Optical Telescope Element

Optical Telescope Element (OTE) is one of three major sections of the James Webb Space Telescope, a large infrared space telescope launched on 25 December 2021, consisting of its main mirror, secondary mirrors, the framework and controls to support the mirrors, and various thermal and other systems.

The OTE collects the infrared light and directs it to the science instruments in Webb's second major section, the Integrated Science Instrument Module (ISIM). The OTE has been compared to being the "eye" of the telescope and the backplane of it to being the "spine". The third major section of the JWST is the Spacecraft Element (SE), which includes the spacecraft bus and sunshield.

The OTE has a three-mirror anastigmat (TMA) design, with an effective f/20 focal ratio and focal length of 131.4 meters (431 ft). The primary mirror is a tiled assembly of 18 hexagonal elements, each 1.32 meters (4.3 ft) from flat to flat. When properly aligned, this combination yields an effective aperture of 6.5 meters (21 ft) and a total collecting surface of 25.4 square meters (273 ft). The secondary mirror is a convex circular mirror with a diameter of 0.74 meters (2.4 ft), and it feeds into the Aft Optics Subsystem (part of OTE), which contains the tertiary mirror (fixed) and the Fine Steering Mirror (FSM, movable). All mirrors are made of gold-plated beryllium, due to its low weight, structural stability at low temperatures and high infrared reflectivity.

The principal subcontractor for the JWST optics was Ball Aerospace, and the mirror development team included Brush Wellman (forming), Axsys Technologies (shaping) and L-3 Communications SSG-Tinsley (polishing). The components of OTE were integrated by L3Harris Technologies to form the final system.