From one position, what share of the sky can JWST see?

From one position, JWST can see about 40 percent of the sky. This is its field of regard: the portion of the sky it can point toward while keeping its sunshield between the telescope and the Sun, Earth, and Moon.

The limit is set by Webb’s thermal design, not by the size of its mirrors or cameras. Webb observes infrared light, so its instruments and mirror must remain extremely cold. The five-layer sunshield blocks heat and light from the bright side, but the telescope cannot simply turn in any direction without exposing its sensitive hardware to those sources.

The 40 percent figure describes the area accessible at one time, not the amount Webb can photograph in a single exposure. Individual instruments have much smaller fields of view. As Webb orbits the Sun, Earth, and Moon’s apparent positions change relative to the telescope, allowing its accessible region to shift.

A common mix-up is to confuse field of regard with total sky coverage. Webb can observe almost the entire sky over roughly six months, although any individual target may be unavailable during part of that cycle because of the Sun-avoidance constraints.