A square mirror would send a lot of the light out of the central region.Įach of Webb's mirrors has an individual designation. A oval mirror, for example, would give images that are elongated in one direction. Finally, a roughly circular overall mirror shape is desired because that focuses the light into the most compact region on the detectors. Symmetry is good because there need only be 3 different optical prescriptions for 18 segments, 6 of each (see above right diagram). If the segments were circular, there would be gaps between them. The hexagonal shape allows for a roughly circular, segmented mirror with 'high filling factor and six-fold symmetry.' High filling factor means the segments fit together without gaps. (Webb's secondary mirror is 0.74 meters in diameter.) Why hexagonal? Each of the 18 hexagonal-shaped mirror segments is 1.32 meters (4.3 feet) in diameter, flat to flat. The mirror would then unfold after launch. The Webb Telescope team also decided to build the mirror in segments on a structure which folds up, like the leaves of a drop-leaf table, so that it can fit into a rocket. Watch a video on how the telescope deployed after launch More information on Webb's launch configuration. When Webb launched, both wings were stowed in this position, which enabled the mirror to fit into the launch vehicle. Each wing holds three of its primary mirror segments.

The James Webb Space Telescope is shown with one of its two 'wings' folded.

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