An investigation of the optical fields of 35 3CR radio sources to faint limiting optical magnitudes

The fields of 35 3CR radio sources, for which detailed radio structural information is available, have been investigated using the 200-in. Hale telescope to a uniform limiting apparent magnitude of about 23.5 or 24. Plates of 34 of these fields are presented. An analysis of the identification content of a complete sample of 46 3CR radio sources shows that the identifications are 78 per cent complete to this limiting magnitude. All the identifications fainter than apparent magnitude 20 are radio galaxies. Various problems which hamper the identification of radio sources are discussed in the light of these results. It is concluded that, while some of the non-identifications may be due to gross asymmetries of the radio source structures, some of the unidentified sources must still lie beyond the present plate limits.