Optical astrometric positions of 59 northern ICRF radio sources

Context.To investigate the link between the International Celestial Reference Frame (ICRF) and its optical Hipparcos-based representation on the northern hemisphere. Aims.To present results of a pilot investigation on the astrometry of 59 northern ICRF sources. Methods.We used the 0.6 m Zeiss telescope at Belogradchik Observatory, Bulgaria. The optical CCD source positions were referred to the UCAC2 catalog. Improved astrometric methods were applied including telescope shifting between exposures, ($x,\,y$) Gaussian measurements within 1 full width at half maximum, and the observation of a large number of frames per source. The huge amount of data generated was treated with a new astrometric package, PRAIA (Platform for Reduction of Astronomical Images Astrometrically). Results.Average and standard deviation for the optical minus radio position offsets were +6 mas (51 mas) and +7 mas (57 mas) for RA and Dec, respectively. The errors of all ($x,\,y$) measurements from Gaussian fits displayed a typical distribution with respect to magnitude, dependent on the sky transparency. For the ICRF objects, the ($x,\,y$) errors per source ranged from 50 mas to 100 mas. The RA and Dec reduction mean errors were 46 mas. Conclusions.No large scale systematic errors with respect to RA or Dec were found within the attained position precision. Comparison with three independent telescope$/$catalog datasets shows that the precision obtained here represents an important improvement on previous works. In comparison with the southern hemisphere, more astrometry on ICRF sources are needed in the north, and it is shown here that the continuation of this program can fill this need