The James Webb Space Telescope

[mfb]

Mirror alignment has begun. All actuators work, now it's a matter of getting all of them to the right positions.

 

[Oldman too]

Some great links on upcoming programs.

https://www.cmu.edu/mcs/news-events/2021/1220_webb-telescope-walker.html

https://www.stsci.edu/jwst/science-execution/program-information.html?id=2608

https://www.stsci.edu/jwst/science-execution/approved-programs/cycle-1-go

https://www.scientificamerican.com/...r-of-extraordinary-science-has-been-revealed/

 

[mfb]

T+3 weeks

Where is Webb added a mirror position tracker. The launch position was -12.5 mm, the nominal position is 0 mm. The mirrors move by about 1 mm per day, except A6 and A3 which will be moved separately later. About a week to go for most segments.

The cold side is now below -200 C.
Edit: I found an expected cooldown timeline

 

[sophiecentaur]

The mirrors move by about 1 mm per day,

That's some very low gearing, I think. But things, in space, tend not to be intuitive.

 

[Grinkle]

https://blogs.nasa.gov/webb/2022/01/13/mirror-mirroron-its-way/

"At full speed, it takes about a day to move all the segments by just 1 millimeter. It’s about the same speed at which grass grows!"

 

[mfb]

That's some very low gearing, I think. But things, in space, tend not to be intuitive.

The actuators move in steps of a few nanometers - the precision needed for the mirror alignment. A centimeter is millions of steps. To keep the operation simple they move one at a time, too.

Sure, it would have been possible to add some extra actuators for the coarse alignment, but it would have been useless extra complexity. Cooling down JWST will take longer anyway, two weeks to move the mirrors is not an issue.

-5 mm

 

[BWV]

The actuators move in steps of a few nanometers - the precision needed for the mirror alignment. A centimeter is millions of steps. To keep the operation simple they move one at a time, too.

Sure, it would have been possible to add some extra actuators for the coarse alignment, but it would have been useless extra complexity. Cooling down JWST will take longer anyway, two weeks to move the mirrors is not an issue.

-5 mm

so when fully deployed, the viewing time is scheduled based to some extent on the direction the telescope points as the Earth orbits the sun? What about pointing the thing perpendicular to the orbital plane? That would take weeks to accomplish?

 

[phyzguy]

so when fully deployed, the viewing time is scheduled based to some extent on the direction the telescope points as the Earth orbits the sun? What about pointing the thing perpendicular to the orbital plane? That would take weeks to accomplish?

No. The whole telescope/sunscreen rotates driven by reaction wheels. This can happen quickly. At any point in time it can see nearly half the sky. See the pictures I posted in post #117.

 

[mfb]

Most mirrors have reached their nominal position, A3 and A6 have moved by half of the distance.
NASA's 1 mm/day estimate was very conservative.

 

[Borg]

And soon, the L2 insertion burn will be occurring - 94% of the way there today.

 

[Oldman too]

https://ntrs.nasa.gov/citations/20190002647
150 Pages of "fun facts" on Space Mirror Tech and related cool stuff.

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

 

[russ_watters]

So, what is Webb doing today? Where is Webb says it arrives at L2 with a velocity of 0.135 mi/s around 2pm EST. I've googled and found conflicting answers about whether there is a burn. Is there one?

It would need to have a thruster pointing "up" to do a burn to slow down, but wouldn't that be generating a lot of heat on the "cold" side?

https://www.republicworld.com/scien...-before-orbit-insertion-read-articleshow.html

https://space.stackexchange.com/questions/57702/james-webb-orbit-insertion

 

[Grinkle]

I see this from NASA.

https://blogs.nasa.gov/webb/2022/01/21/webbs-journey-to-l2-is-nearly-complete/

"On Monday, Jan. 24, engineers plan to instruct NASA’s James Webb Space Telescope to complete a final correction burn that will place it into its desired orbit, nearly 1 million miles away from the Earth at what is called the second Sun-Earth Lagrange point, or “L2” for short."

 

[Oldman too]

Houston, we have L2 Orbit. Flawless!

 

[russ_watters]

I see this from NASA.

https://blogs.nasa.gov/webb/2022/01/21/webbs-journey-to-l2-is-nearly-complete/

"On Monday, Jan. 24, engineers plan to instruct NASA’s James Webb Space Telescope to complete a final correction burn that will place it into its desired orbit, nearly 1 million miles away from the Earth at what is called the second Sun-Earth Lagrange point, or “L2” for short."

Thanks, so my questions are:

1. What direction relative to the spacecraft was the burn? (where was the thruster?)
2. Did it generate heat that could be problematic for the cold (cooling) instruments?

