The James Webb Space Telescope

[Oldman too]

A couple of JWST "fun facts".
While reading, https://phys.org/news/2022-03-video-astronomers-reveal-remarkable-simulations.html
I learned that the very cool simulations mentioned were made in preparation for Web's science campaign. The YT channel, https://www.youtube.com/channel/UCIqmFQYKpX1qlOnh0zXBfPQ is definitely worth a look, as I said, very cool.

The other fun fact that was new, (to me at least) was that Web will have a partner performing "spotting" functions. The apparent plan is for the SPHEREx mission, due for launch in '04 - '05 to, among other duties,
identify exoplanets of interest for Web to study in detail as well as be involved with the early universe re-ionization period observations.

https://www.jpl.nasa.gov/news/nasa-finalizes-plans-for-its-next-cosmic-mapmaker



I'm amazed that such a complex instrument as JWST is completing so many involved steps without a hitch.

Edit note, I forgot to mention this.
https://academic.oup.com/mnras/article-abstract/506/1/1209/6307520?login=false
Observing the host galaxies of high-redshift quasars with JWST: predictions from the BlueTides simulation

 

[mfb]

Where is Webb has temperature history plots. You can nicely see the effect of MIRI's cryo cooler being switched on.

 

[valenumr]

Where is Webb has temperature history plots. You can nicely see the effect of MIRI's cryo cooler being switched on.

Quick question about that ... MIRI is supposed to operate at around 7 K. Why does it not matter that the main mirrors (and fine steering mirror) are going to stabilize in 35 to 40 K ballpark?

 

[phyzguy]

Quick question about that ... MIRI is supposed to operate at around 7 K. Why does it not matter that the main mirrors (and fine steering mirror) are going to stabilize in 35 to 40 K ballpark?

It is just the MIRI detectors that need to be at 7K in order to operate properly. The detectors don't care how warm the rest of the telescope is.

 

[valenumr]

It is just the MIRI detectors that need to be at 7K in order to operate properly. The detectors don't care how warm the rest of the telescope is.

I appreciate the response, but that's not a very direct answer. I'm trying to reconcile (in my own head) how the mirrors could radiate higher energy above the sensitivity of the sensor. I'm not doubting the science at all, but I don't really understand how that works.

 

[WernerQH]

I'm trying to reconcile (in my own head) how the mirrors could radiate higher energy above the sensitivity of the sensor.

I had the same problem. But the reflectivity of the mirror is certainly not 100%. If it's 99% then 1% emissivity (Kirchhoff's law) can still swamp a faint signal. And luckily the (incoherent) thermal radiation will not be focused on the detector.
I think the discovery of the 3K microwave background did not require liquid helium. :-)

 

[renormalize]

I think the discovery of the 3K microwave background did not require liquid helium.

Not true according to Wikipedia, which says this about Penzias and Wilson detecting the cosmological background radiation:
"To measure these faint radio waves, they had to eliminate all recognizable interference from their receiver. They removed the effects of radar and radio broadcasting, and suppressed interference from the heat in the receiver itself by cooling it with liquid helium to -269 C, only 4 K above absolute zero."

 

[pinball1970]

I think the discovery of the 3K microwave background did not require liquid helium.

Not true according to Wikipedia, which says this about Penzias and Wilson detecting the cosmological background radiation:
"To measure these faint radio waves, they had to eliminate all recognizable interference from their receiver. They removed the effects of radar and radio broadcasting, and suppressed interference from the heat in the receiver itself by cooling it with liquid helium to -269 C, only 4 K above absolute zero."

The same that had pigeon cr4p had liquid Helium??

 

[renormalize]

The same that had pigeon cr4p had liquid Helium??

Yeah, a helium-cooled radio receiver connected to a huge pigeon-infested horn antenna.

 

[WernerQH]

I think the discovery of the 3K microwave background did not require liquid helium.

Not true according to Wikipedia, which says this about Penzias and Wilson detecting the cosmological background radiation:
"To measure these faint radio waves, they had to eliminate all recognizable interference from their receiver. They removed the effects of radar and radio broadcasting, and suppressed interference from the heat in the receiver itself by cooling it with liquid helium to -269 C, only 4 K above absolute zero."

