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.