A characteristic of a telescope, defined as the product of its collecting area in square metres (A) times the field of view in square degrees (Ω); also known as throughput. A large aperture combined with a large field of view results in a large étendue. Telescopes intended for sky surveys require a large étendue so that they can image large areas of sky to faint magnitudes. The Large Synoptic Survey Telescope (LSST) has an exceptionally large étendue of 319 m2 deg2.