The Cosmic Lens All-Sky Survey - I. Source selection and observations

The Cosmic Lens All-Sky Survey (CLASS) is an international collaborative programme which has obtained high-resolution radio images of over 10 000 flat-spectrum radio sources in order to create the largest and best-studied statistical sample of radio-loud gravitationally lensed systems. With this survey, combined with detailed studies of the lenses found therein, constraints can be placed on the expansion rate, matter density and dark energy (e.g. cosmological constant, quintessence) content of the Universe that are complementary to and independent of those obtained through other methods. CLASS is aimed at identifying lenses where multiple images are formed from compact flat-spectrum radio sources, which should be easily identifiable in the radio maps. Because CLASS is radio-based, dust obscuration in lensing galaxies is not a factor, and the relative insensitivity of the instrument to environmental conditions (e.g. weather, 'seeing') leads to nearly uniform sensitivity and resolution over the entire survey. In four observing 'seasons' from 1994 to 1999, CLASS has observed 13 783 radio sources with the Very Large Array (VLA) at 8.4 GHz in its largest 'A'-configuration (0.2-arcsec resolution). When combined with the Jodrell Bank VLA Astrometric Survey (JVAS), the CLASS sample contains over 16 000 images. A complete sample of 11 685 sources was observed, selected to have a flux density of at least 30 mJy in the Green Bank Survey (GB6) catalogue at 4.85 GHz (spanning the declination range 0degrees less than or equal to delta less than or equal to 75degrees and \b\ greater than or equal to 10degrees, excluding the Galactic plane) and a spectral index alpha greater than or equal to -0.5 between the NRAO-VLA Sky Survey (NVSS) at 1.4 GHz and the GB6. A typical 30-s CLASS snapshot reached an rms noise level of 0.4 mJy. So far, CLASS has found 16 new gravitational lens systems, and the JVAS/CLASS survey contains a total of 22 lenses. The follow-up of a small number of candidates using the VLA, MERLIN, the Very Long Baseline Array (VLBA) and optical telescopes is still underway. In this paper, we present a summary of the CLASS observations, the JVAS/CLASS sample, and statistics on sub-samples of the survey. An accompanying paper presents the lens candidate selection, and in a third paper the implications for cosmology are discussed.