The James Webb Space Telescope

[pinball1970]

I just checked, and it's working for me...

Nothing Wrong with link, it's my device
Edit. 'privacy error'
My desk top is usually fine. I'll try tomorrow

 

[mfb]

Is this an issue?

It probably means some sensor reading was different from the expectation and they spent another day understanding this. Not unusual with such a complex device.

Tensioning of the first layer has started now:
Webb Team Moving Forward With Sunshield Tensioning

This critical step in the observatory’s complex sequence of deployments resumed after Webb mission managers paused deployment operations on Saturday to allow for team rest, and then again on Sunday to make adjustments to Webb’s power subsystem and to alter the observatory’s attitude to lower the temperature of the motors that drive the tensioning process.

 

[JLowe]

How do they warm the motors? With sunlight?

 

[mfb]

They changed the attitude, the orientation of the spacecraft in space (relative to the Sun, in particular).

First layer tightened successfully, second layer in progress.

Edit: Second and third layer done
Four and five will follow tomorrow. Once this is done 3/4 of the single points of failure will have been passed.

 

[Borg]

Tensioning completed at 11:59 a.m. EST today! Big sigh of relief on this one. Next up - Secondary Mirror Deployment.

 

[mfb]

Secondary mirror deployed
It's now a telescope.

Next is the "Aft Deployed Instrument Radiator", followed by the two wings of the primary mirror as last deployment steps. Both together are expected to take about two days. The rest of the cruise time will be spent on finer alignment of the optical elements, early calibration and so on.

 

[sophiecentaur]

Secondary mirror deployed
It's now a telescope.

That's a relief. They can now, at least use it for something to justify its existence. Things can only get better.

 

[pinball1970]

That's a relief. They can now, at least use it for something to justify its existence. Things can only get better.

So it can still work/semi work like this? If the Primary mirror fails?
I thought the secondary reflected light from the primary?
I was confused when NASA (and Mfb) said "We now have a telescope 600,000 miles from earth"

 

[mfb]

12 of the 18 segments were already launched in their final arrangement. 3 on each side will fold to join the central area. If they fail to deploy for whatever reason it's still a big primary mirror, just 1/3 of the light collection and a bit of resolution is lost.

 

[Borg]

Northrop Grumman was the prime contractor for the telescope and they have an interesting article on the Primary Mirror Backplane Support Structure (PMBSS) that holds everything together.

The backplane will hold the weight of the mirror and carry 5,300 pounds of telescope optics and instruments. PMBSS stands 24 feet tall, is nearly 20 feet wide and weighs 2,180 pounds. As the names suggests, it supports the primary mirror as well as Webb’s instruments, the center section and the wing assemblies — which weigh more than 7,300 pounds combined, more than three times the weight of the backplane itself.

Another wrinkle of development and testing was ensuring that while the telescope’s 18 mirrors move, the backplane remains steadier than a surgeon’s hand, especially because no operating room is as cold as space. The team demonstrated to NASA that PMBSS should not vary more than 38 nanometers — about 1/1,000 the diameter of a human hair — keeping the mirror stable, Atkinson said. To put that into scale, if JWST’s mirrors were as large as the distance between New York and Los Angeles, the tolerance error of movement from the backplane could be no more than one inch.

 

[mfb]

NASA's twitter account for JWST has blocked the NASA accounts for Sun, Earth and Moon:



The Aft Deployable Instrument Radiator has been deployed, the wings of the primary mirror as last unfolding steps will follow Jan 7 and 8 US time.

 

[phyzguy]

@KurtLudwig , here is some more detail on your question about Webb pointing. If it points straight "up" relative to the sunshield, then the secondary will be in sunlight, as you said earlier. So the field of view at any point in time is limited to the swath shown in these figures, although over the course of the year it can see the entire sky. There is more discussion here:
https://space.stackexchange.com/questions/56076/why-does-jwst-have-such-a-big-blind-spot

 

[collinsmark]

@KurtLudwig , here is some more detail on your question about Webb pointing. If it points straight "up" relative to the sunshield, then the secondary will be in sunlight, as you said earlier. So the field of view at any point in time is limited to the swath shown in these figures, although over the course of the year it can see the entire sky. There is more discussion here:
https://space.stackexchange.com/questions/56076/why-does-jwst-have-such-a-big-blind-spot

Indeed.

