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. But so do other hot items, including Web’s own electronics and optics hardware. Cooling of the four instrument detectors 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 why Web’s detectors need to be cold is to suppress something called dark current, or electric current created by vibrations 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. These false signals can drown out the right signals that astronomers want to find. Since temperature is a measure of how fast the atoms in the detector vibrate, a reduction in 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 colder than the other instruments to fully eliminate that effect. For each degree the instrument temperature rises, the dark current increases by a factor of about 10.
When MIRI reached a cold 6.4 kelvin, the researchers began a series of checks to make sure the detectors were working as expected. As a physician searches for any signs of illness, the MIRI team looks at data describing the health of the instrument and then gives the instrument a series of commands to see if it can perform tasks correctly. This milestone is the culmination of work done by scientists and engineers at several institutions beyond Related