James Webb Telescope

smartcooky said:
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.
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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.
 
smartcooky said:
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.
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That operating temperature, by the way, is below 50 K (−223.2 °C; −369.7 °F). That's pretty darn cold.
 
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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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?
 
arthwollipot said:
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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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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RecoveringYuppy said:
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").
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Thank you RecoveringYuppy. I'll pass that along to the other forum.
 
arthwollipot said:
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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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.

Hard to grasp the concept? The video animation might help



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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smartcooky said:
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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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.
 
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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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.
 
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.
 
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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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

Earth-Sun-fromL2.jpg

From L2, the Earth covers about 85% of the sun's disk
 
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smartcooky said:
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

[qimg]https://www.dropbox.com/s/n4ldwypkiaz5zjv/Earth-Sun-fromL2.jpg?raw=1[/qimg]
From L2, the Earth covers about 85% of the sun's disk
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What is that, an umbra for ants?
 
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.
 
arthwollipot said:
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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Lighting levels at L2 would be comparable to those at Ceres where the Dawn mission went.
 
RecoveringYuppy said:
Lighting levels at L2 would be comparable to those at Ceres where the Dawn mission went.
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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.
 
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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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/15392...ameras-on-board-to-livestream-its-deployment/
 
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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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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