A radio optical reference frame. IV - Additional radio source positions in the Northern Hemisphere

Seventy-seven extragalactic radio sources north of -2-degrees declination have been observed using Mark III Very-Long Baseline Interferometry during 13 experiments from 1988 October to 1989 November. Radio positions derived from these observations supplement an ongoing project to define and maintain an all-sky radio/optical reference frame of 400 or more extragalactic sources with milliarcsecond (mas) accurate radio and optical positions. Positions for 11 new sources are presented along with improved positions for 54 sources already in the reference-frame catalog. An additional five sources have been determined to be unsuitable reference-frame objects. Observations of seven calibration sources tie the new positions to the existing catalog. The radio positions of the new sources have for-mal mean errors of approximately 0.7 mas in each coordinate. Sources for which we report improved positions now have formal errors less-than-or-equal-to 1 mas, an improvement in some cases by as much as 85%. The complementary optical program is addressed briefly.