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Description
The positions are the combined results of observations of all participants in the Southern Reference Star (SRS) observing campaign from 1961 tO 1973. Preliminary catalogs were compiled at both the U.S. Naval Observatory (Washington) and the Pulkovo Observatory (Leningrad). The two compilations were compared, reconciled and combined to give the catalog described here. The proper motions have been culled from a number of different sources and should not be regarded as part of the SRS observed catalog. The proper motions are given principally as a means to take into account the small differences in epoch of observation between the SRS and the 2nd Cape Photographic Catalog plate material, which is at most about two years. Because of the inhomogeneous nature of the proper-motion sources and the likely presence of magnitude related systematic errors in the proper motions, they may not safely be used to extend the SRS system of positions very far from the mean epoch of observation which is about 1968. A discussion by T. Corbin of all the SRS observations including those made before and after the 1961 to 1973 campaign and their reduction to the FK4 system is at an advanced stage of preparation and will yield the desired high quality proper motions required for transferring the system to more distant epochs. Two sets of positions and proper motions are given. One set is referred to the equator and equinox of B1950.0 and the other to the equator and equinox of J2000.0. The positions are referred to the mean epoch of observation in both cases. The B1950.0 positions were linked directly to the system of FK4 by observational techniques. The J2000.0 data are based on a transformation of the B1950.0 data. The transformation was accomplished in two steps. First, FK5-FK4 differences referred to the B1950.0/FK4 system were added to the SRS positions and proper motions. The FK5-FK4 differences are those given by the analytical coefficients of H. Schwan (Astronomisches Rechen-Institut, Heidelberg). Second, the reduction from B1950.0 to J2000.0 was accomplished by using the conversion procedure described in the 1988 edition of THE ASTRONOMICAL ALMANAC (pg. B42). The (FK5-FK4) differences are given along with the average FK4 magnitude used as the input argument to Schwan's subroutines. An average FK4 magnitude was estimated for each SRS position from the average magnitude of all FK4 stars in an area on the celestial sphere bounded by a four-hour interval in right ascension and a 20-degree interval in declination centered on the SRS. All SRS less than 10 deg from the pole were treated as if they had been at -80 deg declination for the calculation of the average FK4 magnitude only. This transformation procedure gives a first order reduction from the FK4 to the FK5 system. A more precise reduction of many of the SRS catalogs observed from 1961 to 1973 is possible from a re-discussion of the FK4 star observations made concurrently with the SRS observations on a nightly basis. A program to reduce first the SRS and then the AGK3R to the FK5 system as directly as possible, going back to the nightly observations of FK4/FK5 when they are available, is now in progress at the observatory in Washington.
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