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Old 11th January 2022, 05:05 PM   #481
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Originally Posted by smartcooky View Post
However, the telescopes isn't just allowed to cool down by itself. The cooling is controlled by specially placed electric heating strips. At first, this may seem counter-intuitive - why would you use heating strips to cool down an object. Well, if the telescope cools too fast, then as it shrinks (yes, it will shrink slightly as it cools) water vapour in and on parts of the telescope could freeze and become trapped. The heaters are used to manage the cooling so that everything shrinks carefully and evenly, allowing trapped water to escape as gas into the vacuum of space and not freeze as ice onto mirrors or detectors.
Thanks for that. I was actually wondering if there was some water condensation on the telescope (which would freeze into ice, of course), and about outgassing. And it makes sense too that materials shrink when they get colder. I hope it doesn't cause a problem. Think about what happens to your car's windows on a cold winter day. Hopefully there's no frost on the mirrors.
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Old 11th January 2022, 06:24 PM   #482
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Originally Posted by smartcooky View Post
Once the sunshield is deployed, the telescope and scientific instruments begin to cool a little quicker than before, but it will still take several weeks to cool down to the operating temperature and remain stable.
That operating temperature, by the way, is below 50 K (−223.2 C; −369.7 F). That's pretty darn cold.
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Old 11th January 2022, 11:22 PM   #483
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Mirror testing starts today

The testing of the 18 mirror segments and the secondary mirror will begin soon.

In this phase of the preparations, each of the primary mirror segments and the secondary mirror, will be unlocked one at a time. It will be verified that each of the 18 segments and the secondary mirror will move as required and each will be driven through its full range of adjustment. They are moved by six actuators that are attached to the back of each mirror piece. The primary mirror segments also have additional actuators at their centres to adjust their curvature

As has been explained previously, these segments move at the speed of grass growth, so this is a multi-day multi-step activity to activate and move each of the 18 primary mirror segments out of their launch configuration.
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Old 12th January 2022, 05:02 AM   #484
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There's a very short NASA video that explains how the mirrors are aligned.

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Old 12th January 2022, 05:00 PM   #485
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smartcooky, you're a smart cookie. Can you clarify something for me for a discussion I'm having at another forum?

JWST does not sit at the L2 point, correct? It orbits L2. Is this orbit fully within Earth's shadow? Or does the sun shield, like, actually shield it from the sun?
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Old 12th January 2022, 05:09 PM   #486
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Originally Posted by arthwollipot View Post
JWST does not sit at the L2 point, correct? It orbits L2. Is this orbit fully within Earth's shadow? Or does the sun shield, like, actually shield it from the sun?
The orbit JWST is heading toward is fully outside the shadow of Earth and Moon. It is not directly at L2. If it were directly at L2 it would be outside of Earth's full shadow (the "umbra") but inside partial shadow (the "penumbra" or "antumbra" depending on just how close to exact L2).

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Old 12th January 2022, 05:17 PM   #487
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Originally Posted by RecoveringYuppy View Post
The orbit JWST is heading toward is fully outside the shadow of Earth and Moon. It is not directly at L2. If it were directly at L2 it would be outside of Earth's full shadow (the "umbra") but inside partial shadow (the "penumbra").
Thank you RecoveringYuppy. I'll pass that along to the other forum.
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Old 12th January 2022, 06:32 PM   #488
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Originally Posted by arthwollipot View Post
smartcooky, you're a smart cookie. Can you clarify something for me for a discussion I'm having at another forum?

JWST does not sit at the L2 point, correct? It orbits L2. Is this orbit fully within Earth's shadow? Or does the sun shield, like, actually shield it from the sun?
What it does is orbit the sun in an orbit that is gravitationally affected by the forces that create the L2 point. The end result is that is appears to orbit L2 from our point of view as we look at L2.

