What is JWST's focal ratio?

JWST's focal ratio is f/20.2.

Focal ratio is the focal length divided by the telescope’s effective aperture. Webb has an effective focal length of about 131.4 meters and an aperture of roughly 6.5 meters, producing a ratio close to 20.2. In practical terms, this is a long-focus optical system: the telescope’s focal length is about 20 times its effective aperture.

The number does not mean Webb is a conventional camera lens or that every instrument has the same field of view. Webb’s three-mirror anastigmat optics fold the light path inside the observatory, allowing a very long effective focal length within a spacecraft that had to fit inside an Ariane 5 rocket. Individual instruments then use their own optical assemblies and detectors.

The focal ratio is also easy to confuse with light-collecting power. Webb gathers light primarily because of its large 6.5-meter segmented mirror; f/20.2 describes the geometry of the optical system. Hubble, by comparison, has a different optical design and focal ratio, so the two observatories cannot be compared by f-number alone.