L2 Legrange Point (WHTM) – Ever since the Webb Telescope lifted off in December, one of the main goals has been to keep it cool. And then cooler. And then even cooler then that.

The Webb has four instruments designed to photograph infrared light-wavelengths the human eye cannot see. In a sense, it’s imaging heat waves. The problem is, any warm objects emit light in the infrared spectrum-and that includes components on the Webb Telescope.

There’s also a phenomenon called dark current to deal with. This is an electric current created by the vibration of atoms in the detectors themselves. It can give the sensors a false reading, as if they’ve been hit by light from an outside source, while obscuring real signals. Dropping the temperature means atoms vibrate less, so there’s less dark current.

So for the imaging instruments to work properly, they need to be cooled, a lot. Three of the main instruments can function at a relatively balmy 40 degrees Kelvin (minus 388 Farenheit), but the Mid-Infrared Instrument (MIRI) needs its temperature dropped to around 6.7 Kelvins. (Zero degrees Kelvin is what’s referred to as absolute zero, the point where particles in a substance become motionless. It’s equal to minus 273.15 degrees on the Celsius scale, or minus 459.67 degrees Fahrenheit. In other words, 6.7 kelvin is really, really cold.)

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The instruments were first cooled passively, by keeping them in the shade as the telescope traveled to, then settled into, its orbit. Sitting in the cool of the telescope’s tennis-court-sized sun shield, the instruments slowly radiated heat, eventually dropping to about 90 Kelvins, or roughly minus 298 F/minus 183 C.

Getting cooler required some tech-an electrically powered cryocooler, which cools instruments by circulating helium gas. Last week, the team passed a particularly challenging milestone called the “pinch point,” where the cryocooler’s ability to remove heat was at its lowest. Once through the pinch point, the MIRI instrument dropped from 15 kelvins (minus 433 F, or minus 258 C) to 6.4 kelvins (minus 448 F, or minus 267 C).

With the instruments now down to their operating temperatures, team members can run some more tests, taking more pictures that can be used to further calibrate the instruments. When that’s all done, we can look forward to seeing the Webb’s first scientific imaging later this year.