The LBDS Hercules sample of mJy radio sources at 1.4 GHz - I. Multicolour photometry

The results are presented of an extensive programme of optical and infrared imaging of radio sources in a complete subsample of the Leiden-Berkeley Deep Survey. The LBDS Hercules sample consists of 72 sources observed at 1.4 GHz, with flux densities S-1.4 greater than or equal to 1.0 mJy, in a 1.2 deg(2) region of Hercules. This sample is almost completely identified in the g, r, i and K bands, with some additional data available at J and H, The magnitude distributions peak at r similar or equal to 22 mag, K similar or equal to 16 mag and extend down to r similar or equal to 26 mag, K similar or equal to 21 mag. The K-band magnitude distributions for the radio galaxies and quasars are compared with those of other radio surveys. At S-1.4 GHz less than or similar to 1 Jy, the K-band distribution does not change significantly with radio flux density. The sources span a broad range of colours, with several being extremely red (r - K greater than or similar to 6). Though small, this is the most optically complete sample of mJy radio sources available at 1.4 GHz, and is ideally suited for studying the evolution of the radio luminosity function out to high redshifts.