NASA’s James Webb Telescope reaches a new level of cool

Webb views the universe using infrared, a wavelength important for peering through gas and dust to see distant objects.

It has a large mirror and a suite of instruments all protected by a five-layer sunshield which unfurls to the size of a tennis court. The mirror is segmented into 18 hexagonal pieces on a hinged structure – it’s the largest mirror to be flown in space! It’s so large that it is folded up for launch and has to unfold itself in space.

In January 2022 the Webb telescope fully deployed its 21-foot, gold-coated mirror and on March 11, the Webb team completed the stage of alignment known as ‘fine phasing’. At this key stage, every optical parameter that has been checked and tested is performing at, or above, expectations. The team also found no critical issues and no measurable contamination or blockages to Webb’s optical path. The observatory can successfully gather light from distant objects and deliver it to its instruments without issue.

On April 7, Webb’s Mid-Infrared Instrument (MIRI) – a joint development by NASA and ESA (European Space Agency) – reached its final operating temperature below seven kelvins (-266°C).

All four of Webb’s instruments detect infrared light and the distant galaxies and stars the telescope is viewing emit infrared light. But so do other warm objects, such as Webb’s own electronics and optics. The instruments’ detectors need to be cooled down to stop those infrared emissions.

The MIRI detects longer infrared wavelengths than the other three instruments, which means it needs to be even colder. MIRI’s ability to detect longer infrared wavelengths also makes it more sensitive to dark current.

Dark current, or electric current created by the vibrations of atoms in the detectors, mimics detector signals giving the false impression that they’ve been hit by an external light source. The MIRI needs to be cool enough to fully remove this effect.

It did initially cool off in the shade of Webb’s sunshield but to get it to less than seven kelvins required an electrically powered cryocooler. Webb itself cannot get the MIRI to the required temperature by passive means alone, so it carries a cryocooler that is solely dedicated to cooling MIRI’s detectors.

The team passed a particularly challenging milestone called the ‘pinch point’, where the instrument went from 15 kelvins (-258°C) to 6.4 kelvins (-267°C). It is at this point that the scientists make a series of checks to make sure the detectors were operating as expected.

Now that the instrument is at operating temperature, the team will take test images of stars and other objects to confirm the instruments functionality.