The James Webb Space Telescope

[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.

 

[Office_Shredder]

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

https://www.planetary.org/space-policy/nasa-budget

Funding varies year-to-year, but generally about 50% of NASA's annual budget is spent on human spaceflight activities, 30% on robotic missions and scientific research, and the remainder split between aeronautics, technology development programs, staff salaries, facilities management, and other overhead.

 

[BWV]

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

Well it would have been considerably more expensive to put a crew on the JWST

Seriously, do contingency plans exist for a manned mission out to L2 for emergency servicing? Hubble had to be repaired by a human back in the 90s.

 

[Borg]

Well it would have been considerably more expensive to put a crew on the JWST

Seriously, do contingency plans exist for a manned mission out to L2 for emergency servicing? Hubble had to be repaired by a human back in the 90s.

No - see my post above. There may be a robotic mission someday to replenish fuel but that would be more than 10 years from now.

 

[sophiecentaur]

but that would be more than 10 years from now.

Apparently, they used up less fuel than anticipated and the latest estimate is a 20 year lifespan. Fab or what?

 

[hutchphd]

This calibration capability was built specifically because of the previous Hubble mirror flaw.

I believe it was required because of the nature of the novel segmented berylium miorror. Do you have a reference for this particular assertion?

 

[Borg]

I believe it was required because of the nature of the novel segmented berylium miorror. Do you have a reference for this particular assertion?

It's from the video that I referred to in my earlier post. It depends on how you interpret that. Saying that it was specifically built because of the Hubble mirror flaw was my wording and may be a bit over-stated. However, the video goes into great detail about the issue with Hubble and how the engineers did not want a repeat of that since they wouldn't be able to repair this one with a mission.

 

[JLowe]

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.

Yes, and I'm glad they did. I'm only saying, that's a lot of moving parts that have to work.

 

[valenumr]

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).

It's quite comparable to rolling a ball up a hill, only we don't want to hit the peak at L2. If it drifts past, it will just continue to drift further out. The propellent will be used now and again to roll Webb back close to the peak from time to time (among other things) as it falls back toward earth. Perhaps we should call it Sisyphus (this is pretty standard for the many other observatories at L2).

 

[sophiecentaur]

It's quite comparable to rolling a ball up a hill,

Or the old pub game of shove ha'penny; just one go to get the coin up to the line and not a nanometre over it.

 

[hutchphd]

Except this game is 3D. I likethe graphics here
https://en.wikipedia.org/wiki/Halo_orbit