If you'd like to see an animation of its rotation and discussion of rotational constraints, I'll repost this video from Post #18 28. It's set to go right when Scott Manely starts discussing this subject matter (around 6:24). Rewind it a bit for more context.

 

[sophiecentaur]

So the field of view at any point in time is limited to the swath shown in these figures, although over the course of the year it can see the entire sky.

So that's pretty good use of all the resources. No Earth-bound telescope can do that; they're all limited by their horizon and JW has (hopefully) many years to do the job. I hope all those Duracells will hold out.
PS I wouldn't like to have to design a circular sun shield for JW.

 

[phyzguy]

The port wing has been deployed successfully!

https://blogs.nasa.gov/webb/2022/01/07/first-of-two-primary-mirror-wings-unfolds/

 

[Grinkle]

Sorry if this was addressed elsewhere in the thread - how warm an object can JWST image? Will there be any objects we might expect to observe (thinking of inside-our-solar-system objects) that will be too warm and saturate something in the signal chain? Or does JWST have a very large dynamic range and the low end of temp observation did not come at the cost of being able to observe warmer objects?

 

[anorlunda]

Sorry if this was addressed elsewhere in the thread - how warm an object can JWST image?

Well, it goes all the way from cold comets to stars.. That's pretty wide. It says "dust warmed by starlight", so I presume that dust not warmed is too cold to detect.

https://jwst.nasa.gov/content/observatory/instruments/miri.html
MIRI covers the wavelength range of 5 to 28 microns. Its sensitive detectors will allow it to see the redshifted light of distant galaxies, newly forming stars, and faintly visible comets as well as objects in the Kuiper Belt. MIRI's camera will provide wide-field, broadband imaging that will continue the breathtaking astrophotography that has made Hubble so universally admired. The spectrograph will enable medium-resolution spectroscopy, providing new physical details of the distant objects it will observe.

The ISIM Includes The Following Instruments:​

• Near-Infrared Camera, or NIRCam - provided by the University of Arizona
• Near-Infrared Spectrograph, or NIRSpec - provided by ESA, with components provided by NASA/GSFC.
• Mid-Infrared Instrument, or MIRI - provided by the European Consortium with the European Space Agency (ESA), and by the NASA Jet Propulsion Laboratory (JPL)
• Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph , or FGS/NIRISS - provided by the Canadian Space Agency.

 

[sophiecentaur]

In the end it will be down to resolving power and relative luminosity of other objects ‘nearby’. And exposure / processing.

 

[websterling]

The starboard wing is deployed and latched.

https://blogs.nasa.gov/webb/2022/01/08/primary-mirror-wings-deployed-all-major-deployments-complete/

The telescope is now structurally fully deployed. Who would have thought it would all work?

 

[sophiecentaur]

It would be churlish if me to quote “many a slip twixt cup and lip.”
I’m not relaxed yet. Have you seen that image of the path from present position to its orbit?
All fingers and toes crossed still.

 

[valenumr]

The starboard wing is deployed and latched.

https://blogs.nasa.gov/webb/2022/01/08/primary-mirror-wings-deployed-all-major-deployments-complete/

The telescope is now structurally fully deployed. Who would have thought it would all work?

Truly a marvel of science, technology, and engineering. It's basically 3/4 the way there in distance, half way in time, and fully deployed. I'm excited but it sucks that we're going to wait for the next 5 months.

 

[Office_Shredder]

The telescope is now structurally fully deployed. Who would have thought it would all work?

For ten billion dollars, I hope the people that built the thing.

 

[phyzguy]

The telescope is now structurally fully deployed. Who would have thought it would all work?

Still a long way to go before we are taking images. I agree it is great news so far, but as @sophiecentaur said, there is still a ways to go. I won't celebrate until we get some good images. Remember Hubble?

 

[mfb]

Mars is the only object where saturation can be a concern even at the minimal viable observation time. Everything else is fine.

Two weeks to go until the L2 halo orbit insertion. Progress from now on will be in many incremental steps with less visibility.

