An image of an elliptical galaxy.

Assembling the Webb Telescope’s Mid-Infrared Eye

enlarge / The dust in this galaxy is shaded red and requires MIRI instruments to resolve.

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) is considered groundbreaking for more than one reason. Of the four instruments on the JWST, it is the only one that makes observations in the mid-infrared range (5 to 28 microns); the other three are near-infrared instruments with a wavelength range of 0.6 to 5 microns. To reach these wavelengths, MIRI must remain in the coldest position of any instrument on the JWST, which means it essentially sets the requirements for the telescope’s cooling system.

The stunning images captured by MIRI are a testament to the extraordinary feat of engineering within, a feat achieved through meticulous transatlantic teamwork and coordination overcoming formidable challenges.

Make Miri

“I remember someone telling me early on that the instrument would never be built. Some people at NASA looked at the block diagram of our management structure and said it would never work,” recalls Professor George Rieke, who led MIRI’s science team.

MIRI was built by the Jet Propulsion Laboratory and a European consortium involving multiple institutions. While the control software and detector electronics were developed at JPL in the US, the main subsystems of the instrument were developed in the UK, France, Germany, Belgium, Netherlands, Denmark, Sweden, Ireland, Spain and Switzerland.

Although everything finally fell into place, MIRI’s European Principal Investigator, Professor Gillian Wright, was at times a little nervous. One of them is about the possibility of U.S. budget cuts affecting the program. “Because it’s a 50-50 partnership, there’s something America needs to offer. Sometimes I think, ‘I hope they actually do that,'” she said.

Wright also said the U.S. government’s International Traffic in Arms Regulations (ITAR) restrictions created some hurdles, especially early on. “By definition, space hardware is [ITAR]. We would have liked to know more about what the US has to offer. But it’s a struggle because of ITAR restrictions,” she added.

The team faced other challenges related to military use, starting with MIRI’s imaging detectors, which convert mid-infrared light into electrical signals. “We used the type of detectors that were developed in the US for military purposes. When we started developing MIRI, the military had moved to other types. So it wasn’t very supported,” recalls Rieke.

He said the MIRI team had to work with manufacturers to restore the critical steps in making the detectors. “Getting these detectors when the manufacturer leaves them out is the scary part,” he said.

keep cool

The second challenge was to make sure the detector was working properly by reaching a temperature of 7 Kelvin (266ºC below freezing). It might not sound like it, but that’s well below the 37 Kelvin (-236º C) achieved by the JWST radiant cooler.

According to Wright, the coolers have the potential to put the MIRI project at risk. Initially, the MIRI team designed a thermos-like container filled with liquid hydrogen to keep the instrument cool. However, this system, which can cool MIRI for 5 to 10 years, is heavy. “The observatory exceeded its mass budget. One way to save mass is to dismantle this system and replace it with an active cooling mechanism,” Wright said.

The decision brings up a different set of problems. “After the MIRI design was confirmed, this was a significant change. While active cooling technology was already being developed for other future missions, it had not been designed for the JWST and MIRI until then. This was a risk because the technology was developed faster than the telescope’s The rest are about five years late,” Wright said.

However, the cloud of uncertainty has been lifted thanks to what Wright calls “the excellent work of JPL.”

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