The James Webb Space Telescope

[phyzguy]

I think wobbling the mirrors one at a time is exactly how they determined which spot goes with which mirror.

 

[sophiecentaur]

Yes. And the positioning is so precise that they can get back accurately to the starting point.
Much harder when they are looking at an interference pattern of multiple beams.

 

[anorlunda]

The topic of this video is the mirror actuator mechanism that allows such precise adjustments. Very clever mechanical engineering. The video producer 3D printed a replica to test and demonstrate which IMO is also very clever.

 

[Oldman too]

https://blogs.nasa.gov/webb/

 

[sophiecentaur]

JWST power supply; any left over?
The life of the vehicle is said to be limited by the amount of fuel available. Also, they have been ultra careful with the navigation so much less fuel was used and its estimated life is now double what was expected.

My question is, though, why didn't they use an electrical propulsion system, powered by PV panels, for station keeping? The potential surface it sits on has a very gentle slope so the energy needed would be very little, at anyone time. A few m² of PV panels (or less?) could provide an endless electrical supply for an Ion Drive, for instance.

Perhaps the project started so long ago that the decision to use a rocket engine was made with 'older' technology in mind.

On the same sort of topic, how much power does the refrigeration system use? I heard that it only works at all because the passive system keeps the refrigerant cold enough already. Other low noise imagers on other missions have needed to take fuel with them to achieve the same cooling facility.

 

[hutchphd]

My guess: ions would condense out on mirror. Just a guess.

 

[anorlunda]

My question is, though, why didn't they use an electrical propulsion system, powered by PV panels, for station keeping?

There are too many factors for us to speculate on the optimum choice. Mass, simplicity, reliability, contamination (as @hutchphd suggests), plus more I can't think of.

But the pressure to make JWST last a long time is deprecated by these four additional projects coming "soon" that may surpass JWST's capabilities.

https://www.physicsforums.com/threa...es-webb-space-telescope.1008536/#post-6558722

 

[sophiecentaur]

My guess: ions would condense out on mirror. Just a guess.

Yikes - I never thought of that - however, the ions would be traveling much faster than rocket ejecta so would they not disperse more?

 

[hutchphd]

I presume there is a distribution of speeds emitted... is there a slow tail? Also do you build up a noticeable opposite charge on the spacecraft ? I really have no idea here as to the numbers

 

[Oldman too]

Curiosity led me to look into the Ion drive as far as issues of compatibility with JWST. Its all way over my head but this link seems to have an abundance of info on Ion drive development as well as pro vs. con points. Perhaps someone more academically inclined may get something useful from it. I believe the attitude is controlled mainly with reaction wheels rather than boosters, I could be wrong about this but it seems this is the the way I remember it.
https://aip.scitation.org/doi/10.1063/5.0010134
An update, it seems reaction wheels as well as boosters are used.

https://jwst-docs.stsci.edu/jwst-ob...acecraft -bus/jwst-attitude-control-subsystem

https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst- spacecraft -bus/jwst-propulsion

 

[valenumr]

They must be able to identify each mirror without 'turning off' the others. Wobbling it a bit is the only way I could think of. No wonder it's all taking a long time. It's going to be even more of a problem when all the images are in one place; wobbling will have a much more subtle effect when an image is in amongst all the others. A very painstaking system and (yet again) very impressive. Unlike Apollo, there is no room for any seat of the pants stuff here.

If I understand correctly, mostly they just adjust the telescope's pointing, not so much the mirrors to get the initial state. I could be wrong, but it is a reasonable proposition that none of the primary mirrors have the same aim prior to alignment. It only took a handful of captures to identify which mirror corresponded to which target. Don't be so pessimistic.

 

[sophiecentaur]

If I understand correctly, mostly they just adjust the telescope's pointing, not so much the mirrors to get the initial state. I could be wrong, but it is a reasonable proposition that none of the primary mirrors have the same aim prior to alignment. It only took a handful of captures to identify which mirror corresponded to which target. Don't be so pessimistic.

Not “pessimistic”, just saying that it’s easier to spot one image moving about on its own and one image moving amongst 17 others when they are all on the same basic spot. Wouldn’t you also expect an interference pattern?

 

[berkeman]

Don't be so pessimistic.

I dunno, seems like a pretty hard problem to me unless you can cover up individual mirrors somehow (which they can't). Anybody know how often they plan to re-calibrate the alignments? Once a "day", once a year, etc.?

 

[hutchphd]

Wouldn’t you also expect an interference pattern?

The alignment of each optical axis seems almost "easy". If fact at first cosideration the most difficult part if this design would seem to me to get the distance to the center of each mirror exactly adjusted. Each of those (d~1m) mirrors will produce a Rayleigh limit $$\theta =1.22\frac \lambda d$$ but for the coherently adjusted group of 18 where $$D\approx 4d$$ then the diffraction spot gets smaller by 4. How do they get that distance correct?

 

[collinsmark]

I dunno, seems like a pretty hard problem to me unless you can cover up individual mirrors somehow (which they can't). Anybody know how often they plan to re-calibrate the alignments? Once a "day", once a year, etc.?

According to the article below, they'll do a check every few days, it seems.

The alignment of each optical axis seems almost "easy". If fact at first cosideration the most difficult part if this design would seem to me to get the distance to the center of each mirror exactly adjusted. Each of those (d~1m) mirrors will produce a Rayleigh limit $$\theta =1.22\frac \lambda d$$ but for the coherently adjusted group of 18 where $$D\approx 4d$$ then the diffraction spot gets smaller by 4. How do they get that distance correct?

