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(AΩ, étendue)
(measure of how wide and deep a telescope observes)

Etendue (or étendue) is a measure of a combination of the width and depth (how far into space) that a telescope's view captures. For a typical telescope, it is calculated as the collection area (typically π/4 × aperture² minus the cross section of any obstruction within the telescope) times the field of view (FOV), with units of m²deg², and is often referred to as . The measure is especially of interest for surveys, capturing a limit on how much of space the telescope is effectively viewing at any instant, so telescopes designed for surveys tend to have large etendues. The FOV clearly indicates how much of the sky is viewed, and the collection area indicates the light collecting ability, thus its magnitude limit and the distance it effectively covers. Another independent factor is the sensitivity and resolution of the sensor: ideally its sensitivity is high and its resolution is chosen to match the FOV and aperture. Examples:

Rubin Observatory (LSST)319 m²deg²
Roman Space Telescope29.3 m²deg²
Palomar 48 Inch Telescope50 m²deg²
TESS21.9 m²deg²
Kepler Telescope4.2 m²deg²
Pan-STARRS PS115 m²deg²

Etendue is also often cited for arrays of telescopes (e.g., Pan-STARRS), multiplying by the number of telescopes if they are identical.

Further reading: