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James Webb Telescope

RecoveringYuppy said:
And for all we know it wasn't one line all the way back. We are pretty certain that Eukarya (that's basically all life large enough to see and even some you can't see) was the product of a merger of lines. Bacteria have lateral gene transfer which doesn't make for well defined lines. Whatever came before might be the same. I certainly don't expect that all the chemicals of the simplest life form we know of now could have been produced simultaneously in one decisive reaction.
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That reminds me: mitochondria have their own DNA. They were probably originally a separate lifeform that got subsumed in a symbiotic relationship with a host organism.
 
The Aft Deployable Instrument Radiator has successfully deployed. This increases the radiator area by about 50% and we should now see the instrument radiator temperature tracking downwards a little. quicker.
 
Darat said:
Have you read about how long it will take to adjust the individual mirror cells to make the big mirror? A month! The precision required is :jaw-dropp
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:jaw-dropp = 2.5 nanometres (or about 1/10,000th the thickness of an averag human hair)

https://www.planetary.org/articles/jwst-first-images

Why will it take six months to see JWST's first science images?

"It will take the JWST team three months to very slowly and carefully align the 18 mirror segments. How slow? One of the telescope scientists described it to me this way: “the mirror segments move at the same rate that a blade of grass grows.” This delicate but time-consuming process brings the 18 separate images together to create one sharp image."​
 
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Trebuchet said:
I'm a mechanical engineer, and I find it hard to imagine how to design something to move that slowly!
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Really small gear - really big gear - axle - really small gear - really big gear, etc?

This is the Lego designers version. Probably not suitable for deep space teloscopy :)
 
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Okay, can someone explain to me what's happening here? I can't quite work it out.
 
3point14 said:
:D

and

:p
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Sorry, couldn't help it. :)

I assume you are actually wondering about the movement of the individual mirror cells? There are those here who can explain it way better than me but essentially, the large mirror is not one solid surface. It is made up of a number of smaller mirrors that were "folded up" during the launch and have to be positioned precisely relative to each other to provide a clear image. This is a long and very slow process which has just been initiated.
 
3point14 said:
Okay, can someone explain to me what's happening here? I can't quite work it out.
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The screw is advancing quickly through the channel by engaging the threads. It's pushing the nut ahead of it, which would normally mean the nut would be advancing as quickly as the screw.

However, the screw itself is hollow, and threaded inside. The nut is engaging with the threads inside the screw, which are at a different ratio of "turns to advance" than the outer threads. So while the nut advances, it ends up advancing more slowly than the screw.

Thus you can turn the screw at a speed that's convenient to your motor, but drive the nut at a speed that's convenient to whatever it is the nut is driving. Very slowing aligning mirrors to extremely tight tolerances, for example.
 
3point14 said:
Okay, can someone explain to me what's happening here? I can't quite work it out.
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I assume you mean the video?

First, just think of the simple part: the bolt on the right hand side.

The bolt on the right is moving in to the block as you turn it which is a perfectly normal thing that everyone has probably done. If it's a bolt that that has 8 threads per inch it take 8 turns to advance an inch in to the object it's being screwed into.

That by itself can give you fairly fine positioning of something. To move the tip of the bolt 1/8" of an inch you turn it once. Turn it half a turn and you get a movement of the tip of the bolt 1/16".

But then that bolt also has a screw (on the left side of the video). That screw is threaded such that it is retracting in to the bolt as the bolt is turning. That slows down the progress of the tip of that screw so that you have even finer control.
 
theprestige said:
The screw is advancing quickly through the channel by engaging the threads. It's pushing the nut ahead of it, which would normally mean the nut would be advancing as quickly as the screw.

However, the screw itself is hollow, and threaded inside. The nut is engaging with the threads inside the screw, which are at a different ratio of "turns to advance" than the outer threads. So while the nut advances, it ends up advancing more slowly than the screw.

Thus you can turn the screw at a speed that's convenient to your motor, but drive the nut at a speed that's convenient to whatever it is the nut is driving. Very slowing aligning mirrors to extremely tight tolerances, for example.
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RecoveringYuppy said:
I assume you mean the video?

First, just think of the simple part: the bolt on the right hand side.

The bolt on the right is moving in to the block as you turn it which is a perfectly normal thing that everyone has probably done. If it's a bolt that that has 8 threads per inch it take 8 turns to advance an inch in to the object it's being screwed into.

That by itself can give you fairly fine positioning of something. To move the tip of the bolt 1/8" of an inch you turn it once. Turn it half a turn and you get a movement of the tip of the bolt 1/16".

But then that bolt also has a screw (on the left side of the video). That screw is threaded such that it is retracting in to the bolt as the bolt is turning. That slows down the progress of the tip of that screw so that you have even finer control.
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Thank you both. It now makes sense. It just wouldn't add up for some reason.
 
Here, this might help to visualize how very small movements can be made.

Gear%26Worm.png


This is a worm gear and drive plate for an astronomical telescope. I allows the rotational axis of the brass plate to be rotated very slowly and very accurately

Ideally, in this case, the large brass gear has 1,436 teeth (the number of minutes in a sidereal day). If you rotate the worm gear one rotation, it rotates the drive plate 1/1436th of a rotation, so if you rotate the worm gear at a rate of 1 rpm, the drive plate will rotate one rotation per sidereal day.

Mount this on the equatorial axis of an equatorially mounted telescope, and a star the field of view will remain pretty much in the same place indefinitely.

Its a little more complicated than this in reality, but that is the basics.

NOTE: this drive plate looks more like it has 718 teeth, so the worm gear would have to be rotated at ½ rpm to turn the plate one rotation per sidereal day.


Trebuchet: Does this give you any ideas for winding up one of your Weapons of "Mass" destruction?
 
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