Following Webb’s arrival at its orbital destination around Lagrange Point 2 (L2) on January 24, the mission operations team began working its way through a critical series of steps: powering on all the science instruments, turning off heaters to begin a long cooldown process, and ultimately capturing the first photons on Webb’s primary camera to enable a months-long alignment of the telescope.
While the MIRI instrument and some instrument components were powered on in the weeks after Webb’s December 25 launch, the team didn’t finish turning on the remaining three instruments – NIRCam, NIRSpec, and FGS/NIRISS – until the past few days.
The mission operations team’s next major step is to turn off instrument heaters. The heaters were necessary to keep critical optics warm to prevent the risk of water and ice condensation. As the instruments meet pre-defined criteria for overall temperatures, the team is shutting off these heaters to allow the instruments to restart the months-long process of cooling to final temperatures.
When NIRCam reaches 120 kelvins (approximately -244 degrees Fahrenheit, or -153 degrees Celsius), Webb’s optics team will be ready to begin meticulously moving the 18 primary mirror segments to form a single mirror surface. The team has selected the star HD 84406 as its target to begin this process. It will be the first object NIRCam “sees” when photons of light hit the instrument’s powered-on detectors. The process will essentially create an image of 18 random, blurry points of light. For the first few weeks of mirror alignment, the team will keep the instrument trained on the star while they make microscopic adjustments to the mirror segments; ultimately that collection of 18 blurry dots will become a focused image of a single star. Cooling of the telescope and instruments will also continue over the next month, with the near-infrared instruments ultimately reaching 37-39 kelvins. The cryocooler will cool MIRI to 6 kelvins in the following months.
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In conjunction with JWST, I’ve seen explanations of Lagrange points, which
A) are something that I think every high school graduate should be familiar with
B) can easily be googled, without any JWST related explanations (not that such explanations are a bad thing)
I have even seen questions and explanations of why JWST is slowing down!!
On the other hand, when I read
“The heaters were necessary to keep critical optics warm to prevent the risk of water and ice condensation”
I must wonder,
A) What would be the source of the water molecules?
Air pockets trapped in the instruments, prior to leaving Earth’s atmosphere?
B) Why is it safe to turn the heaters off now?
Because the air pockets (presuming that they’re the source) will have dissipated?
Pretty much every surface that’s exposed to Earth’s sea-level atmosphere has a small amount of water that has been adsorbed onto the surface from humidity in the air. When exposed to vacuum, this water will desorb, but some of that water vapor released will condense onto the coldest surface around. So, the heaters are to make sure that the sensitive surfaces aren’t the coldest surface around.
(thruster firings can also release some vapor, known as “plume effects”.)
After a week or two in space, most of the water vapor released will have escaped into the vacuum, so you can stop heating the sensitive instruments.
Thanks, nice explanation!
Yes, nice explanation!
I don’t understand the -244 temperature. The current temperature of WEBB is -344. This is what is displayed on the NASA web site “where is WEBB”. Is this for a different instrument?
-344 F refers to the temperature on the cold side of the spacecraft sunshield. -244 F refers to the temperature read on the NIRCam instrument itself, which is not the same.
Can we send President Trump up with a bottle of champagne to christen it?
President Trump can’t go, we need him here in case we ever get that thieving idiot out of the Oval Office.
All goes according to plan.
According to the NASA Where is Webb? site, NIRCam has attained 116K, allowing HD 84406 acquisition and mirror alignment to commence. The site now displays temperatures of all instruments in C and K.
This is so great reading. Thank you so much 👍