CLASS: Cosmic Lens All-Sky Survey

The Cosmic Lens All-Sky Survey (CLASS), is an international (USA, UK and Netherlands) collaborative program aimed at obtaining high-resolution radio images of over 10000 at-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, powerful constraints can be placed on the cosmography (ie. expansion rate, mean density, and cosmological constant) of the Universe. CLASS is aimed at identifying lenses where multiple images are formed from compact at-spectrum radio sources, which should be easily identiiable in the radio maps. In three observing \seasons" in 1994, 1995, and 1998, CLASS has observed over 12,000 radio sources. When combined with the JVAS survey, the CLASS sample contains over 15,000 images and at least 17 lenses, with 11 in CLASS so far as well as a number of candidates still being followed up. In this poster, we present a summary of the CLASS observations, the JVAS-CLASS sample, and statistics on subsamples of the survey. Overview The Cosmic Lens All-Sky Survey (CLASS) is an international (USA, UK and Netherlands) collaborative project to map more than 10,000 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, powerful constraints can be placed on the cosmography (ie. expansion rate, mean density, and cosmological constant) of the Universe. CLASS is aimed at identifying lenses where multiple images are formed from compact at-spectrum radio sources. The lens conngurations should be easily identiiable in the radio maps. Thus, CLASS is most eecient at nding galaxy-mass lenses (which will dominate the counts for surveys not targeted at clusters) with separations of around a few arcseconds. CLASS will also be able to detect more extreme lensed systems with larger separations, which are due to clusters of galaxies. Of course, the CLASS database will also contain a rich sample of radio galaxies and quasars for the study of AGN phenomena. The Very Large Array(VLA) is being used as the primary instrument for the CLASS survey. In its largest \A" connguration, the VLA provides high-quality images with an angular resolution of 0.2 arcseconds at a typical observing frequency of 8.5 GHz. In the rst three 1