What is the diameter of JWST's primary mirror?
Answer
6.5 meters
The diameter of JWST's primary mirror is 6.5 meters. That broad collecting surface allows Webb to gather far more faint infrared light than smaller space telescopes, helping it study the first galaxies, forming stars, and distant exoplanet systems.
Webb's mirror is not a single solid disc. It is assembled from 18 hexagonal segments made of lightweight beryllium and coated with a thin layer of gold, which reflects infrared light efficiently. The segments fold for launch and are aligned after deployment in space to act as one precisely shaped mirror.
A common mix-up is confusing the mirror's diameter with its area or with Hubble's 2.4-meter primary mirror. Webb's 6.5-meter mirror gives it roughly six times Hubble's light-collecting area. Its sunshield and extremely cold operating environment are equally important because infrared observations can be overwhelmed by heat from the spacecraft itself.