James Webb solar panels deploy early

[AE-NMidlands]

It's like being in a vacuum flask, no convection and hardly any conduction to worry about, so if you can stop the radiative energy then all the heat leaks out into space which has an ambient temp of about 5 deg K IIRC.
Desp

I understand the "no convection," but it's not really obvious how a few (or multiple) reflecting layers are going to stop all that radiated heat.
With no convection possible at all, it must depend on the efficiency of the reflective surfaces and attrition!
Surface 1 reflects 99.9%. Surface 2 receives 0.1% radiated from the back of #1, reflects 99.9% and passes 0.01% (0.00001 of the original?) on to the next layer. Maybe it is that simple.
Presumably the 6 Kelvins is as low as it is worth trying to achieve if deep space is 5K anyway. How would you use any active cooling to get below it? Unless you could somehow transport the energy to a radiator to beam it off into space, whereupon the 5K ambient would warm it back up anyway!
A

[spread-tee]

I think that is how it works, an Aluminised surface in a reflecting scope can easily reflect 88% of the radiation that hits it, in visible light that is, so if you have 7 layers as the Webb does of I think it is a gold reflective layer then you should be able to reflect almost all of the energy beaming in from the Sun. I don't know how they deal with all the different wavelengths though. Maybe as the scope is working in the IR the shorter wavelengths don't interfere or heat the sensors so much??

Desp

[Andy]

It's like being in a vacuum flask, no convection and hardly any conduction to worry about, so if you can stop the radiative energy then all the heat leaks out into space which has an ambient temp of about 5 deg K IIRC.
Desp

I understand the "no convection," but it's not really obvious how a few (or multiple) reflecting layers are going to stop all that radiated heat.
With no convection possible at all, it must depend on the efficiency of the reflective surfaces and attrition!
Surface 1 reflects 99.9%. Surface 2 receives 0.1% radiated from the back of #1, reflects 99.9% and passes 0.01% (0.00001 of the original?) on to the next layer. Maybe it is that simple.
Presumably the 6 Kelvins is as low as it is worth trying to achieve if deep space is 5K anyway. How would you use any active cooling to get below it? Unless you could somehow transport the energy to a radiator to beam it off into space, whereupon the 5K ambient would warm it back up anyway!
A

There was a video somewhere showing how the layers worked. If I remember rightly they are also aligned so radiation bounces out side ways from the layers as well.

https://webb.nasa.gov/content/observato ... hield.html

[nowty]

There was a video somewhere showing how the layers worked. If I remember rightly they are also aligned so radiation bounces out side ways from the layers as well.

https://webb.nasa.gov/content/observato ... hield.html

After the first layer, its the only way to remove the residual heat.

[nowty]

There is also an active cooler on some of the instruments too.
https://webb.nasa.gov/content/about/inn ... ooler.html

[AE-NMidlands]

News of a micro-meteorite strike:
https://www.bbc.co.uk/news/science-environment-61744257

A tiny rock fragment has hit the new James Webb Space Telescope's main mirror.
The damage inflicted by the dust-sized micrometeoroid is producing a noticeable effect in the observatory's data but is not expected to limit the mission's overall performance.
/snip/
The incident appears to have occurred sometime between 23 and 25 May.
Analysis indicates the mirror segment known as C3 - one of the 18 beryllium-gold tiles that make up Webb's 6.5m-wide primary reflector - was struck. The contact left a "dimple" in the segment, Nasa told the Reuters news agency.
The speed at which things move through space means even the smallest particles can impart a lot of energy when colliding with another object. Webb has now been hit five times with this latest event being the most significant.

It was on the morning BBC radio news, but I couldn't find it reported before now.
A

[spread-tee]

A few dings on the reflective surfaces wouldn't affect the wavefront too much, but if there is too much dust on the mirror it could scatter quite a bit of light. I don't know if the Hubble scope suffered too much like this over the years.

Desp

[nowty]

A few dings on the reflective surfaces wouldn't affect the wavefront too much, but if there is too much dust on the mirror it could scatter quite a bit of light. I don't know if the Hubble scope suffered too much like this over the years.

Desp

Webb has an open design; its mirrors are not guarded by the kind of tubular baffle seen on other space telescopes, such as Hubble. Instead, the reflectors sit behind one giant sunshield that allows them to maintain the stable, cold temperatures needed to detect infrared light.

The possibility of micrometeoroid hits was anticipated and contingencies like this were incorporated into the choice of materials, the construction of components and the different modes of operating the telescope.

"We always knew that Webb would have to weather the space environment, which includes harsh ultraviolet light and charged particles from the Sun, cosmic rays from exotic sources in the galaxy, and occasional strikes by micrometeoroids within our Solar System," said Paul Geithner, technical deputy project manager at Nasa's Goddard Space Flight Center in Greenbelt, Maryland.

"We designed and built Webb with performance margin - optical, thermal, electrical, mechanical - to ensure it can perform its ambitious science mission even after many years in space."

Engineers will adjust the positioning of the affected mirror segment to cancel out a portion of the introduced distortion, but they can't remove it all.

[spread-tee]

Engineers will adjust the positioning of the affected mirror segment to cancel out a portion of the introduced distortion, but they can't remove it all.

That doesn't make any sense to me, if that segment is moved at all it wont focus the light in the correct place and so will not contribute anything to the final image

But as they say this kind of damage is to be expected, and after all the secondary mirror will cast a big shadow on the primary which acts exactly the same as a big ding.

Hopefully the engineers will hand it over to the scientists soon?

Desp

[spread-tee]

https://www.theguardian.com/science/202 ... -telescope

wow, just wow

fascinating seeing the gravitational lensing and how clean the images are, I know they will have gone through a lot of sprucing up but even so it is plain to see that they ground the mirrors to the right shape this time

Desp