What is the focal length of JWST in meters?

The focal length of JWST is 131.4 meters.

This is Webb’s effective focal length: the optical distance over which its mirror system brings incoming parallel light to focus. It is remarkably long compared with the telescope’s physical dimensions because Webb uses a folded three-mirror optical design rather than a simple straight tube.

The Optical Telescope Element is a three-mirror anastigmat, containing primary, secondary, and tertiary mirrors, plus a fine-steering mirror for tiny pointing corrections. Its effective focal ratio is f/20. The primary mirror is about 6.5 meters across, yet the complete folded optical path produces a 131.4-meter effective focal length.

Focal length is not the same as mirror diameter, telescope length, or distance from Webb to its target. It helps determine image scale and field of view. Webb’s instruments then divide and analyze the focused infrared beam for imaging, spectroscopy, and wavefront sensing.