In this illustration, NASA’s multi-layered solar shield of the James Webb Space Telescope extends beneath the observatory’s honeycomb mirror. The sun shield is the first step in cooling Webb’s infrared instruments, but the Mid-Infrared Instrument (MIRI) requires extra help to reach operating temperature. Credit: NASA GSFC / CIL / Adriana Manrique Gutierrez
NASA’s James Webb Space Telescope will see the first galaxies to form after the Big Bang, but to do so, its instruments must first cool down – really cold. On April 7, Webb’s Medium Infrared Instrument (MIRI), a joint development of NASA and ESA (European Space Agency), reached its final operating temperature below 7 Kelvin (minus 447 degrees Fahrenheit or minus 266 degrees Celsius).
Along with the Web’s other three instruments, MIRI initially cooled in the shadow of Web-sized tennis court-sized sun shields, dropping to about 90 Kelvin (minus 298 F or minus 183 C). But falling to less than 7 kelvins requires an electrically powered cryocooler. Last week, the team went through a particularly challenging stage, called a “pinch point”, when the instrument went 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 worked very hard to develop the pinch point procedure,” said Analin Schneider, MIRI project manager at NASA’s Southern California Jet Propulsion Laboratory. “The team was both excited and nervous to get into critical work. In the end, it was a textbook procedure, and the performance of the cooler is even better than expected. ”
The light beam coming from the telescope enters MIRI through the mirror located at the top of the instrument and acting as a periscope. A series of mirrors then redirects the light to the bottom of the instruments, where a set of 4 spectroscopic modules are located. Once there, the light beam is divided by optical elements, called dichroic, into 4 beams corresponding to different parts of the middle infrared region. Each beam enters its own integral field unit; these components separate and reformat the light from the entire field of view, ready to be scattered in spectra. This requires the light to fold, bounce and split many times, making this probably one of Webb’s most complex light paths. To complete this amazing journey, the light from each beam is scattered by gratings, creating spectra that are then projected onto 2 MIRI detectors (2 beams per detector). Incredible engineering achievement! Credit: ESA / ATG medialab
The low temperature is necessary because all four Webb instruments detect infrared light – wavelengths slightly longer than human eyes can see. Distant galaxies, stars hidden in dust cocoons, and planets outside our solar system emit infrared light. But they also make other hot items, including Webb’s own electronics and optical hardware. Cooling the detector of the four instruments and the surrounding hardware suppresses these infrared emissions. MIRI detects longer infrared wavelengths than the other three instruments, which means it must be even colder.
Another reason Webb 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. The dark current mimics a real signal in the detectors, giving the false impression that they were struck by light from an external source. These false signals can drown out the real signals that astronomers want to find. Because temperature is a measure of how fast the atoms in the detector vibrate, lowering 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 currents, so it must be cooler than other instruments to completely eliminate this effect. For each degree the temperature of the instrument increases, the dark current increases by a factor of about 10.
NASA is testing the MIRI thermal shield of the Webb Telescope in a thermal vacuum chamber at NASA’s Goddard Space Flight Center in Greenbelt, MD. Credit: NASA
After MIRI reached a cool 6.4 kelvins, scientists began a series of tests to make sure the detectors were working as expected. Like a doctor looking for signs of illness, the MIRI team looks at data describing the health of the instrument, then gives the instrument a series of commands to see if it can perform the tasks properly. This stage is the culmination of the work of scientists and engineers at many institutions in addition to the JPL, including Northrop Grumman, who built the cryocooler, and NASA’s Goddard Space Flight Center, which oversees the integration of MIRI and the cooler to the rest of the observatory.
“We’ve spent years practicing for now, going through the commands and inspections we’ve done on MIRI,” said Mike Ressler, a MIRI project scientist at JPL. “It was like a movie script: everything we had to do was recorded and rehearsed. When the test data came out, I was thrilled to see that it looked exactly as expected and that we had a solid tool.
There are still challenges the team will have to face before MIRI can begin its scientific mission. Now that the instrument is at operating temperature, team members will make test images of stars and other known objects that can be used to calibrate and verify the operation and functionality of the instrument. The team will conduct these preparations along with the calibration of the other three instruments, delivering the first scientific images to the Web this summer.
“I am extremely proud to be part of this group of highly motivated, enthusiastic scientists and engineers from across Europe and the United States,” said Alistair Glass, MIRI Instrument Scientist at the UK Astronomy Technology Center (ATC) in Edinburgh, Scotland. This period is our “test of fire”, but it is already clear to me that the personal connections and mutual respect we have built in recent years are what will help us deliver a fantastic instrument to the world astronomy in the next few months. community. “
More about the mission
The James Webb Space Telescope is an international program led by NASA with its partners, ESA and the Canadian Space Agency.
MIRI was developed through a 50-50 partnership between NASA and ESA. JPL is leading the US effort on MIRI, and a multinational consortium of European astronomical institutes is contributing to ESA. George Ricke of the University of Arizona is the head of MIRI’s research team. Gillian Wright is MIRI’s European Principal Investigator.
Laszlo Tamas with the UK ATC manages the European consortium. The development of the MIRI cryocooler was led and operated by JPL, in collaboration with Northrop Grumman in Redondo Beach, California, and NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
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