The Danjon astrolabe at Santiago, Chile, was modified in 1989 by Chollet & Noël (1993) (Noël & Chollet 1990). The classical transparent prism was replaced by two CERVIT reflecting prisms which now permit observations at and zenith distances. Both prisms can be interchanged in a few minutes without further adjustment of the optical system. The modifications introduced in the astrolabe permit also observations of the Sun. For solar observations a filter made of transparent CERVIT with a chrome-nickel layer of density close to 5, is installed in front of the objective. A program of astrometric observations of the Sun at and zenith distances is in progress since 1990 (Chollet & Noël 1993; Noël 1993, 1994, 1995).
A narrow observing zone of less than wide in declination, and spurious variations of the instrumental zenith distance due to thermal deformations of the transparent prism, were some of the drawbacks of the classic Danjon astrolabe. Both drawbacks have been diminished with the modifications introduced in the astrolabe of Santiago. A zone of about wide in declination can now be observed at zenit distance, and the rather low thermal coefficient of CERVIT provide a more stable instrumental reference. On the other hand, some disturbing effects inherent to astrolabes with transparent prism, do not exist in an astrolabe with reflecting prisms (Kovalevski 1990).
Series of fundamental stars were observed at between 1990 and 1994 in order to obtain the instantaneous local latitude and UT0 for the reduction of the solar observations in the FK5 system. As a test of the results obtained at zenith distance with the modified astrolabe and as a contribution to the research of the Fundamental Reference System, we present here an evaluation of systematic differences in the sense Astrolabe-FK5. These differences are compared with similar ones obtained with the photoelectric meridian circle of the U.S. Naval Observatory at Black Bierch, New Zealand (Corbin 1991).
Table 1: Systematic differences in right ascension and declination as a function of declination: Astrolabe-FK5, J1991.9