The differences in right ascension plotted in Fig. 1 as a function of declination show a clear agreement between the astrolabe results obtained at zenith distance at Santiago and those obtained at Black Bierch, New Zealand, with the automatic meridian circle of the U.S. Naval Observatory (Corbin 1991). Since it is rather improbable that both instrumental systems defined by quite different astrometric techniques could be affected by similar systematic errors, one can conclude that the shapes of the curves in Fig. 1 (click here) are not due to instrumental artifacts, but they are representing a systematic error in right ascension of FK5 which varies as a function of declination.
Figure 2: Systematic differences in declination as a function of declination: Instrument-FK5
With respect to the systematic differences in declination which are given in Table 1 (click here) and are plotted in Fig. 2 (click here), the astrolabe and meridian observations show a similar general trend; however, it is exagerated in the astrolabe results. We think that the declination results of the astrolabe could be strongly affected by spurious effects of refraction which are quite prevalent at large zenith distance as in this case. If these effects are introducing distortions of the star zenith distance residuals which are similar at the east and west observations, then, according to Eqs. (1) and (2) they will be canceled when computing the right ascension results, but they should have a strong effect in the results in declination, as it is apparent if one compares the results given in Figs. 1 (click here) and 2 (click here).
To conclude, one can say that according to the results presented in this paper and in spite of a rather large zenith distance of observation, the astrolabe is still a reliable instrument to disclose systematic errors in right ascension of star catalogues which vary as a function of declination. However, the results obtained to research systematic errors in declination should be considered more cautiously.
This work was partially financed by Fondo Nacional de Ciencia y Tecnología (FONDECYT), Santiago, Chile, under research project 1940414. The program of the astrolabe at Santiago is a joint collaboration research project in astrometry at the southern hemisphere between the European Southern Observatory (ESO) and Universidad de Chile.