Bringing together the mid-infrared eyes of the Webb Telescope

Bringing together the mid-infrared eyes of the Webb Telescope

Zoom in / The dust in this galaxy, shaded in red, required the MIRI instrument to resolve.

There is more than one reason why the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) is considered a pioneer. Of the four JWST instruments, it is the only one that observes in the mid-infrared range, from 5 to 28 microns; the other three are near-infrared devices with a wavelength range of 0.6 to 5 microns. To reach these wavelengths, MIRI had to be kept the coldest of any instrument on JWST, essentially setting the requirements for the telescope’s cooling system.

The stunning images taken by MIRI are a testament to the remarkable engineering feats that were achieved there, feats that were achieved by overcoming formidable challenges through painstaking teamwork and transatlantic coordination.

Doing MIRI

“I remember being told in the early days that the instrument will never be built. Some people at NASA looked at the block diagram of our management structure and said it will never work,” Professor George Rieke, who leads ​​the MIRI scientific team. remembered

MIRI was built jointly by the Jet Propulsion Lab and a European consortium involving several institutions. While the control software and detector electronics were developed at JPL in the USA, major subsystems of the instrument were developed in the UK, France, Germany, Belgium, the Netherlands, Denmark, Sweden, Ireland, Spain and Swiss.

Although everything finally fell into place, there were moments when Professor Gillian Wright, who is MIRI’s European Principal Investigator, was a little nervous. One of them was about the possibility of budget cuts in the US affecting the project. “Because it was a 50-50 partnership, there were some things that the U.S. had to provide. There were times when I thought, ‘I hope they really do,'” he said.

Wright also said the US government’s International Trade in Arms Regulation (ITAR) restrictions created some obstacles, especially in the early days. “By definition, space hardware falls [ITAR]. We would have liked a little more information about what the US had to offer. But it was a struggle because of the ITAR restrictions,” he added.

The team also faced other challenges related to military uses, starting with MIRI’s imaging detectors, which convert mid-infrared light into electrical signals. “We were using a type of detector that was developed in the US for military purposes. By the time we started developing MIRI, the military had moved on to other types. So it didn’t have strong support,” Rieke recalled.

He said the MIRI team had to work with the manufacturer to recover a key step in the construction of the detectors. “Getting these detectors when the manufacturer dropped them was the scary part,” he said.

Keep things fresh

The second challenge was to ensure that the detectors worked properly by reaching a temperature of 7 kelvin (266º C below the freezing point). It may not sound like it, but it is much lower than the 37 kelvin (-236 º C) achieved by JWST’s radiative coolers.

According to Wright, the cooler had the potential to put the MIRI project at risk. Initially, the MIRI team had designed a thermos-like container filled with liquid hydrogen to keep the instrument cool. However, this system, which could cool MIRI for five to 10 years, was very heavy. “The observatory was over its mass budget. One way to save mass was to take that system out and replace it with an active cooling mechanism,” Wright said.

This decision raised a different set of problems. “This was a significant change that came late after the MIRI design had been confirmed. Although active cooling technology had been developed for other future missions, it had not been designed for JWST and MIRI until back then. It was a risk because the technology development started about five years behind the rest of the telescope,” Wright said.

However, the cloud of uncertainty was lifted because of what Wright called “the excellent work of JPL.”

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