Syst eme de r ef erence stellaire : eets syst ematiques Stellar reference system: Systematic eects

We discuss two dierent ways to determine the systematic eects on the star's observations obtained with the astrolabe. The rst one is related to the absolute determination of the declinations got through stars belonging simultaneously to 30 and 45 zenith distance programmes. The absolute declinations were obtained from a global reduction including all the data. The second way uses a sum of chosen functions, added to the observational results in zenith distance obtained with astrolabes, which represent both the systematic dierences and between catalogues of stars and the instrumental eects I. With this method it is possible to associate several dierent catalogues as only one. The interest is that it allows the utilization of even single passage stars to get systematic dierences and , what is not possible in classical procedure of astrolabe data reduction.