The latest update on the James Webb Space Telescope literally sent a shiver down my spine! The telescope’s Mid-Infrared Instrument (MIRI) has now reached its operating temperature of a chilly 7 kelvins (7 deg above absolute 0, or -266 degrees C,-447 degrees F).
MIRI has now been turned on and is undergoing initial checkouts.
This frigid temp is colder than JWST’s other three instruments need to be, since MIRI detects longer infrared wavelengths than the rest of the instruments. But still, all the instruments need to reach extremely low temperatures — less than 40 K (-223 degrees Celsius, -369.4 degrees Fahrenheit).
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Most of the telescope and its instruments rely on JWST’s massive sunshield to shield it from the heat from the Sun and Earth to cool it down, as well as passive cooling — taking advantage of the frigid temperatures in deep space. Getting to the temperatures required for MIRI is not possible by passive means alone, so Webb carries an innovative cryocooler, dedicated to the task of cooling MIRI’s detectors so that it can see farther into the infrared than the other instruments.
Infrared light is basically thermal radiation, and the telescope itself has a certain temperature and continually radiates heat that would interfere with the measurements taken by the instrument’s sensors. Cooling down the entire telescope – including the four instruments’ detectors and the surrounding hardware — suppresses those infrared emissions. This allows the distant objects to be detected, without any interference from the other nearby sources.
Last week, the team passed a particularly challenging milestone called the “pinch point,” when the instrument goes from 15 kelvins (minus 433 F, or minus 258 C) to 6.4 kelvins (minus 448 F, or minus 267 C).
“The MIRI cooler team has poured a lot of hard work into developing the procedure for the pinch point,” said Analyn Schneider, project manager for MIRI at NASA’s Jet Propulsion Laboratory in Southern California. “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.”
NASA says that 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 longer infrared detectors are 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.
Scientists and engineers are now doing a series of checks to make sure the detectors are operating as expected. They are also sending commands to determine if it can execute tasks correctly.
“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 at JPL. “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.”
Now, MIRI 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. The team will conduct these preparations alongside calibration of the other three instruments, delivering Webb’s first science images this summer.