Webb launches: a folded mirror becomes an observatory

The James Webb Space Telescope was built through cooperation among NASA, ESA and the Canadian Space Agency, primarily for infrared observation. Its approximately 6.5-metre primary mirror comprises 18 hexagonal segments, with a sunshield keeping the optics and instruments cold.

Infrared measurements investigate distant galaxies, star formation and planetary atmospheres. Because the equipment itself emits thermal radiation, temperature control affects the ability to detect faint signals.

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Webb launched on an Ariane 5 from French Guiana on December 25, 2021. Its mirror and sunshield were folded inside the fairing. After separation, it travelled toward the region around Sun–Earth L2.

About 1.5 million kilometres beyond Earth in the direction away from the Sun, this region allows an orbit in which the shield keeps the main heat sources on one side of the observatory.

References: [1]

During the journey, teams deployed the sunshield, secondary mirror and primary-mirror wings. Deployment established the structure, but precision adjustment was still needed to merge the images produced by individual segments and obtain a sharp focus.

Cooling, optical alignment and observing-mode tests continued for months. Ground systems analysed test data and converted measurements into further adjustments as the observatory entered scientific operation.

References: [1]

Rigby and colleagues’ 2023 performance paper reviewed six months of commissioning across the optics, instruments, spacecraft and ground system. Measured sensitivity, image quality and stability supplied the basis for observing plans beyond pre-launch predictions.

Cameras and spectrometers produce images and wavelength information. These measurements support studies of early galaxies and stars, as well as atmospheric composition inferred from spectral changes when planets cross their stars.

References: [1]