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Tags astronomy , telescopes

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Old 25th December 2021, 06:16 PM   #241
RecoveringYuppy
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Originally Posted by Mike Helland View Post
"This orbit (which takes Webb about 6 months to complete once) keeps the telescope out of the shadows of both the Earth and Moon. "
Yeah, but not for solar power reasons. Again, you could read the sources cited.
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Old 25th December 2021, 06:19 PM   #242
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Originally Posted by Puppycow View Post
Huh. If that's true, I wonder why they chose solar panels as a power source. Couldn't they have gone with a nuclear source?
It's not true. L2 gets sunlight and they could have designed it be right at L2 if there was a good reason to. They want it where it is for consistent temperature. Getting more sunlight is a secondary concern.
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Old 25th December 2021, 06:27 PM   #243
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Originally Posted by RecoveringYuppy View Post
It's not true. L2 gets sunlight and they could have designed it be right at L2 if there was a good reason to. They want it where it is for consistent temperature. Getting more sunlight is a secondary concern.
Consistent sunlight leads to consistent temperature, and consistent power.
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Old 25th December 2021, 06:31 PM   #244
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So a nuclear power source would not have extended the telescope's useful lifetime, because it would still have to use thrusters. Right?

ETA: and also it would give off waste heat, and that might be a problem for some of the instruments that have to be kept very cold?
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Last edited by Puppycow; 25th December 2021 at 06:33 PM.
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Old 25th December 2021, 06:39 PM   #245
RecoveringYuppy
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Originally Posted by Puppycow View Post
So a nuclear power source would not have extended the telescope's useful lifetime, because it would still have to use thrusters. Right?
Sort of. Both nuclear and solar would have very long life times. So the reason nuclear wouldn't help extend the lifetime is because solar potentially lasts forever too.

Thruster fuel is the planned (or should I say hoped for) limitation, though.

Originally Posted by Puppycow View Post
ETA: and also it would give off waste heat, and that might be a problem for some of the instruments that have to be kept very cold?
Possibly. But it would presumably be on the "bus" side of the heat shield, not the instrument side. Not sure how big a problem that would actually present.

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Old 25th December 2021, 06:52 PM   #246
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This video is about the engineering behind the telescope. It has a click-baity title, but the video itself is quite good.

https://www.youtube.com/watch?v=aICaAEXDJQQ
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Old 25th December 2021, 07:17 PM   #247
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This one is called "JAMES WEBB TELESCOPE Orbit & Trajectory Explained - Where Is It Flying To?" A 6-minute video.

https://www.youtube.com/watch?v=Dxl3nXbOJzQ
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Old 25th December 2021, 07:48 PM   #248
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A little over 12 hours in, and it completes an important step:

https://blogs.nasa.gov/webb/2021/12/...rrection-burn/

Quote:
At 7:50 pm EST, Webb’s first mid-course correction burn began. It lasted 65 minutes and is now complete. This burn is one of two milestones that are time critical — the first was the solar array deployment, which happened shortly after launch.

This burn adjusts Webb’s trajectory toward the second Lagrange point, commonly known as L2. After launch, Webb needs to make its own mid-course thrust correction maneuvers to get to its orbit. This is by design: Webb received an intentional slight under-burn from the Ariane-5 that launched it into space, because it’s not possible to correct for overthrust. If Webb gets too much thrust, it can’t turn around to move back toward Earth because that would directly expose its telescope optics and structure to the Sun, overheating them and aborting the science mission before it can even begin.

Therefore, we ease up to the correct velocity in three stages, being careful never to deliver too much thrust — there will be three mid-course correction maneuvers in total.

After this burn, no key milestones are time critical, so the order, location, timing, and duration of deployments may change.
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Old 26th December 2021, 01:14 AM   #249
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Originally Posted by Beelzebuddy View Post
I really wish NASA would stop doing this prototype-only stuff. If we can justify designing a single sun-shaded telescope operating at a lagrange point a million kilometers from Earth, why not send three and have a bit of redundancy?
I think this is right.
Eta an early thread post and the emergence of trillion dollar tech companies says this current approach with a small budget is tragic. I hope to be very wrong.

