Critical instrument on Webb Telescope reaches -266°C operating temperature

The Mid-Infrared Instrument (MIRI) reached -266°C on April 7, just over three months after its launch on Christmas day in 2021.

Along with Webb’s three other instruments, MIRI initially cooled off in the shade of Webb’s tennis-court-size sunshield, dropping to about -183°C, but further temperature drops required an electrically powered cryocooler.

Analyn Schneider, project manager for MIRI, said: “The team was both excited and nervous going into the critical activity. In the end it was a textbook execution of the procedure and the cooler performance is even better than expected.”

The low temperature is necessary because all four of Webb’s instruments detect infrared light – wavelengths slightly longer than those that human eyes can see.

Distant galaxies, stars hidden in cocoons of dust and planets outside our solar system all emit infrared light. So do other warm objects, including Webb’s own electronics and optics hardware. Cooling down the four instruments’ detectors and the surrounding hardware suppresses those infrared emissions. MIRI detects longer infrared wavelengths than the other three instruments, which means it needs to be even colder.

Another reason Webb’s detectors need to be cold is to suppress something called dark current, or electric current created by the vibration of atoms in the detectors themselves.

Dark current mimics a true signal in the detectors, giving the false impression that they have been hit by light from an external source. Those false signals can drown out the real signals astronomers want to find. Since temperature is a measurement of how fast the atoms in the detector are vibrating, reducing the temperature means less vibration, which in turn means less dark current.

MIRI’s ability to detect longer infrared wavelengths also makes it more sensitive to dark current, so it needs to be colder than the other instruments to fully remove that effect. For every degree the instrument temperature goes up, the dark current goes up by a factor of about 10.

Once MIRI reached its target temperature, the team began a series of checks and tests to make sure the detectors were operating as expected.

“We spent years practicing for that moment, running through the commands and the checks that we did on MIRI,” said Mike Ressler, project scientist for MIRI. “It was kind of like a movie script: everything we were supposed to do was written down and rehearsed. When the test data rolled in, I was ecstatic to see it looked exactly as expected and that we have a healthy instrument.”

There are still more challenges that the team will have to face before MIRI can start its scientific mission.

Now that the instrument is at operating temperature, team members will take test images of stars and other known objects that can be used for calibration and to check the instrument’s operations and functionality.