1 Introduction

The Chinese Photoelectric Astrolabe Mark I (PHA I) was installed at Shaanxi Astronomical Observatory in 1971, and started to work in 1972. The PHA I worked at almucantar with zenith distance of $30^\circ$ before the 1990s, mainly for Earth Orientation Parameters (EOP) measurements, but partly for star catalogues. Since French astronomers (Guinot et al. 1961) published their first creative results of 571 star positions observed with the Danjon astrolabe, many astrolabe catalogues have been published, and have played quite good role for compilation of the FK5 system. Most of those catalogues have blind zones with declination width of about 25 degrees. As most astrolabes in the world, the PHA I also had a blind zone in declination determination. In order to do more and better work for star catalogues, and to eliminate the blind zone in declination measurements, both CSAO and VS NIIFTRI had signed a scientific cooperation agreement about jointly using the PHA I in Irkutsk, Russia for star catalogue and EOP measurements in Sep. 1993. To do this, CSAO revised their PHA I with a new combined prism which could work on almucantar with zenith distance of $45^\circ$ in 1993-94. According to the agreement, the PHA I was installed at the astronomical base of VS NIIFTRI in Irkutsk in Oct. 1995, and started to work in Nov. 1995.