HST snapshot survey of 3CR radio source counterparts .1. Intermediate redshifts

We have obtained images of 267 3CR radio galaxies and quasars by conducting a snapshot survey with HST's WFPC2 through a broadband red (F702W) filter. This is the first in a series of papers resulting from this survey, describing and presenting the basic data. Here we focus on the 77 radio galaxies within the redshift range 0.1 < z < 0.5 (91% of the total number of 3CR radio galaxies within this redshift range). The images show that on the 0.''1 scale of the HST observations there is a wide variety of structure in radio galaxy morphology. At least 30% of the galaxies show dust, either well-defined dust lanes or chaotically distributed throughout the galaxy. We find disturbed morphologies, such as multiple nuclei and tails of emission, that could indicate merging systems. There is an obvious optical synchrotron jet in 3C 346, and another eight jet candidates are present.