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The orbit is completely outside the Earths shadow. IIRC, the angle between the Earth and the Sun from JWST's position anywhere in its Halo orbit is about 33
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Old 12th January 2022, 06:34 PM   #489
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Originally Posted by smartcooky View Post
What it does is orbit the sun in an orbit that is gravitationally affected by the forces that create the L2 point. The end result is that is appears to orbit L2 from our point of view as we look at L2.
Yes, but my question was about whether it was in direct sunlight, or in Earth's shadow. The answer appears to be yes, it is in direct sunlight.
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Old 12th January 2022, 06:42 PM   #490
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Originally Posted by arthwollipot View Post
Yes, but my question was about whether it was in direct sunlight, or in Earth's shadow. The answer appears to be yes, it is in direct sunlight.
Not sure who you're talking to with this but point out that there is no day/night cycle there either so JWST gets about twice as much sun as the Hubble or anything else in low Earth orbit.
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Old 12th January 2022, 06:54 PM   #491
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Fingers crossed. This step is going to take a while so I'll probably focus on other stuff for a while and check back in later. Looks like we have about 10 days to wait, and the L2 insertion will happen in about 11 days.
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Old 12th January 2022, 07:02 PM   #492
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Originally Posted by RecoveringYuppy View Post
Not sure who you're talking to with this but point out that there is no day/night cycle there either so JWST gets about twice as much sun as the Hubble or anything else in low Earth orbit.
Yes, good point.
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Old 12th January 2022, 09:11 PM   #493
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Originally Posted by arthwollipot View Post
Yes, but my question was about whether it was in direct sunlight, or in Earth's shadow. The answer appears to be yes, it is in direct sunlight.
Well, even if it was stationary at L2, technically it still would not be "in" the Earths shadow. The Earth's umbral shadow only extends 1.4 million kilometres, but L2 is 1.5 million kilometres from the earth. It would be in Earth's antumbra - the Earth's disk would appear wholly within the sun's disk, rather like an annular solar eclipse.


ETA: this might help to visualize


From L2, the Earth covers about 85% of the sun's disk
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Old 12th January 2022, 09:33 PM   #494
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Originally Posted by smartcooky View Post
Well, even if it was stationary at L2, technically it still would not be "in" the Earths shadow. The Earth's umbral shadow only extends 1.4 million kilometres, but L2 is 1.5 million kilometres from the earth. It would be in Earth's antumbra - the Earth's disk would appear wholly within the sun's disk, rather like an annular solar eclipse.


ETA: this might help to visualize

https://www.dropbox.com/s/n4ldwypkia...omL2.jpg?raw=1
From L2, the Earth covers about 85% of the sun's disk
What is that, an umbra for ants?
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Old 12th January 2022, 09:41 PM   #495
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Originally Posted by theprestige View Post
What is that, an umbra for ants?
From Latin ante, "before"

The antumbra is the lighter part of a shadow that forms at a certain distance from the object casting the shadow. It is involved in annular solar eclipses and planet transits.

It only exists if the light source has a larger diameter than the object.

https://www.timeanddate.com/eclipse/...ra-shadow.html
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Old 12th January 2022, 10:24 PM   #496
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Thanks for all the info, smartcooky. The issue actually formed a side discussion about why there were no cameras overviewing the telescope's deployment so that we could see what was going on.
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Old 12th January 2022, 10:32 PM   #497
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Originally Posted by arthwollipot View Post
Thanks for all the info, smartcooky. The issue actually formed a side discussion about why there were no cameras overviewing the telescope's deployment so that we could see what was going on.
Lighting levels at L2 would be comparable to those at Ceres where the Dawn mission went.
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Old 12th January 2022, 10:42 PM   #498
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Originally Posted by RecoveringYuppy View Post
Lighting levels at L2 would be comparable to those at Ceres where the Dawn mission went.
And I'm not sure whether a visible light camera exists that would work at temperatures below 50 K, but I suspect that the main reason there are no vehicle-facing cameras is that it wasn't judged worth the extra weight and cost, and wouldn't provide that much of a benefit beyond what existing sensors provide.
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Old 13th January 2022, 05:26 AM   #499
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Originally Posted by arthwollipot View Post
And I'm not sure whether a visible light camera exists that would work at temperatures below 50 K, but I suspect that the main reason there are no vehicle-facing cameras is that it wasn't judged worth the extra weight and cost, and wouldn't provide that much of a benefit beyond what existing sensors provide.
Those are all valid reasons. Plus, having to run wiring harnesses around moving parts for little benefit. They actually did try to put cameras on a mockup, but found the other sensors were more useful. Plus the fact its too dark on one side of the telescope and too bright on the other.

