The James Webb Space Telescope

JWST Observatory: The Optical Telescope Element (OTE)

The Optical Telescope Element (OTE) is the eye of the JWST Observatory. The OTE gathers the light coming from space and provides it to the science instruments. JWST needs a large mirror to collect as much light as possible to see galaxies from the beginning of the Universe. The JWST scientists and engineers have determined that a primary mirror 6.5 meters (21.3 feet) across is needed to measure the light from these galaxies.

The OTE has 2 other components: the Fine Steering Mirror (FSM) and the structural pieces to hold everything together [the secondary mirror support structure (SMSS) and the primary mirror backplane assembly (PMBA)]. The OTE tertiary mirror and the fine steering mirror are both contained within an OTE subsystem known as the Aft Optics Subsystem. The PMBA, in addition to holding the OTE together will be where the science instrument, in the Integrated Science Instrument Module (ISIM), is installed in the Observatory.

No launch vehicle (rocket) is large enough to hold a 6.5-meter mirror if it was all one piece. The JWST team decided to build the mirror with 18 hexagonal primary mirror assembly segments, which can be folded up (shown at left) to fit into the launch vehicle and then unfold after launch.

This animation shows how JWST can fold up to fit in the launch vehicle.

This animation shows JWST unfolding and deploying in space.

So that the segments work together as a single large mirror, the 18 segments have been divided into 3 groups of six mirrors, each group having a slightly different shape (prescription). A system known as Wavefront Sensing & Control has been developed, as well. The image at right shows the location of the different mirrors within the primary mirror.

In addition to making the mirror small enough to fit into a rocket, the JWST team also has to make it light enough to be launched. If the Hubble Space Telescope's mirror (2.4 meters) was scaled to be large enough for JWST, it would be too heavy to launch into orbit. The JWST team had to find new ways to build the mirror so that it would be light enough.