VLBA Observations of Radio Reference Frame Sources. II. Astrometric Suitability Based on Observed Structure

We present simultaneous dual-frequency Very Long Baseline Array 2 and 8 GHz observations of 133 of the 560 extragalactic sources for which positions were reported by Johnston et al. These observations represent the second in a series of observations intended to image the entire set of sources presented by Johnston et al. and, together with previously reported observations, bring the total number of sources observed so far to 169. We use the data to quantify the magnitude of the expected effect of intrinsic source structure on astrometric bandwidth synthesis VLBI observations. We also define a source "structure index," which can be used as an estimate of the astrometric quality of the sources.