The James Webb Space Telescope

[anorlunda]

This video is very enjoyable. It analyzes the apparent trajectory of JWST as seen from Earth. It's a good reminder of how inadequate our simple mental models of orbits, trajectories, and frames of reference are.

 

[valenumr]

Ooooh, we're half way there.
Oooh-oh, living on a prayer!

No but really, all good vibes to JWST. Everything is going great.

 

[KurtLudwig]

Momentum flap deployed, sunshield cover removed. We are passing many of the single points of failure now.

The cold side has warmed up a bit since my last post, it's now -45 degrees at the mirror and -145 degrees at the instruments. I expect it to cool down again once the sunshield deployment starts.The telescope part rotates/tilts, the sunshield part stays where it is because it needs to keep shielding the Sun. Reaction wheels take care of that.

An L2 halo orbit has to be perpendicular to the L2/Sun direction because only these two dimensions are an attractive potential. You can't orbit L2 along the L2/Sun direction. About 800,000 km, as you can find on the Wikipedia page.

If the telescope rotates with respect to the sun-shield, then the secondary mirror will be in sunlight. Please explain in detail.
If the telescope tilts, it and the secondary mirror will still be shielded.

 

[mfb]

The requirement to stay in the shadow limits the observable range in the sky. Here is a discussion and here are more technical details (PDF download). Within +-5 degrees of roll the sunshade can keep its 3D orientation, for more it has to rotate around the JWST/Sun axis. It keeps its orientation relative to the Sun independent of the pitch angle.

New blog updates:
Webb Ready for Sunshield Deployment and Cooldown
First of Two Sunshield Mid-Booms Deploys
With Webb’s Mid-Booms Extended, Sunshield Takes Shape

The sunshield is now fully extended. It's not under tension yet, but it's enough to cool down the telescope side significantly. Tensioning the five layers will take at least two days.

That's a massive chunk of single points of failures:

all 107 membrane release devices associated with the sunshield deployment — every single one of which had to work in order for the sunshield to deploy — have now successfully released.

 

[valenumr]

The requirement to stay in the shadow limits the observable range in the sky. Here is a discussion and here are more technical details (PDF download). Within +-5 degrees of roll the sunshade can keep its 3D orientation, for more it has to rotate around the JWST/Sun axis. It keeps its orientation relative to the Sun independent of the pitch angle.

New blog updates:
Webb Ready for Sunshield Deployment and Cooldown
First of Two Sunshield Mid-Booms Deploys
With Webb’s Mid-Booms Extended, Sunshield Takes Shape

The sunshield is now fully extended. It's not under tension yet, but it's enough to cool down the telescope side significantly. Tensioning the five layers will take at least two days.

That's a massive chunk of single points of failures:

We are getting through all the scary parts, and it's going great!

 

[valenumr]

If the telescope rotates with respect to the sun-shield, then the secondary mirror will be in sunlight. Please explain in detail.
If the telescope tilts, it and the secondary mirror will still be shielded.

If I am picturing correctly, the flap corrects for pitch. But I don't really know. It is a very interesting concept if one thinks about it.

 

[valenumr]

Happy new year everyone!

 

[JLowe]

We are getting through all the scary parts, and it's going great!

I think it's all scary. Until data arrives at Earth with the expected quality, it ain't over.

 

[anorlunda]

The animations in this NASA video illustrate very well what is happening in these 29 days.

Until data arrives at Earth with the expected quality, it ain't over.

Can you hold your breath for 5 months until the temperature stabilizes?

https://safir.jpl.nasa.gov/BeyondSpitzerConf/proceedings/sessionB/Parrish.pdf

 

[mfb]

Deployment Timeline Adjusted as Team Focuses on Observatory Operations

They focus on the electrical system before tensioning the sunshield (no earlier than Jan 3).

The mirror temperature dropped to -116 C and the instrument data point has reached -192 C (81 K).

 

[pinball1970]

Deployment Timeline Adjusted as Team Focuses on Observatory Operations

They focus on the electrical system before tensioning the sunshield (no earlier than Jan 3).

The mirror temperature dropped to -116 C and the instrument data point has reached -192 C (81 K).

Is this an issue?

 

[phyzguy]

If the telescope rotates with respect to the sun-shield, then the secondary mirror will be in sunlight. Please explain in detail.

Looking at this "top" view, my understanding is that the telescope tilts to get pointing in one axis (equivalent to altitude in an Earth-bound alt-az telescope), and that the entire telescope+sunscreen rotates around the axis pointing at the sun to get pointing in the other axis (equivalent to azimuth).

 

[anorlunda]

Is this an issue?

The opening sentence in the linked article says "Taking advantage of its flexible commissioning schedule" That doesn't sound like an issue to me.

 

[pinball1970]

The opening sentence in the linked article says "Taking advantage of its flexible commissioning schedule" That doesn't sound like an issue to me.

Where's Webb and links not working for me. A few things on YT mentioned the same thing but I prefer to get it from the horses mouth. At least from someone who understands the horse.
Not an issue, good.

 

[berkeman]

Where's Webb and links not working for me.

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

 

[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/