Folding Mirrors
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. The mirror would then unfold after launch. Each of the 18 hexagonal-shaped mirror segments is 1.32 meters (4.3 feet) in diameter, flat to flat. (Webb's secondary mirror is 0.74 meters in diameter.)
Why hexagonal?
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. If the segments were circular, there would be gaps between them. Symmetry is good because there need only be 3 different optical prescriptions for 18 segments, 6 of each (see above right diagram). Finally, a roughly circular overall mirror shape is desired because that focuses the light into the most compact region on the detectors. A oval mirror, for example, would give images that are elongated in one direction. A square mirror would send a lot of the light out of the central region.
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