Thanks for the correction. But at least the antenna did not require liquid helium.

 

[collinsmark]

For what it's worth, I have a backyard telescope and astronomy camera. Even when it's a balmy 21 deg C (70 deg F) outside I still cool my camera's sensor down to -10 deg C (14 deg F) using a Peltier junction device inside the camera.

I don't cool down the whole telescope or even the whole camera. 'Just the camera's sensor. That's all that's needed to reduce thermal noise. (Of course, I'm imaging using the visible spectrum. I would have to cool more things down if I was shooting in the deep infrared.)

The camera's sensor relies on some active electronic trickery to produce a potential energy well for each pixel. Photons of sufficient energy can strike the camera's sensor and knock an electron into the well.

But sometimes, due to the thermal vibrations of atoms that make up the sensor's electronics itself, electrons can "accidentally" get bounced into the well. This gets interpreted as a photon detection, but it really isn't. It's just thermal noise. This is sort of analogous to how a violently boiling pot of water might splash liquid water outside the pot, even though the water droplet splashes didn't leave the pot by being converted to steam.

 

[Oldman too]

Here is an interesting take on the next "pinch point" coming up.
https://blogs.nasa.gov/webb/2022/04/06/webbs-mid-infrared-instrument-cooldown-continues/

“Over the last couple weeks, the cryocooler has been circulating cold helium gas past the MIRI optical bench, which will help cool it to about 15 kelvins. Soon, the cryocooler is about to experience the most challenging days of its mission. By operating cryogenic valves, the cryocooler will redirect the circulating helium gas and force it through a flow restriction. As the gas expands when exiting the restriction, it becomes colder, and can then bring the MIRI detectors to their cool operating temperature of below 7 kelvins. But first, the cryocooler must make it through the ‘pinch point’ – the transition through a range of temperatures near 15 kelvins, when the cryocooler’s ability to remove heat is at its lowest. Several time-critical valve and compressor operations will be performed in rapid succession, adjusted as indicated by MIRI cryocooler temperature and flow rate measurements. What is particularly challenging is that after the flow redirection, the cooling ability gets better as the temperature gets lower. On the flip side, if the cooling is not immediately achieved due to, for example, larger than modeled heat loads, MIRI will start warming.

 

[anorlunda]

What is particularly challenging is that after the flow redirection, the cooling ability gets better as the temperature gets lower. On the flip side, if the cooling is not immediately achieved due to, for example, larger than modeled heat loads, MIRI will start warming.

The boldness of the JWST project plan continues to amaze me. So many things needed to go right, I thought the project was doomed. But so far, it has been an outstanding success.

Kudos to the project team.

 

[Grinkle]

So does radiated energy from the relatively warm mirror dissipate sufficiently to avoid warming the sensor before getting to the sensor? Is that why the mirror can be 20K or so warmer than the sensor? I guess the mirrors radiated thermal energy won't be focused at all?

I don't see how the statement that the sensor doesn't care how warm the mirror is can be correct. If the mirror is warm enough, it will warm anything that has LoS to it with radiated thermal energy, and of course there is conduction via whatever is connecting the mirror to the rest of the telescope.

 

[phyzguy]

I appreciate the response, but that's not a very direct answer. I'm trying to reconcile (in my own head) how the mirrors could radiate higher energy above the sensitivity of the sensor. I'm not doubting the science at all, but I don't really understand how that works.

I think I see your question. The JWST MIRI instrument images from about 5 to about 25 microns. At 25 microns, the effective blackbody temperature is about 115K. So the mirrors at 35-40K are radiating essentially nothing at 25 microns. It's not like the CMB instruments, which are measuring microwave radiation at a few degrees K. They need to be very cold otherwise they would radiate into the wavelength being measured. The reason the MIRI detectors need to be at 7K is to make sure that all of the arsenic doping in the detectors is "frozen out" and there are no free carriers. Another question above is why don't the mirrors at 35-40K warm up the MIRI detectors at 7K. The answer is that they do a little, but the amount of power radiated by an object at 35K is extremely small. The MIRI detectors have a cryocooler that removes the heat radiated and conducted from the surrounding warmer objects. I think conduction is probably dominant over radiation at 35K.

