Optical positions for 21 3C objects

Optical positions relative to AGK have been obtained for 21 3C objects using (i) direct photographs taken with the Cambridge 17/24-inch Schmidt telescope; (ii) positions measured by Murray, Tucker and Clements and modified by our recalibration of their secondary reference stars and (iii) positions measured from 48-inch Mt Palomar Schmidt Telescope plates. The external standard errors have been estimated by comparison with the most accurate radio positions resulting in: for R.A. ± 0.15 arc sec, and for Dec a value varying between ±0.12 arc sec at δ = +40° and ±0.20 arc sec at δ = +77° and +4°.