NASA / reuters
During the lens alignment process of the James Webb Space Telescope (JWST), its infrared camera continued to cool down, andReached operating temperature in the last week. Among them, the lowest temperature required is the Mid-Infrared Instrument (MIRI), which requires -266 degrees Celsius, or an ultra-low temperature of only 6.4 degrees above absolute zero.
JWST starts by using its tennis-court-sized “visor” to isolate infrared heat sources from the sun and the earth, which alone can reduce the temperature of the backlit surface to -183 degrees Celsius (90K). But even lower than this, an electrically powered active cooling device circulates ultra-low temperature helium gas through the MIRI instrument, further reducing it to about -258 degrees Celsius (15K). But in this last stage from -258 degrees Celsius to -266 degrees Celsius, it is not feasible to circulate helium alone, but to use additional valves to restrict the flow of helium. When the trapped helium is released from pressure at the other end, it instantly expands and reduces pressure to cooler temperatures. The operation of this valve requires the rapid and accurate sequential movement of multiple components at extremely low temperatures, and is the most troublesome item of all instrument preparation.
This was the procedure done last week, and once the temperature dropped to -266 degrees Celsius, MIRI didn’t need much energy to maintain a low operating temperature, while the passive cooling of the sun visors alone was sufficient for the other three instruments. There are two main reasons why infrared sensors are so sensitive to heat. One is Webb’s own electronic and optical parts, which actually heat up slightly during operation. If this heat is not removed, it will interfere with the accuracy of the instrument; the other reason is to suppress the so-called “dark current”. —— This is the random vibration of the atoms of the photoreceptor itself, which produces a signal similar to being hit by a photon, forming “noise” in the picture. Lowering the temperature reduces the vibration of the atoms and suppresses the amount of dark current.
After completing the cooling last week, MIRI has passed all the instrument health checks, and the next step is to photograph some stars that are familiar to astronomers for instrument calibration and functional confirmation. The first official photos are still expected to arrive this summer.
Cool news! Webb’s MIRI instrument recently passed through its critical “pinch point” and cooled to just a few kelvins above absolute zero, which is the coldest you can go: https://t.co/jjE7xTal0O
Wondering why MIRI is extremely chill? Thread ❄️ pic.twitter.com/a9l7lcZ645
— NASA Webb Telescope (@NASAWebb) April 13, 2022