Cosmic Lens All-sky Survey

The Cosmic Lens All-sky Survey (CLASS) was a 1990s/2000s radio astronomical survey to discover and characterize galaxy-mass gravitational lensing amongst over ten thousand promising radio sources. Very Large Array (VLA) radio images of the sources were produced, after which about a hundred and fifty of the most promising were followed up using MERLIN, the VLBA, HST, and Keck. Twenty two such gravitational lenses were discovered.

The acronym CLASS is also used for the Cosmology Large Angular Scale Surveyor, a more recent array of four microwave telescopes surveying CMB polarization, located at the Llano de Chajnantor Observatory on the Atacama Desert, Chile.