The world is still reeling from the release of the James Webb Space Telescope‘s (JWST) first images. These provided a comprehensive overview of the kind of science operations that Webb will conduct over its 20-year mission. They included the most sensitive and detailed look at some iconic astronomical objects, spectra from an exoplanet atmosphere, and a deep field view of some of the most distant galaxies in the Universe. Since their release, we’ve also been treated to glimpses of objects in the Solar System captured by Webb‘s infrared instruments.
Meanwhile, the JWST collaboration released a full report titled titled “Characterization of JWST science performance from commissioning,” in which they examined everything Webb has accomplished so far and what they anticipate throughout the mission. This paper recently appeared online and covers everything from the telescope’s navigation and pointing to the performance of its many instruments. An interesting tidbit, which was not previously released, is how Webb suffered a series of micrometeoroid impacts, one of which caused “uncorrectable change” in one mirror segment.
The team behind this study included researchers from the three participating space agencies – NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA) – and from the mission’s many partner agencies. These include the Space Telescope Science Institute (STScI), the Niels Bohr Institute, the Max-Planck-Institut für Astronomie (MPIA), the UK Astronomy Technology Centre (UK ATC), the National Research Council Canada (NRCC), the Instituto Nacional de Técnica Aeroespacial (INTA), the Centro de Astrobiología (CAB), and many aerospace companies, universities, research institutes, and agencies worldwide.
The main components of the JWST’s primary mirror. Credit: NASA/STScI
The paper they compiled assesses the JWST performance during the six-month commissioning period before it entered service on July 12th, 2022. This consisted of characterizing the observatory’s on-orbit performance, the JWST’s design and architecture, and the pre-launch predicted performance. These were then compared to the performance of the spacecraft, telescopes, science instruments, and ground system. Section 4 of the Report, Optical Performance, addresses how Webb’s various instruments functioned during the commissioning period.
The JWST’s primary mirror consists of eighteen hexagonal segments arranged in a honeycomb configuration. Each segment is composed of gold-plated beryllium, and all are aligned to ensure the highest resolution and sensitivity possible. The overall performance is measured in terms of Wavefront Error (WFE), which refers to how light collected by the telescope’s mirrors deviates from the expected wavelength of light. The overall extent is determined by calculating the collected light’s deviation from the Root-Mean-Square (RMS) error – the spherical average of the entire wavefront.
This is expressed mathematically using the units of the particular wavelength, measured in nanometers (nm) when dealing with Infrared wavelengths. Section 4.7 addresses micrometeoroid impacts and their potential effect on Webb’s long-term optical performance. The assessment begins by reminding readers that any spacecraft will inevitably encounter micrometeoroids, then lists how several impacts were expected during the commissioning period:
“During commissioning, wavefront sensing recorded six localized surface deformations on the primary mirror that are attributed to impact by micrometeoroids. These occurred at a rate (roughly one per month) consistent with pre-launch expectations. Each micrometeoroid caused degradation in the wavefront of the impacted mirror segment, as measured during regular wavefront sensing. Some of the resulting wavefront degradation is correctable through regular wavefront control; some of it comprises high spatial frequency terms that cannot be corrected.”
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