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