolabe can be used as a producer of stellar catalogues.
Acknowledgements
The authors would like to thank Wilson Monteiro for most of the observations taken in this observational programme. V.A.F. Martin is also grateful to Dr. L.B.F. Clauzet (in memorium) for his incentive and several aids at the beginning of her job in astrometry and also to Dr. P. Benevides-Soares for his help in the global reduction method. Financial support from CNPq, FAPESP, CAPES and "Fundo Bunka de Pesquisa/1993" are gratefully acknowledged.