Last edited by Samson; 26th December 2021 at 01:19 AM.
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Old 26th December 2021, 01:32 AM   #250
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Originally Posted by Mike Helland View Post
Not a conspiracy, but kind of eye opening:

https://www.space.com/16000-spy-sate...opes-nasa.html
That was several years ago. One will be used for the Nancy Grace Roman telescope.

https://www.nasa.gov/feature/goddard...cope-completed. Probably didn't end up saving much (if any) money, but there was a lot of pressure to try to come up with a use.
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Old 26th December 2021, 01:50 AM   #251
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Originally Posted by Puppycow View Post
So a nuclear power source would not have extended the telescope's useful lifetime, because it would still have to use thrusters. Right?
Both for occasional stationkeeping and for angular desaturation. Magnetic fields are too weak to use magnetic torque (like Hubble), so it has to be thrusters.
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Old 26th December 2021, 02:13 AM   #252
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Originally Posted by BowlOfRed View Post
Both for occasional stationkeeping and for angular desaturation. Magnetic fields are too weak to use magnetic torque (like Hubble), so it has to be thrusters.
So, how many times in a month will the thrusters fire?
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Old 26th December 2021, 02:40 AM   #253
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Originally Posted by Mike Helland View Post
So, how many times in a month will the thrusters fire?
Only a couple. The spacecraft is constrained in where it can be for everything to work. It is planned to fire them every 21 days.

The better the orbit is modeled, the less fuel will be used. But the radiation pressure on the sun shield complicates things. And even the planned observation targets (which change the orientation of the shield) are not expected to be known that far in advance. So fuel use is expected to be greater than than what would be required for an "optimal" orbit.

https://ntrs.nasa.gov/api/citations/...0140007519.pdf
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Old 26th December 2021, 04:09 AM   #254
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https://mobile.twitter.com/marinakor...67236244750345

Quote:
Unlike Hubble, Webb isn't designed to be fixed by astronauts. But it can be refueled robotically. Zurbuchen says that "once this telescope is deployed, I'm going to put all the effort towards developing that technology, and so within the 10-year lifespan, we can go refuel it."
Quote:
A few days ago I asked Bill Ochs, JWST's project manager at NASA, what he would say if Elon Musk offered to try to refuel JWST someday, which would be a very Musk thing to say. Ochs said, "Yeah, go for it."
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Old 26th December 2021, 05:19 AM   #255
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Well if it's working as it should, a refueling mission might be well worth it.

If there are other problems than simply needing more fuel though, it might not be worth it.
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Old 26th December 2021, 10:35 AM   #256
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Wasn't it stated upthread that there is no 'dock'? How can it be refuelled if that is the case?
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Old 26th December 2021, 11:08 AM   #257
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Would have thought better to start on a JW Mk 2 than refuel a 10 year old Mk 1.
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Old 26th December 2021, 11:25 AM   #258
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Originally Posted by RecoveringYuppy View Post
Yeah, but not for solar power reasons. Again, you could read the sources cited.
No, actually solar power is relevant here.

https://en.wikipedia.org/wiki/Lagrange_point#L2
"For example, the angular radius of the sun as viewed from L2 is arcsin(695.5×103/151.1×106) ≈ 0.264°, whereas that of the earth is arcsin(6371/1.5×106) ≈ 0.242°. Looking toward the sun from L2 one sees an annular eclipse. It is necessary for a spacecraft, like Gaia, to follow a Lissajous orbit or a halo orbit around L2 in order for its solar panels to get full sun."
So yeah, you still get sunlight at L2, but you get a lot less.

Note also that while L2 is unstable, it's a saddle point instability, not a maximum. If you displace from L2 perpendicular to the sun-earth-L2 direction, then the net gravitational force pushes you back towards L2, it's only displacements along the direction of sun-earth-L2 which are unstable. So you can orbit L2 for free, as long as that orbit is perpendicular to the sun-earth-L2 direction. Which is what they're doing, and which then allows them to get full sunlight. You only need fuel to correct for variations along that direction, which you would still need even at L2.
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Old 26th December 2021, 11:27 AM   #259
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Originally Posted by Darat View Post
Would have thought better to start on a JW Mk 2 than refuel a 10 year old Mk 1.
If the cost of a refueling mission is, say, 10% of the cost of the JW, seems like a refueling mission would be worth it. And given the cost and complexity of JW, I wouldn't be surprised if it's in that ballpark. The launch cost is a minority of the total cost.
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Old 26th December 2021, 01:05 PM   #260
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Originally Posted by welshdean View Post
Wasn't it stated upthread that there is no 'dock'? How can it be refuelled if that is the case?
It's presumably got something that can be latched on to even if it's not a traditional "dock" such as the ISS would have. Presumably some of the structure that mates it to the launch vehicle is sturdy enough to support a docking.

What I can't find is a reliable statement that the fuel tanks have a robotic compatible way of refilling. There is an alternative that something could dock and then take over the positioning of the satellite. I don't know what complications there might be with that approach.

And sending a human presumably changes the cost by a lot, not to mention the risk.

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Old 26th December 2021, 01:14 PM   #261
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https://apnews.com/article/space-lau...dd437fc0ed5b33

Quote:
NASA is shooting for 10 years of operational life from Webb. Engineers deliberately left the fuel tank accessible for a top-off by visiting spacecraft, if and when such technology becomes available.
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Old 26th December 2021, 01:24 PM   #262
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Yeah, thanks for that Pixel42, but I'm afraid I see too much wrong from such sources to consider that reliable.
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Old 26th December 2021, 02:04 PM   #263
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Originally Posted by slyjoe View Post
That's a great site.

