Wavefront Sensing and Control on the James Webb Space Telescope

Roeland P. van der Marel, marel@stsci.edu

The optical telescope element (OTE) of the James Webb Space Telescope has a three-mirror anastigmat, f/20 design, consisting of a primary, secondary, and tertiary mirror. A fine-steering mirror provides line-of-sight 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 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 wavelength range, from 0.6 to 29 microns.

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 microns can be met.

The primary mirror consists of 18 semi-rigid, hexagonal mirror segments. Together, these make up a quasi-hexagonal, 6.5 m diameter, two-ring configuration with a collecting area of 25 square meters. The segments are attached to a stable, rigid, graphite-composite backplane structure. At launch, the primary mirror is folded on two chords, allowing 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 OTE deploys over the course of a week, starting approximately five days after launch. Then a 1–2 month commissioning process to optimize the telescope image quality begins. The goal of this process is to align the primary segments into a co-phased state, and to align the primary and secondary mirrors with the aft optics system, which is fixed and houses the tertiary mirror, the fine-steering mirror, and a central baffle.