https://www.universetoday.com/153927...ts-deployment/
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Old 13th January 2022, 01:06 PM   #500
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Originally Posted by arthwollipot View Post
Yes, but my question was about whether it was in direct sunlight, or in Earth's shadow. The answer appears to be yes, it is in direct sunlight.
The telescope requires electricity to run the computers, actuators, reaction wheels, and communications equipment. That comes from a solar panel and a set of rechargeable batteries. Having the telescope in full sun means it can get power from an smaller solar array than if it was in shadow.
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Old 13th January 2022, 06:15 PM   #501
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Originally Posted by smartcooky View Post
L2 is not 100% stable. There is also no telling which direction it will go once it leaves... it could just as easily fall in towards the earth.
I'd expect that 20 or so years from now, when the end is near, they'll give it a final push in a desired direction.
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Old 13th January 2022, 06:19 PM   #502
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Originally Posted by Trebuchet View Post
I'd expect that 20 or so years from now, when the end is near, they'll give it a final push in a desired direction.
Kessler Syndrome probably being the greater concern than meteoritic landfall.
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Old 13th January 2022, 06:25 PM   #503
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Originally Posted by Trebuchet View Post
I'd expect that 20 or so years from now, when the end is near, they'll give it a final push in a desired direction.
Why? Why not just leave it there? It's not in anyone's way.
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Old 13th January 2022, 06:29 PM   #504
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Originally Posted by theprestige View Post
Kessler Syndrome probably being the greater concern than meteoritic landfall.
Joke?

No concern for either I expect.
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Old 13th January 2022, 06:42 PM   #505
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Originally Posted by RecoveringYuppy View Post
Joke?

No concern for either I expect.
Serious.

JWST accidentally crossing LEO, hitting something, and triggering an orbital debris cascade, is probably a much bigger concern than some JWST component surviving reentry and hitting something valuable on the ground.
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Old 13th January 2022, 06:51 PM   #506
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Originally Posted by theprestige View Post
much bigger concern
You mean pedantically less small?
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Old 13th January 2022, 06:55 PM   #507
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Originally Posted by RecoveringYuppy View Post
You mean pedantically less small?
I mean, when the time comes to send it somewhere, sending it away from earth is a colossally better choice than otherwise, for obvious reasons having to do with Kessler Syndrome.
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Old 13th January 2022, 08:13 PM   #508
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Originally Posted by arthwollipot View Post
Why? Why not just leave it there? It's not in anyone's way.
L2 isn't stable, so it won't stay there.

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Old 13th January 2022, 08:22 PM   #509
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Originally Posted by Dave Rogers View Post
L2 isn't stable, so it won't stay there.
Okay then. Good reason.
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Old 14th January 2022, 09:34 AM   #510
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Originally Posted by RecoveringYuppy View Post
You mean pedantically less small?
Now I'm imagining a tragicomic conversation between two NASA engineers.

"Well, we don't want it falling back into LEO somehow and accidentally triggering Kessler Syndrome."

"That's not even a concern! Space is huge, according to Douglas Adams. We could send it literally almost anywhere, and not have to worry about it falling back towards Earth."

"Great! Should be no problem at all to avoid the one bad outcome."

