The JWST telescope has a three-mirror anastigmatic design, with a 25 square meter collecting area (equivalent to approximately a 6 m circular primary). The effective focal ratio is f/16.67. A Fine Steering Mirror provides accurate pointing and image stabilization. Two primary mirror "wings" and a tripod structure supporting the secondary mirror are deployed during orbit insertion.

The primary mirror consists of 18 hexagonal segments (~1.3m flat-to-flat side), in two rings around the center, resulting in a 6.5m flat-to-flat diameter. Each segment is relatively stiff and will be figured to have the correct off-axis surface at the nominal cryogenic 40 K temperature of the primary. Each segment will be made of Beryllium.

The wavefront sensing and control subsystem must align the segments so that their wavefronts match properly, creating a diffraction limited image for a 6.5-m telescope, not simply overlapping images from 18 individual 1.3 meter telescopes. Each segment will have six actuators to change its radius of curvature (focal length). The secondary mirror has 6-degrees of adjustment for collimation and overall focus. Once aligned (about 2-3 months after launch), the telescope will be diffraction limited at 2 microns (Strehl ratio of 0.80, dominated by large scale errors) and have an encircled energy of 75% within a 0.15 arcsecond radius at 1 micron (dominated by sub-segment errors). It is estimated that the backplane will be sufficiently stable during slews that the wavefront control adjustments will be needed less than once every two weeks. Wave Front Sensing techniques and algorithms have been collaboratively developed by the Jet Propulsion Laboratories, Ball Aerospace, and the STScI.