The optical telescope element (OTE) of the James Webb Space Telescope has a three-mirror anastigmat design, consisting of a primary, secondary, and tertiary mirror. The effective focal ratio is f/20. A fine-steering mirror provides accurate pointing and image stabilization. The design of the OTE was chosen to provide a high quality image to the integrated science instrument module (ISIM), which houses the Webb science instruments.
The primary mirror consists of 18 hexagonal mirror segments (with ~1.3m flat-to-flat sides). They are arranged in two rings around the center, resulting in quasi-hexagonal shape with a 6.5m flat-to-flat diameter. The collecting area is 25 square meters (equivalent to approximately a 6 m circular primary).
The segments are attached to a stable, rigid, graphite-composite backplane structure. Each segment is relatively stiff and will be figured to have the correct off-axis surface at the nominal cryogenic 40K temperature of the primary. Each segment has six actuators to adjust its position and a seventh to change its radius of curvature (focal length). The secondary mirror has 6-degrees of adjustment for collimation and overall focus.
At launch, the two "wings" of the primary mirror are folded to allow it to fit in the fairing of the Ariane launch vehicle. Similarly, the secondary mirror is attached to a deployable tripod support structure, which latches into position after launch.
The mirrors are made of beryllium, which has an extremely small variation in its coefficient of thermal expansion over temperatures of 30-80K. This makes the telescope optics intrinsically stable for small temperature variations. A thin gold coating provides high infrared reflectivity over a broad spectral bandpass, from 0.6 to 29 μm.
All the optical elements of the OTE are in advanced stages of production. Cryo-polishing has been completed for an engineering test unit of a primary mirror segment. It has achieved an excellent wavefront error, which demonstrates that the requirements for Webb to deliver diffraction-limited performance at a wavelength of 2 μm can be met. See below for images of the Primary mirror segment EDU and the flight tertiary mirror coated in gold.
JWST Primary Mirror EDU coated in gold and JWST Flight Tertiary Mirror