2 The program and the instrument

The first observing program involving 668 star transits contains 12 groups. Since the program was used for both EOP measurements and a star catalogue, it contains 496 basic stars and 172 catalogue stars (both from FK5 and FK5supp.). The program was observed for 19 months, and 329 clear nights, with 1146 groups in total. Some winter groups were observed 132 times at the maximum, and summer groups only 57 times at the minimum.

The PHA I is a fully-automatic astrolabe, controlled by an IBM PC 286 computer which could also be used for photon-counting data reduction and data solution after observation. The PHA I is equipped with two photon-counting detectors which could reach faint stars to mag. 9.5 with an equivalent aperture of 75 mm for each pupil of two images (the primary mirror aperture of the PHA I is 150 mm). It has an inductosyn with precision of 3'' in azimuth positioning. The PHA I could work for more than 12 hours in winter at temperatures down to $-35^\circ$C with some electronic parts being heated partly. Since the combined prism is made of zerodur, the zenith distance of the almucantar is very stable either over a year, or during a night (see Tables 1 and 2). This feature benefits not only its observing precision, but declination measurements as well, making the closing error k of zenith distance (being 0.217'' for 12 groups) much smaller than that of the Danjon astrolabe. The mean RMS of 1146 groups observed in 329 nights with mean temperature of $-8.3^\circ$C is $\pm 0.230''$ which is equivalent to $\pm 0.190''$ of RMS for zenith distance of $30^\circ$. This is excellent precision under quite low temperature.

  

Table 1: Observations in each month

  

Table 2: Group differences

Since the data of astronomical time and latitude measured by the PHA I are very good in both precision and accuracy, the PHA I had quite large weights in the Russian EOP service system among about 20 optical instruments in the year 1996. Besides this, the data also have been used to get corrections in both right ascension and declination for 200 stars, which are listed in order of declination in the appendix.