The JWST telescope has a three-mirror anastigmatic design, with a 29.4 square meter collecting area (equivalent to approximately a 6.5 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 36 hexagonal segments (1m flat-to-flat side), in three rings around the center, resulting in a 7m flat-to-flat diameter. Each segment is relatively stiff and will be cryo-figured to have the correct off-axis surface at the nominal 40 K temperature of the primary. Each segment will be made of either Beryllium or ultra-low expansion glass depending on the results of the Advanced Mirror System Demonstrator program. The mirror technology will be chosen in the third quarter of 2003.

The wavefront sensing and control subsystem must align the segments so that their wavefronts match properly, creating a diffraction limited 7-m telescope, not simply overlapping images from 36 individual 1 meter telescopes. Each segment can be moved in tip, tilt, and piston with three actuators and can have its radius of curvature (focal length) adjusted using a single actuator. 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 0.84, dominated by large scale errors) and have an encircled energy of 82% within a 0.15 arcsecond radius at 1 micron (dominated by sub-segment errors). It is estimated that through the use of semi-rigid mirror segments wavefront control adjustments are needed less than once a month. Wave Front Sensing to keep the mirror in shape is currently studied at the Jet Propulsion Laboratories.