The James Webb Space Telescope's primary mirror is made up of 18 hexagonal segments. Together, these individually controlled pieces form the telescope's 6.5-meter-wide collecting surface.
The segmented design was necessary because a single mirror that large could not fit inside the rocket used to launch Webb. The hexagons fold into a compact arrangement for liftoff, then unfold in space. Each segment has actuators that adjust its position by tiny amounts so all 18 reflectors can work as one optical surface.
The segments are made from beryllium, chosen for its low mass and stable performance at the telescope's extremely cold operating temperature, and coated with gold to improve infrared reflectivity. The number 18 is sometimes confused with the mirror's 18-fold deployment components or with the telescope's other hardware, but it specifically counts the primary-mirror segments. Webb also has a separate, smaller secondary mirror.