Here's why Webb's most sensitive instrument needs cryogenic cooling

Three of Webb's four instruments are aligned, so what's the hold-up?
Brad Bergan
An artist's depiction of Webb.dima_zel / iStock

So close, but so far.

The James Webb Space Telescope is in orbit of the second Lagrange point (L2), with three of its four primary instruments aligned and ready to go — but one remains.

The most unexplored frontiers of space reside in mid-infrared wavelengths. And Webb's Mid-InfraRed Instrument (MIRI) is poised to remedy this empirical blindspot. The reason for this is mainly due to the way our atmosphere interferes with our ability to observe the universe from the ground — where most telescopes have operated since the days of Galileo Galilei.

"Dramatic results in the mid-infrared have come from telescopes in the vacuum of space, where they are cooled to cryogenic temperatures," said Webb's Deputy Senior Project Scientist Jonathan Gardner of NASA's Goddard Space Flight Center, in a blog post from the agency.

But it's already really cold in space, and there's no atmosphere way out beyond the orbit of the moon. Why, then, must MIRI be cryogenically cooled?

Cryocoolerelectronics
Webb's cryocooler electronics, during tests. Source: NASA / JPL-Caltech

How the James Webb Telescope cools its MIRI instrument

Unlike the first three instrument alignments on the James Webb Space Telescope, the MIRI will take longer to bring to full readiness, since it employs a different kind of sensor that requires incredibly low temperatures to operate. So low that an onboard cooler and heater are required to keep it in the target temperature range.

The other three instruments have reached their operating temperatures between 34 and 39 kelvins, but for MIRI to work, it needs to be brought down to 7 kelvins. This is accomplished with a specialized cryocooling system. "Over the last couple of weeks, the cryocooler has been circulating cold helium gas past the MIRI optical bench, which will help cool it to about 15 kelvins," said Bret Naylor and Konstantin Penanen in a joint statement from NASA's Jet Propulsion Laboratory, in an agency post.

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"Soon, the cryocooler is about to experience the most challenging days of its mission," continued Naylor and Penanen. "By operating cryogenic valves, the cryocooler will redirect the circulating helium gas and force it through a flow restriction. As the gas expands when exiting the restriction, it becomes colder, and can then bring the MIRI detectors to their cool operating temperature of below 7 kelvins."

Webb Cryocooler
The cooling device for Webb's MIRI, in a test chamber. Source: NASA / JPL-Caltech / Flickr

Why Webb's MIRI instrument must be kept below 7 kelvins

The James Webb Telescope's MIRI instrument is highly sensitive — so sensitive that it needs to be extremely cold to suppress infrared background "noise", including the heat from the instrument itself. "The detectors inside each scientific instrument, that convert infrared light signals into electrical signals for processing into images, need to be cold to work just right," said a different NASA blog page.

In general, the longer the wavelength of infrared light, the colder each allocated detector needs to be to execute this conversion, without itself creating too many random "noise" electrons. MIRI can see mid-infrared light at wavelengths from 5 to 28 microns, which means its Arsenic-doped Silicon detectors must be kept below 7 kelvin to work correctly.

This temperature of outer space is typically 2.7 kelvins, but within Webb, where instruments and computers are active, it can jump a little above that — which is why "passive cooling" (the way other instruments remain cool from the exposure to space alone) just isn't enough. Thus, the James Webb Telescope uses a cryocooler specifically to cool its MIRI mid-infrared instrument down to sufficient temperatures, below 7 kelvin.

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