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Old 14th January 2022, 10:11 AM   #511
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Originally Posted by theprestige View Post
Now I'm imagining a tragicomic conversation between two NASA engineers.

"Well, we don't want it falling back into LEO somehow and accidentally triggering Kessler Syndrome."

"That's not even a concern! Space is huge, according to Douglas Adams. We could send it literally almost anywhere, and not have to worry about it falling back towards Earth."

"Great! Should be no problem at all to avoid the one bad outcome."

. . .

"You had ONE JOB, Kevin!"
You realize the JWST is like 4 times the distance from the Earth to the Moon.

That things not going to "fall" back to Earth any sooner than the moon will.
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Old 14th January 2022, 10:49 AM   #512
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Originally Posted by Mike Helland View Post
You realize the JWST is like 4 times the distance from the Earth to the Moon.

That things not going to "fall" back to Earth any sooner than the moon will.
My point is that when there's really only one way things could go at all wrong, it's probably not a bad idea to just rule out that one risk. Especially when it's apparently so easy to do so.

And L2 isn't a stable orbit. Unlike the moon, something at L2 is going to leave that orbit and go somewhere else sooner rather than later. That "somewhere else" is extremely unlikely to be anywhere near Earth. Which is good. It should also make the end of life mission planner's job really easy. All I'm saying is, it's still their job, no matter how easy it is.
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Old 14th January 2022, 04:36 PM   #513
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Originally Posted by arthwollipot View Post
Yes, but my question was about whether it was in direct sunlight, or in Earth's shadow. The answer appears to be yes, it is in direct sunlight.
Yup. I'm pretty sure that's one of the operational parameters of the mission (0% eclipse/shadow). I can guess that moving into shadow at some point would cause thermal stress on something and affect the delicate alignment.
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Old 14th January 2022, 04:42 PM   #514
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Originally Posted by arthwollipot View Post
And I'm not sure whether a visible light camera exists that would work at temperatures below 50 K, but I suspect that the main reason there are no vehicle-facing cameras is that it wasn't judged worth the extra weight and cost, and wouldn't provide that much of a benefit beyond what existing sensors provide.
I imagine decades ago when the vehicle was designed folks weren't used to just tossing a camera on for media views. Might think harder about that today.

But this would be a tough place to do it anyway. I'd guess a camera on the cold side can't see anything (nothing is lit). While a camera on the warm side doesn't have much photogenic to look at.
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Old 14th January 2022, 10:07 PM   #515
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https://phys.org/news/2022-01-life-earth.html

A link to latest theory on origin of life to help Webb seek spectra I guess.
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Old 14th January 2022, 10:09 PM   #516
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Originally Posted by Samson View Post
https://phys.org/news/2022-01-life-earth.html

A link to latest theory on origin of life to help Webb seek spectra I guess.
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Old 15th January 2022, 12:12 AM   #517
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If anyone's interested, the Where's Webb page now has a Mirror Segment Deployment Tracker feature up:

https://webb.nasa.gov/content/webbLa...l?units=metric

Quote:
Mirror Segment Deployment Tracker
Nominal Event Time: Launch + 18-28 days

Status: Ongoing

The adjacent image tracks the progress of the individual primary mirror segments (A,B,C) and the secondary (SM) mirror as they move upward 12.5mm from their stowed launch position to a deployed state where they are ready for the mirror alignment process. View a full screen copy.

The mirrors move in very small increments over the course of ~10 days to complete their deployment. They move approximately 1mm per day each. For more detail on this process read this blog entry.

Each primary mirror segment has an ID which consists of a letter (A,B,C) followed by a number. The letter denotes one of 3 different 'prescriptions' for each group of primary mirror segments.

NOTE: Segment A3 and A6 will be moved separately at the end of the process because their position sensors are read out in a different way.
Most of the segments other than A3 and A6 have moved 2.5 mm so far.

