James Webb Telescope

RecoveringYuppy said:
You mean pedantically less small?
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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!"
 
theprestige said:
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!"
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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.
 
Mike Helland said:
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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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.
 
arthwollipot said:
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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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.
 
arthwollipot said:
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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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.
 
If anyone's interested, the Where's Webb page now has a Mirror Segment Deployment Tracker feature up:

https://webb.nasa.gov/content/webbLaunch/whereIsWebb.html?units=metric

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.
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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.
 
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?
 
Samson said:
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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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_Webb_Space_Telescope

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]
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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/material/150227_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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Puppycow said:
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_Webb_Space_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/material/150227_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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25.4µm 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 µm, while black hair is believed to be the thickest with an average thickness of 56–181 µm.."​

If we take a median value it would be around 100µM, and 1/10000 of that is, hey presto, 10nm

https://scitechdaily.com/james-webb-space-telescope-mirror-mirroron-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: -206°C (down from -198°C yesterday)
Instrument Radiator: -201°C (same as yesterday)

About 15 degrees to go.
 
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