The coldest instrument in the web space telescope reaches the operating temperature below minus 447 ° F

In this illustration, the multilayer solar shield on NASA’s James Webb space telescope extends below the observatory’s honeycomb mirror. The sun visor is the first step in cooling Web’s infrared instruments, but the Mid-Infrared Instrument (MIRI) requires additional assistance to reach its operating temperature. Credit: NASA GSFC / CIL / Adriana Manrique Gutierrez

Webb MIRI Spectroscopy Animation

The light beam coming from the telescope enters the MIRI through the pick-off mirror, which is located at the top of the instrument and acts as a periscope. Then a series of mirrors redirects the light towards the bottom of the instruments, where a set of 4 spectroscopic modules is located. Once there, the light beam of optical elements called dichroics is divided into 4 beams corresponding to different parts of the mid-infrared region. Each beam enters its own integrated field unit; these components split and reformat the light from the entire field of view, ready to be scattered in spectra. This requires the light to be folded, bounced and split many times, making this probably one of Webb’s most complex light paths. To complete this amazing journey, the light is scattered from each beam of gratings, creating spectra that then project onto 2 MIRI detectors (2 beams per detector). A fantastic engineering art! Credit: ESA / ATG medialab

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.

NASA is testing the Web Telescope's MIRI thermal shield

NASA tests the Web Telescope’s MIRI thermal shield in a thermal vacuum chamber at NASA’s Goddard Space Flight Center in Greenbelt, MD. Credit: NASA

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

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