I do have a question on the day 30+ orbit.

The orbit for Webb is roughly circular at the L2 Lagrange point. Since the L2 point doesn't have mass there, the Webb is not orbiting a body in space (like the earth). So, how does it maintain its orbit without constant thruster firing? It must be in constant acceleration, no?
Lagrange points are actually the only stable orbits, where the gravitational forces balance out. However the complications of other massive bodies makes L2 less than perfectly stable.
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Old 26th December 2021, 02:56 PM   #264
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Originally Posted by slyjoe View Post
That's a great site.

I do have a question on the day 30+ orbit.

The orbit for Webb is roughly circular at the L2 Lagrange point. Since the L2 point doesn't have mass there, the Webb is not orbiting a body in space (like the earth). So, how does it maintain its orbit without constant thruster firing? It must be in constant acceleration, no?
Its not really a circular orbit, its the way the object at L2 orbits the sun under the influence of gravitational forces of the Earth and Sun combined that makes it appear as if it is circling the L2 point from our perspective.

Scott Manley has a great video that describes halo orbits, and how the forces at Lagrange points keep objects there stable.

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If you don't want to watch the whole 13½ minute video, the bit about L2 starts at 11:00, and there is an excellent graphic that exactly answers your question starting at 12:21.
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Old 26th December 2021, 03:12 PM   #265
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Originally Posted by Pixel42 View Post
Originally Posted by RecoveringYuppy View Post
Yeah, thanks for that Pixel42, but I'm afraid I see too much wrong from such sources to consider that reliable.
Agree with RY here. There was an early plan to have some kind of rudimentary docking apparatus to allow a spacecraft to grip it for servicing and refuelling, but that plan has been abandoned

https://jwst.nasa.gov/content/about/faqs/faq.html

"Why is Webb not serviceable like Hubble?

Hubble is in low-Earth orbit, located approximately 375 miles (600 km) away from the Earth, and is therefore readily accessible for servicing. Webb will be operated at the second Sun-Earth Lagrange point, located approximately 1 million miles (1.5 million km) away from the Earth, and will therefore be beyond the reach of any crewed vehicle currently being planned for the next decade. In the early days of the Webb project, studies were conducted to evaluate the benefits, practicality and cost of servicing Webb either by human space flight, by robotic missions, or by some combination such as retrieval to low-Earth orbit. Those studies concluded that the potential benefits of servicing do not offset the increases in mission complexity, mass and cost that would be required to make Webb serviceable, or to conduct the servicing mission itself.
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Old 26th December 2021, 04:16 PM   #266
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Originally Posted by smartcooky View Post
Its not really a circular orbit, its the way the object at L2 orbits the sun under the influence of gravitational forces of the Earth and Sun combined that makes it appear as if it is circling the L2 point from our perspective.

Scott Manley has a great video that describes halo orbits, and how the forces at Lagrange points keep objects there stable.

YouTube Video This video is not hosted by the ISF. The ISF can not be held responsible for the suitability or legality of this material. By clicking the link below you agree to view content from an external website.
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If you don't want to watch the whole 13½ minute video, the bit about L2 starts at 11:00, and there is an excellent graphic that exactly answers your question starting at 12:21.
Nothing at all wrong with 13.5 minutes of Manley!
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Old 26th December 2021, 09:57 PM   #267
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Originally Posted by RecoveringYuppy View Post
It's presumably got something that can be latched on to even if it's not a traditional "dock" such as the ISS would have. Presumably some of the structure that mates it to the launch vehicle is sturdy enough to support a docking.

What I can't find is a reliable statement that the fuel tanks have a robotic compatible way of refilling. There is an alternative that something could dock and then take over the positioning of the satellite. I don't know what complications there might be with that approach.

And sending a human presumably changes the cost by a lot, not to mention the risk.
I'm thinking they could send a fully-fueled thruster pack to the telescope, dock with it, and used that for station-keeping after the main tank goes dry. The robotic tanker has to be able to maneuver on its own anyway, and do the docking maneuver as well, and it's less complicated than trying to transfer fuel from one tank to another.

In theory that attachment could be designed to accept another robotic tank, or they could send a larger than necessary tanker with enough fuel to keep the telescope in place for, say, fifty years.

I'm also thinking that this is an obvious solution, so it's either been considered and rejected for technical reasons, or it's on the list of things to do.
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Old 26th December 2021, 10:09 PM   #268
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When can we expect the first pictures?
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Old 26th December 2021, 10:38 PM   #269
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Originally Posted by alfaniner View Post
When can we expect the first pictures?
6 months until science operations.