In addition:
"Where is Webb" still shows a cruising speed of 0.1255 mi/s (presumably the orbital speed?) and quite a large curve to orbit insertion (graphical license or freefall path?). And what's the orbit diameter? What are the corrections going to look like (perpendicular delta-V?)

 

[mfb]

It would need to have a thruster pointing "up" to do a burn to slow down, but wouldn't that be generating a lot of heat on the "cold" side?

It came in sideways, the thrust was somewhat in the general direction of the Sun. It's a three-dimensional problem and thinking of just slowing down/speeding up isn't representing the geometry of the orbit. Here is a good 2D projection. The displayed velocity is probably the velocity relative to L2, which is non-zero as it orbits L2 now.

 

[russ_watters]

It came in sideways, the thrust was somewhat in the general direction of the Sun. It's a three-dimensional problem and thinking of just slowing down/speeding up isn't representing the geometry of the orbit. Here is a good 2D projection. The displayed velocity is probably the velocity relative to L2, which is non-zero as it orbits L2 now.

You might have been writing that while I was writing the prior post. That's the graphic I was referring to. So, it's fairly accurate? Googling for the topography of the Lagrange points, it is difficult to see how it is pulled into a roughly circular if unstable orbit (per the graphic). For example:
https://upload.wikimedia.org/wikipedia/commons/e/ee/Lagrange_points2.svg

Anyway, it hadn't occurred to me that the reported speed and distance weren't along the same line. Though I guess at first they were closer to it.

 

[phyzguy]

This site says that, "Webb's orbit is represented in this screenshot from our deployment video (below), roughly to scale; it is actually similar in size to the Moon's orbit around the Earth! This orbit (which takes Webb about 6 months to complete once)...". If we assume that the "orbit" around L2 is circular with a radius of 400,000 km (moon's orbit SMA is 380,000 km), and that it takes 180 days to complete, then the speed would be 0.16 km/sec, and its current speed is 0.20 km/sec. So it all makes sense.

 

[Oldman too]

 

[sophiecentaur]

1. What direction relative to the spacecraft was the burn? (where was the thruster?)

In the same way as getting to the right distance without retro firing, starting off (early deployment) can give the craft the right 'lateral' velocity so that it will be doing 30 days' worth of 'corkscrew' motion on the way. It will ideally not lose any of that translational / orbital energy as it slows down due to (radial with respect to Earth and Sun) potential energy. If the orbit around L2 takes six months (?) then it will have done around 360/6 = 60 degrees of corkscrew on the journey and arrived 'hanging there' and gently moving around in a large ellipse.

So it all makes sense.

(Yep. After a bit of brain ache and arm waving.)
Navigating is space does have some advantages in that there's no wind, air pressure or rain to deal with so it's more predictable.

 

[mfb]

You do have radiation pressure and solar wind. They can lead to larger trajectory uncertainties, especially with uncontrolled spacecraft . An old Falcon 9 upper stage might hit the Moon in March. It launched a NASA spacecraft to L1, somewhat similar to Ariane 5's launch of JWST to L2. People are studying it to get a better estimate. If it hits then the impact will be used to study the Moon, just like the many deliberate impacts in the past.

 

[sophiecentaur]

Ah yes. Solar wind does vary too. It will always be outwards tho’ and must affect station keeping a bit.

 

[Devin-M]

The way I heard it described was the launch rocket gave it slightly less velocity than was needed to reach the final orbit, so rather than turning around and thrusting to slow down (which would expose the mirror and instruments to the Sun’s heat), the final burn had to add a small amount of speed in the direction it was already moving (the telescope has been slowing down continuously relative to Earth since launch).

“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. “

https://blogs.nasa.gov/webb/2021/12/25/the-first-mid-course-correction-burn/

 

[Arjan82]

You know, it just dawned on me that I finally understand the Dune song:
"
I was sent to
outer space
To find another happy place
Now I'm left here all alone
Million miles away from home
"
It was about Webb!

 

[Arjan82]

There is a new video out from Launch Pad Astronomy explaining the Web orbit really well (I think):

 

[bland]

This one is also pretty good with some 3D topography.

 

[mfb]

Mirror alignment has begun

 

[mfb]

NASA released an alignment picture. 18 images of a star, one from each segment. They already know which image is from which segment. The alignment will put the star in focus in each of them and then move them all to the same spot.

 

[phyzguy]

NASA released an alignment picture. 18 images of a star, one from each segment. They already know which image is from which segment. The alignment will put the star in focus in each of them and then move them all to the same spot.

This may not seem like much, but to me this is a huge step. It means the detectors are working and they are able to read out the data from the detectors and send it back to Earth.

 

[sophiecentaur]

This may not seem like much, but to me this is a huge step. It means the detectors are working and they are able to read out the data from the detectors and send it back to Earth.

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.

 

[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.