etendue

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 factor regarding survey-effectiveness that is not covered by etendue is the sensitivity and resolution of the sensor: ideally its sensitivity is high and its resolution is chosen to match the telescope's etendue. Example etendue determinations:

A telescope's aperture and mirrors provide a focal plane with some etendue value, but instruments (e.g., imagers) invariably use less than the entire focal plane, and in many cases, only a small fraction of it, effectively yielding a substantially smaller etendue. For example, JWST's FOV is generally described as a 333 square-arcminute rectangle (which clearly ignores portions of a larger circular area of equal focusing ability), but the largest of that which any JWST instrument uses is 12.24 square arcminutes by NIRSpec. Given JWST's 25.4-m² collecting area, the larger rectangle represents an etendue of 2.34 m²deg², but NIRSpec's etendue calculates as 0.086 m²deg², neither value unusual for a telescope not designed for bulk surveying.

An etendue is often cited for an entire array of telescopes if the individual telescopes are viewing different parts of the sky simultaneously, for increasing the surveying rate (e.g., Pan-STARRS). This etendue is the sum of that of the individual telescopes.