It has to cool down, I think that takes the longest.
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Old 27th December 2021, 04:33 AM   #270
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Originally Posted by alfaniner View Post
When can we expect the first pictures?
You know it's not an optical telescope, right? It's an infra red telescope, it's looking at the heat, rather than the light, that astronomical objects emit. So the pictures it takes won't look much like the Hubble ones. But ground based optical telescopes can now be made which are as good as Hubble, whilst ground based infra red telescopes are not much use as the atmosphere blocks a lot of those frequencies.

But yes, about six months before we see the first results.
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Old 27th December 2021, 04:35 AM   #271
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Originally Posted by Pixel42 View Post
You know it's not an optical telescope, right? It's an infra red telescope, it's looking at the heat, rather than the light, that astronomical objects emit. So the pictures it takes won't look much like the Hubble ones. But ground based optical telescopes can now be made which are as good as Hubble, whilst ground based infra red telescopes are not much use as the atmosphere blocks a lot of those frequencies.

But yes, about six months before we see the first results.
They should be able to take a picture of a highly redshifted galaxy, and color correct it to look something like we would expect.
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Old 27th December 2021, 11:33 AM   #272
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Old 27th December 2021, 12:28 PM   #273
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Originally Posted by slyjoe View Post
Well, there are other reasons not to park it at L2. It's crowded there.
You're probably joking, but it's not the least bit crowded at L2. The volume of space available for the orbits that are useful there is easily a million times the volume of low Earth orbit. And I'm being very conservative there, I think the actual number is somewhere around 50 million times larger than LEO. The region of space we are talking about is fairly thin but it is wider than the Sun. It's freaking enormous.
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Old 27th December 2021, 09:10 PM   #274
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Originally Posted by Pixel42 View Post
You know it's not an optical telescope, right? It's an infra red telescope, it's looking at the heat, rather than the light, that astronomical objects emit. So the pictures it takes won't look much like the Hubble ones.
I imagine they can and will be color-corrected thanks to modern technology, so that they will appear just as we would perceive them with our own eyes, if only they were powerful enough.

I recently watched a video NASA put out from the Perseverance Mars rover. The color in the video was adjusted to make it more pleasant on the eyes. At one point in the video, the narrator shows the difference between pictures with and without color adjustment.

The same sort of color adjustment will be possible for images from the Webb telescope, assuming we actually get any.
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Old 27th December 2021, 09:27 PM   #275
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Originally Posted by Puppycow View Post
I imagine they can and will be color-corrected thanks to modern technology, so that they will appear just as we would perceive them with our own eyes, if only they were powerful enough.

I recently watched a video NASA put out from the Perseverance Mars rover. The color in the video was adjusted to make it more pleasant on the eyes. At one point in the video, the narrator shows the difference between pictures with and without color adjustment.

The same sort of color adjustment will be possible for images from the Webb telescope, assuming we actually get any.
You can see here that gold doesn't reflect short wavelengths all that well, that's why it's gold:



So any redshifted deep enough could be color corrected back.

But anything not redshifted, those shorter wavelengths won't be there to color correct, resulting in a different view.

For example:

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Old 28th December 2021, 12:44 AM   #276
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OK, so one of the wags over at Apollohoax.net posted this. I just had to repost here....

Originally Posted by Bryanpoprobson
*****BREAKING NEWS*******

First image received from the JWST.



😂🤣 I’m sorry I couldn’t resist.
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Old 28th December 2021, 04:05 AM   #277
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Originally Posted by Mike Helland View Post
JWST Where is Webb?

https://jwst.nasa.gov/content/webbLa...ereIsWebb.html

That's pretty cool.
It says that the telescope is now 286 thousand miles from Earth (461 thousand kilometers) or 31% of the distance to the L2 point.

I also read that the high gain antenna was successfully deployed. One of many parts that will need to work as designed.

This is not insignificant as there have been missions in the past which were crippled because an antenna did not successfully deploy. Still many more steps to go of course, but so far, so good.
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Old 28th December 2021, 05:26 AM   #278
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Originally Posted by RecoveringYuppy View Post
Yeah, thanks for that Pixel42, but I'm afraid I see too much wrong from such sources to consider that reliable.
A bit more detail. https://space.stackexchange.com/ques...pace-telescope
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Old 29th December 2021, 03:16 AM   #279
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https://www.youtube.com/watch?v=QeiQEG450gc

How James Webb's Deployments MUST Work (Launch Pad Astronomy)
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Old 29th December 2021, 09:28 AM   #280
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https://www.jwst.nasa.gov/content/we...ereIsWebb.html

With the Aft Sunshield Pallet deployed, the above page now has temperature displays.

ETA: The Deployable Tower Assembly is being raised to move the mirrors away from the sunshield. And NASA says the launch was very good and the corrections used much less propellant than expected, possibly giving the telescope a longer mission life.

https://www.space.com/news/live/jame...escope-updates
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