 

[mfb]

Due to the excellent orbit insertion by Ariane 5, JWST's fuel is now expected to last 20 years.

 

[JLowe]

Amazing how precisely you can aim a skyscraper.

 

[RandyD123]

I was looking at the "Where Is Webb" progress web page and noticed that it shows Webb about halfway through the total number of days to the final L2 location but that distance traveled is about 3/4 of the way.
And wondered why?

 

[Arjan82]

The JWST is constantly moving against the gravitational pull of the Earth (and sun and moon I guess) without any propulsion. So therefore it is moving slower and slower.

 

[sophiecentaur]

I was looking at the "Where Is Webb" progress web page and noticed that it shows Webb about halfway through the total number of days to the final L2 location but that distance traveled is about 3/4 of the way.
And wondered why?

When you throw a stone up in the air, it goes slower and slower as it approaches the high point. That experiment on Earth is very nearly under constant gravitational force and the variation of the g field near L2 is a bit more complicated but the same principle applies; Kinetic Energy turns into Gravitational Potential Energy and, at L2, the GPE changes are very small so KE has to be low.
So why not manoeuvre JWST into place more aggressively? The design choice was to avoid retro thrust, which would involve facing back towards the Sun and frazzling equipment. When you can only 'push' you have to be very gentle with these of the engines and make it arrive in the desired spot traveling verrryy slowly (= a long time to get there).

 

[phyzguy]

Also, there's really no reason to hurry, since the next phase of checkout and calibration will take months. It will arrive at L2 long before it is ready to begin observations.

 

[anorlunda]

They need to allow the temperatures to stabilize, and they need to align the mirrors. Alignment itself is a fascinating story.

http://www.parabolicarc.com/2022/01...-nasas-james-webb-space-telescope-with-light/

When the engineers need to adjust the positions and shapes of the mirror segments to achieve precise alignment, they use the seven actuators (tiny mechanical motors) attached to the back of each one of the mirror segments. For each segment, six of these actuators are placed into groups of two, at three equally spaced points along the outside of the mirror (to adjust the segment’s position), and one is attached to six struts that are connected to each of the hexagonal mirror segment’s corners (to adjust the segment’s shape).

The actuators on each mirror segment are capable of extremely minute movements, which allow engineers to align the entire primary mirror by finely adjusting each mirror segment. “They can move in steps that are a fraction of a wavelength of light, or about 1/10,000th the diameter of a human hair,” explained Feinberg.

These actuators can also be used to precisely reshape each mirror segment to ensure they all match up once aligned. The ability to change the mirror alignment and shape is critical because the mirror must be unfolded from its unaligned stowed position when the telescope deploys. This test verifies the actuators have enough range of movement once they are in space, at their operational temperature of about 40 K (or about minus 388 degrees Fahrenheit / minus 233 degrees Celsius), to put the telescope’s primary mirror into its correct shape so it can accurately survey the universe.

JWST is destined to entertain engineers and scientists for a long time to come. NASA's unmanned space program continues to soar past expectations. Too bad NASA's manned space program is so moribund.

 

[sophiecentaur]

Too bad NASA's manned space program is so moribund.

I would normally respond with something about the lower cost of unmanned stuff but JWST costa packet.

 

[JLowe]

Calibrating seems pretty scary, as each mirror has its own set of motors that have to work. Mirror flaw is what almost doomed Hubble.

 

[Borg]

Calibrating seems pretty scary, as each mirror has its own set of motors that have to work. Mirror flaw is what almost doomed Hubble.

This calibration capability was built specifically because of the previous Hubble mirror flaw. There can't reasonably be a manned mission to fix any optical problems so JWST is designed to be able to overcome optical flaws on its own using this capability.

The Hubble flaw and JWST approach is described starting at the 22 minute mark in the video that collinsmark posted on Dec. 22nd.

 

[phyzguy]

Calibrating seems pretty scary, as each mirror has its own set of motors that have to work. Mirror flaw is what almost doomed Hubble.

Also, remember that the problem with Hubble was that the mirror was fabricated incorrectly, and the problem wasn't caught because it was never completely tested on the ground. That's one of the reasons Webb was delayed so long, because they did extensive testing on the ground.