4 ms·
I would guess it's semiautomatic-- lots of humans squinting at interference patterns, guessing at an adjustment, then waiting 24 hours to get data back. Also n
by sbierwagen 5y ago
I would guess it's semiautomatic-- lots of humans squinting at interference patterns, guessing at an adjustment, then waiting 24 hours to get data back.
Also note that JWST still hasn't cooled down to temp yet! Temps here: https://www.jwst.nasa.gov/content/webbLaunch/whereIsWebb.html https://www.jwst.nasa.gov/content/webbLaunch/whereIsWebb.htm...
Mirror is supposed to be at ~50K, and it isn't there yet. (And the instruments are still at 100K+, a month after the sunshield deployed, and you sure aren't doing anything until the actual cameras are cold!) Every time you run an alignment motor you heat up a mirror segment a little bit, so you do want to run coarse alignment early, then gradually converge on fine alignment as the whole structure approaches design temp.
Remember, JWST is weird. Hubble was one big piece of (warm) glass. Multisegment telescopes down on Earth can be nailed to a hundred ton support structure, but JWST is comparatively big and floppy, since the frame had to be light enough to launch into space. (Every kilogram of mirror support structure is another kilogram of propellant lost, and once all the fuel is gone you don't have an observatory anymore...) Something like that is just going to inherently take a while to calibrate.
- stavros 5y agoWait, how is it still warm? I guess vacuum is a much better insulator than I thought, does it just take way too long to radiate heat?
- citizenpaul 5y agoThe vaccume of space us too hot for the jwt. It actualky has a cooling system to get it colder than space is. I guess it takes time to get that cold.
- stavros 5y agoOh huh, really? That's very interesting, thanks.
- adhesive_wombat 5y agoThe temperature of deep space is 2.7K due to the cosmic microwave background. JWST's cold side is going to be about 40K, so it's substantially warmer than that. I'm not sure how much of that heat is coming through the sunshield and how much is heat from the equipment.
- citizenpaul 5y agoI'm not a physicist I just watched the NASA video explaining that the instruments will need an active cooling system called the cryocooler that cools the stuff below what it can achieve in the vacuum of space on its own.
- adhesive_wombat 5y agoI think that's a pretty poor way for NASA to have put it, because the vacuum of space 1AU from the Sun isn't particularly cold as such. Actually, as a vacuum, it tends to make things there hotter up to a point, not cooler, because you can only radiate that solar heat away. So satellites that are in sunlight, like you are at L2, can easily be at 150°C and even up to 300 depending on various factors. If the JWST were out by, say, Pluto, then you certainly could describe that vacuum as "cold" (inasmuch as a vacuum has a temperature), because you hardly get any sunlight. In fact, Pluto itself has a temperature around 40K.
- sbierwagen 5y agoDesign temp for the MIRI detectors is 6.7K. The front end electronics produce some heat, and are in a metal box sealed against light leaks, so they'll need some active cooling. Skimming this paper https://www.stsci.edu/files/live/sites/www/files/home/jwst/documentation/technical-documents/_documents/paper2.pdf https://www.stsci.edu/files/live/sites/www/files/home/jwst/d... it looks like they expect 10.46mW optical module heat, and they've given themselves 36mW of headroom "for conductive and radiative loads." (Heat conducted from the structure, and radiant heat from the the comparatively warm primary mirror?)
- Gare 5y ago> then waiting 24 hours to get data back Why 24 hours? The JWST is only 5 light-seconds away and has up to 28 Mbit/s downlink.