The average primary mirror temp is -200 C, and the instrument radiator is -201 C.
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Old Yesterday, 09:00 PM   #518
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I would like to query the 1/10,000 the thickness of a human hair for tuning the mirror.
I do not believe this.
Does anyone else have difficulty here?
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Old Yesterday, 09:26 PM   #519
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Originally Posted by Samson View Post
I would like to query the 1/10,000 the thickness of a human hair for tuning the mirror.
I do not believe this.
Does anyone else have difficulty here?
Well, let's check. First step is to look up the thickness of a human hair I suppose:

https://hypertextbook.com/facts/1999/BrianLey.shtml

Of course, it varies, but one figure given is 25.4 μm as a standardized result. The actual thickness varies from hair to hair.

1/10,000 of 25 μm would be 2.5 nm.

Wikipedia says:
https://en.wikipedia.org/wiki/James_...pace_Telescope

Quote:
On January 12, 2022, mirror alignment began. The primary mirror segments are moved away from their protective launch positions. This will take about 10 - 12 days, because the 126 actuator motors are designed to fine-tune the mirror positions at microscopic accuracy (10 nanometer increments) and must each move over 1.2 million increments (12.5mm) during initial alignment.[228][229]
So is 10 nanometers the minimum amount it can move? I'm out of my league here.

I'm pretty sure though that such distances are not unheard of in precision engineering. In semiconductor manufacturing, the distances are measured in nanometers:

https://www.tel.com/museum/magazine/...7_report04_01/

The wiring used in a modern semiconductor chip is so thin that you could fit about a thousand of them (including the semiconductor that goes in between) parallel in the width of a human hair (defined as 25 microns).
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Old Yesterday, 09:50 PM   #520
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Originally Posted by Puppycow View Post
Well, let's check. First step is to look up the thickness of a human hair I suppose:

https://hypertextbook.com/facts/1999/BrianLey.shtml

Of course, it varies, but one figure given is 25.4 μm as a standardized result. The actual thickness varies from hair to hair.

1/10,000 of 25 μm would be 2.5 nm.

Wikipedia says:
https://en.wikipedia.org/wiki/James_...pace_Telescope



So is 10 nanometers the minimum amount it can move? I'm out of my league here.

I'm pretty sure though that such distances are not unheard of in precision engineering. In semiconductor manufacturing, the distances are measured in nanometers:

https://www.tel.com/museum/magazine/...7_report04_01/

The wiring used in a modern semiconductor chip is so thin that you could fit about a thousand of them (including the semiconductor that goes in between) parallel in the width of a human hair (defined as 25 microns).
25.4m would be near the low end.

https://lewigs.com/width-and-thickness-of-human-hair/
"There is no precise value for the thickness of human hair in micrometers. The diameter of human hair is proved to range between 17 μm to 181 μm (Brian, 1999). Flaxen hair (yellowish-gray) is the thinnest with an average width of 17–50 m, while black hair is believed to be the thickest with an average thickness of 56–181 m.."
If we take a median value it would be around 100M, and 1/10000 of that is, hey presto, 10nm

https://scitechdaily.com/james-webb-...roron-its-way/
“Getting there is going to take some patience: The computer-controlled mirror actuators are designed for extremely small motions measured in nanometers. Each of the mirrors can be moved with incredibly fine precision, with adjustments as small as 10 nanometers (or about 1/10,000th of the width of a human hair). Now we’re using those same actuators instead to move over a centimeter. So these initial deployments are by far the largest moves Webb’s mirror actuators will ever make in space."

At full speed, it takes about a day to move all the segments by just 1 millimeter. It’s about the same speed at which grass grows!
IIRC SlyJoe has already explained the engineering of this earlier in the thread.



ETA: also worth noting that the primary mirror temperature has "overtaken" the instrument radiator temperature.

Primary Mirror: -206C (down from -198C yesterday)
Instrument Radiator: -201C (same as yesterday)

About 15 degrees to go.
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