James Webb telescope's coldest instrument reaches operating temperature:
NASA's James Webb Space Telescope will see the first galaxies to form after the Big Bang, but to do that, its instruments first need to get cold—really cold. 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 7 kelvin (minus 447 degrees Fahrenheit, or minus 266 degrees Celsius).
Along with Webb's three other instruments, MIRI initially cooled off in the shade of Webb's tennis-court-size sunshield, dropping to about 90 kelvin (minus 298 F, or minus 183 C). But dropping to less than 7 kelvin required an electrically powered cryocooler. Last week, the team passed a particularly challenging milestone called the "pinch point," when the instrument goes from 15 kelvin (minus 433 F, or minus 258 C) to 6.4 kelvin (minus 448 F, or minus 267 C).
"The MIRI cooler team has poured a lot of hard work into developing the procedure for the pinch point," said Analyn Schneider, project manager for MIRI at NASA's Jet Propulsion Laboratory in Southern California. "The team was both excited and nervous going into the critical activity. In the end it was a textbook execution of the procedure, and the cooler performance is even better than expected."
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 warm objects, including Webb's own electronics and optics hardware. Cooling down the four instruments' detectors and the surrounding hardware suppresses those infrared emissions. MIRI detects longer infrared wavelengths than the other three instruments, which means it needs to be even colder.
(Score: 5, Informative) by Anonymous Coward on Friday April 15 2022, @01:37PM (3 children)
I'm not an IR guy, so I didn't have a good intuitive feel for the background signal you'd get at these temperatures. The JWST instrument they're talking about here, MIRI, has a wavelength range of 5 to 28 microns. That's is right out there in the thermal IR region, of course, and EVERYTHING has a temperature, so everything is radiating in that wavelength range. I was curious what the difference dropping from 90K to 7K, because by itself 90K is pretty damn cold, and I'm used to visible sensors where 90K is way overkill to keep their sensor noise sources down (IIRC, Hubble's cameras run around -100 C = 173K).
If you consider MIRI to be an idealized blackbody source (which, of course all good physicists would), you can integrate Planck's equation over wavelength to find out what the radiance of the sensor is at 90K and 7K, so you can calculate how much signal MIRI would detect just from its own temperature. If you first consider what is the average wavelength of the radiation at these temperatures, from Wien's Law you get about 32 microns at 90K and 414 microns at 7K, so right away you can see that the peak wavelength of radiation is very close to the upper MIRI limit at 90K, but WAY displaced at 7K. So if you punch the numbers in and integrate from 5 to 28 microns (sensitive range of MIRI) you get a background radiance of 0.2 W/m2/sr at 90K and (essentially) zero at 7K. Without digging through design review documents, I don't know why they picked 7K over any other temperature, but my guess is that once you got below whatever their required temperature was, it was what the cooler they chose could get down to.
Ultimately, you have to do the same kind of calculation for all of the structures in the optical path that MIRI can see, such as all of the mirror surfaces, and then calculate how much of that makes it back to MIRI, and that is what will ultimately set its detection threshold because those surfaces aren't going to be as cold as this.
(Score: 0) by Anonymous Coward on Friday April 15 2022, @03:18PM
Interesting!
(Score: 2) by hendrikboom on Friday April 15 2022, @08:19PM (1 child)
It's possible that they chose 7 K because it was the coldest they could figure out how to accomplish, and then realized it was cold enough to make new kinds of observations. So the project got a go-ahead.
I'm quite sure they would have preferred colder. Could it be that the optics couldn't handle longer wavelengths either? Diffraction-limited optics?
-- hendrik
(Score: 1, Informative) by Anonymous Coward on Friday April 15 2022, @08:50PM
One paper I found [stsci.edu] says that the design is the structure around it needs to be below 15.5K with a stability of plus/minus 1K, and they decided to cool everything down to 7K to eliminate troublesome temperature gradients that might result in some misalignments. I still haven't found a reason for 7K on the sensor, which I'm sure is in a sensor characterization paper somewhere. That paper is very interesting, by the way, because you have to go to great lengths to get that heat out without letting any creep back in.
(Score: 1) by pTamok on Sunday April 17 2022, @09:33PM
In case you are wondering, like me, what the "pinch-point" is, here's an easy-to-understand definition:
From Thermal Engineering: What is Pinch Point – Heat Exchanger – Definition [thermal-engineering.org]
The MIRI instrument has a cryocooler to get the sensor temperature below ambient temperature. The instrument uses a three-stage pulse tube (Stirling Engine) cryocooler followed by a Joule-Thomson refrigerator (same fundamental design as most domestic refrigerators, just different working fluid). As the refrigerator cools down the cold side, there comes a point (known as the "pinch-point") where the the temperature difference between hot and cold fluid is minimum: at this point the heat transfer between the hot and cold fluids is at its lowest, and it needs to exceed the rate of heat transfer from the environment into the cold-side, otherwise you can't get the cold-side any colder.
Sources:
https://www2.jpl.nasa.gov/adv_tech/coolers/Cool_ppr/C15-2008%20MIRI%20Cryocooler%20Design.pdf [nasa.gov]
https://jwst.nasa.gov/content/about/innovations/cryocooler.html [nasa.gov]
https://www2.jpl.nasa.gov/adv_tech/coolers/ACTDP_MIRI.htm [nasa.gov]
https://cryocooler.org/resources/Documents/C16/002.pdf [cryocooler.org]
http://ircamera.as.arizona.edu/MIRI/miricooler.pdf [arizona.edu]
https://en.wikipedia.org/wiki/Cryocooler [wikipedia.org]
https://www.researchgate.net/publication/312506402_Basics_of_Joule-Thomson_Liquefaction_and_JT_Cooling [researchgate.net]