This article might shine some light:
https://svs.gsfc.nasa.gov/12753

During coarse phasing, engineers point the telescope toward a bright star and use NIRCam to find any large offsets between the mirror segments (though “large” is relative, and in this case it means mere millimeters). NIRCam has a special filter wheel that can select, or filter, specific optical elements that are used during the coarse phasing process. While Webb looks at the bright star, grisms in the filter wheel will spread the white light of the star out on a detector. Grisms, also called grating prisms, are used to separate light of different wavelengths. To an observer, these different wavelengths appear as parallel line segments on a detector. ​

[...]

During fine phasing, engineers will again focus the telescope on a bright star. This time, they will use NIRCam to take 18 out-of-focus images of that star — one from each mirror segment. The engineers then use computer algorithms to determine the overall shape of the primary mirror from those individual images, and to determine how they must move the mirrors to align them. These algorithms were previously tested and verified on a 1/6th scale model of Webb’s optics, and the real telescope experienced this process inside the cryogenic, airless environment of Chamber A at NASA’s Johnson Space Center in Houston. Engineers will go through multiple fine-phasing sessions until those 18 separate, out-of-focus images become a single, clear image. ​

[...]

The entire alignment process is expected to take several months, and once Webb begins making observations, its mirrors will need to be checked every few days to ensure they are still aligned​

 

[Motore]

Everything about alignment is described here:
https://blogs.nasa.gov/webb/2022/02/03/photons-incoming-webb-team-begins-aligning-the-telescope/

This site (the Webb blog) was referred here several times already, so if you are interested I would advise you to look it up every once in a while.

 

[hutchphd]

This site (the Webb blog) was referred here several times already, so if you are interested I would advise you to look it up every once in a while.

Well now I know and that is indeed a very nicely presented synopsis. I'm pleased I was asking the right questions: it makes the answers more satisfying ! First time I've seen the term "piston difference" which is very descriptive.

 

[valenumr]

This is so cool: https://blogs.nasa.gov/webb/2022/02...8-dots-of-starlight-into-hexagonal-formation/

 

[collinsmark]

First time I've seen the term "piston difference" which is very descriptive.

If you're interested in the actuators' engineering, here is a good video with a working, 3D-printed replica of the actuators:

 

[mfb]

The 18 mirror images are now in the same spot
It's still 18 largely independent images, the mirrors are not aligned to interference yet.

 

[Oldman too]

It just keeps getting better!
https://www.nasa.gov/press-release/...ignment-milestone-optics-working-successfully

 

[pinball1970]

It just keeps getting better!
https://www.nasa.gov/press-release/...ignment-milestone-optics-working-successfully

Great news ! I posted on TIL regarding cooling. EDIT, also..

 

[mfb]

Casual observations of some extremely far away galaxies taken while still aligning the telescope.

That star is magnitude 11, so even amateur telescopes can see it - it's extremely bright compared to typical JWST targets. You can see three pairs of diffraction spikes from the mirror borders and smaller horizontal diffraction spikes from the support of the secondary mirror (the other spikes from that are merged with the mirror segment spikes). These things will be far less prominent in science exposures, as it's obvious from the background galaxies.

 

[fresh_42]

 

[pinball1970]

View attachment 298509

Which bit is what here?

Circled the whole image conventional? On the right, same area with Webb?

 

[fresh_42]

Which bit is what here?

Circled the whole image conventional? On the right, same area with Webb?

It was said to be the first sharp image from Webb in the article.

 

[Oldman too]

On "killing time" while awaiting commissioning... about that commissioning,
https://ntrs.nasa.gov/citations/20205010662

After looking at the last image posted (the background was a nice bonus), I decided to look into some of the instrument details to get an idea of what's in store when the science begins.Reading through this site, https://webbtelescope.org/resource-gallery/articles/pagecontent/filter-articles/how-does-webb-see-the-universe.html
it links me to, https://sci.esa.int/web/jwst/-/45694-nirspec-the-near-infrared-spectrograph
This is a truly amazing instrument, https://jwst.nasa.gov/content/about/innovations/microshutters.htm
"Capable of measuring the spectra of up to 200 objects simultaneously in a 3.4 arcminute × 3.6 arcminute field of view."Looking into Webb's connectivity, it's better than I thought.
"Webb can downlink at least 57.2 gigabytes of recorded science data each day, with a maximum data rate of 28 megabits per second."This video explains its title nicely.

"Webb's Field of Regard"


It's also worth noting that someone must have learned a thing or two from the Hubble primary mirror issue, this looks like a contingency plan should there be a problem with optics on JWST.
https://jwst.nasa.gov/content/about/innovations/wavefront.html

And a couple of random upcoming science proposals.

https://arxiv.org/abs/2111.14865

https://arxiv.org/abs/2109.03829

Cheers, Scott

 

[valenumr]

This a pretty cool comparison of the same shot from spitzer:

 

[hutchphd]

As I recall Spitzer mirror was about 1 meter. So that looks just about perfect! Great

 

[mfb]

It was said to be the first sharp image from Webb in the article.

Not the image you posted. That's Gaia taking a picture of JWST. Note the different diffraction pattern: JWST has 6+2 spikes, not 4.
Here it's in an article

Overview which feature of JWST leads to which diffraction pattern.