JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25.4 m².
If it were built as a single large mirror, this would have been too large for existing launch vehicles.
The mirror is therefore composed of 18 hexagonal segments which will unfold after the telescope is launched.
Image plane wavefront sensing through phase retrieval will be used to position the mirror segments in the correct location using very precise micro-motors.
Subsequent to this initial configuration they will only need occasional updates every few days to retain optimal focus.
This is unlike terrestrial telescopes, for example the Keck telescopes, which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading.
The Webb telescope will use 126 small motors to occasionally adjust the optics as there is a lack of environmental disturbances of a telescope in space.

JWST's optical design is a three-mirror anastigmat, which makes use of curved secondary and tertiary mirrors to deliver images that are free of optical aberrations over a wide field.
In addition, there is a fast steering mirror which can adjust its position many times per second to provide image stabilization.

Ball Aerospace & Technologies Corp. is the principal optical subcontractor for the JWST project, led by prime contractor Northrop Grumman Aerospace Systems, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland.
Eighteen primary mirror segments, secondary, tertiary and fine steering mirrors, plus flight spares have been fabricated and polished by Ball Aerospace & Technologies based on beryllium segment blanks manufactured by several companies including Axsys, Brush Wellman, and Tinsley Laboratories.

The final segment of the primary mirror was installed on 3 February 2016, and the secondary mirror was installed on 3 March 2016.
JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25.4 m². If it were built as a single large mirror, this would have been too large for existing launch vehicles. The mirror is therefore composed of 18 hexagonal segments which will unfold after the telescope is launched. Image plane wavefront sensing through phase retrieval will be used to position the mirror segments in the correct location using very precise micro-motors. Subsequent to this initial configuration they will only need occasional updates every few days to retain optimal focus. This is unlike terrestrial telescopes, for example the Keck telescopes, which continually adjust their mirror segments using active optics to overcome the effects of gravitational and wind loading. The Webb telescope will use 126 small motors to occasionally adjust the optics as there is a lack of environmental disturbances of a telescope in space.
JWST's optical design is a three-mirror anastigmat, which makes use of curved secondary and tertiary mirrors to deliver images that are free of optical aberrations over a wide field. In addition, there is a fast steering mirror which can adjust its position many times per second to provide image stabilization.
Ball Aerospace & Technologies Corp. is the principal optical subcontractor for the JWST project, led by prime contractor Northrop Grumman Aerospace Systems, under a contract from the NASA Goddard Space Flight Center, in Greenbelt, Maryland. Eighteen primary mirror segments, secondary, tertiary and fine steering mirrors, plus flight spares have been fabricated and polished by Ball Aerospace & Technologies based on beryllium segment blanks manufactured by several companies including Axsys, Brush Wellman, and Tinsley Laboratories.
The final segment of the primary mirror was installed on 3 February 2016, and the secondary mirror was installed on 3 March 2016.
James Webb Space Telescope