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**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
**AΩ**. 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 Telescope | 29.3 m²deg² |

Palomar 48 Inch Telescope | 50 m²deg² |

TESS | 21.9 m²deg² |

Kepler Telescope | 4.2 m²deg² |

Pan-STARRS PS1 | 15 m²deg² |

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

http://en.wikipedia.org/wiki/Etendue

https://arxiv.org/abs/1203.0590