 

[Oldman too]

And now for a visual on that cool down.

 

[pinball1970]

I think I see your question. The JWST MIRI instrument images from about 5 to about 25 microns. At 25 microns, the effective blackbody temperature is about 115K. So the mirrors at 35-40K are radiating essentially nothing at 25 microns. It's not like the CMB instruments, which are measuring microwave radiation at a few degrees K. They need to be very cold otherwise they would radiate into the wavelength being measured. The reason the MIRI detectors need to be at 7K is to make sure that all of the arsenic doping in the detectors is "frozen out" and there are no free carriers. Another question above is why don't the mirrors at 35-40K warm up the MIRI detectors at 7K. The answer is that they do a little, but the amount of power radiated by an object at 35K is extremely small. The MIRI detectors have a cryocooler that removes the heat radiated and conducted from the surrounding warmer objects. I think conduction is probably dominant over radiation at 35K.

Yes and MIRI is now at 15K wow!

 

[Jonathan Scott]

Yes and MIRI is now at 15K wow!

I don't know when it was updated, but I'm seeing 6K on the main screen and 5.6K on the temperature plot now, which also mentions:

The cryocooler completed the 15K "pinch point" transition to state 5 to lower MIRI's temperature to below 7K.

 

[pinball1970]

I don't know when it was updated, but I'm seeing 6K on the main screen and 5.6K on the temperature plot now, which also mentions:

I can't get on the site! That's so annoying, most of the instruments were in the 30s K range with MIRI at 17K when I posted a couple of hours ago.

 

[Jonathan Scott]

I'm amused to see that if I pan within the temperature plot, the axes allow for negative Kelvin temperatures!

 

[pinball1970]

I'm amused to see that if I pan within the temperature plot, the axes allow for negative Kelvin temperatures!

6K! Just got it! Wow

 

[valenumr]

I think I see your question. The JWST MIRI instrument images from about 5 to about 25 microns. At 25 microns, the effective blackbody temperature is about 115K. So the mirrors at 35-40K are radiating essentially nothing at 25 microns. It's not like the CMB instruments, which are measuring microwave radiation at a few degrees K. They need to be very cold otherwise they would radiate into the wavelength being measured. The reason the MIRI detectors need to be at 7K is to make sure that all of the arsenic doping in the detectors is "frozen out" and there are no free carriers. Another question above is why don't the mirrors at 35-40K warm up the MIRI detectors at 7K. The answer is that they do a little, but the amount of power radiated by an object at 35K is extremely small. The MIRI detectors have a cryocooler that removes the heat radiated and conducted from the surrounding warmer objects. I think conduction is probably dominant over radiation at 35K.

Awesome. This explanation is great.

 

[valenumr]

6K! Just got it! Wow

I'm in tears. It's almost unbelievable that this has gone so well. I literally can't wait to see what we discover from this marvel of engineering and science.

 

[pinball1970]

On where's Webb it looks like cooling is finished and they are on the final stage before commissioning.
@mfb Images to follow soon? Sooner than expected?

 

[Oldman too]

Images to follow soon? Sooner than expected?

'As we go through the seven steps, we may find that we need to iterate earlier steps as well. The process is flexible and modular to allow for iteration. After roughly three months of aligning the telescope, we will be ready to proceed to commissioning the instruments." Patience Grasshopper

 

[pinball1970]

'As we go through the seven steps, we may find that we need to iterate earlier steps as well. The process is flexible and modular to allow for iteration. After roughly three months of aligning the telescope, we will be ready to proceed to commissioning the instruments." Patience Grasshopper

There is another poster who keeps saying exactly the same thing to me. He says NASA will stick the plan, no short cuts even if everything has ran like clockwork so far.

The project planner in me and critical path person in me always builds in a worse case scenario in terms of deadlines. Expectations and the odd cushion here or there if one can build that in. If one part of production exceeds or meets the best case we have just made ourselves a few days leeway.
This is the mother of all projects.
I don't work for NASA but if I did it would go something like this.

Project Manager (me) 'So this final correction thing how long?'
Scientists.'Best case scenario PM? 5 or 6 days. If all the instruments are in line with previous...'
Me. 'Ok so we call that two weeks. You over there (other scientists) final commissioning. Say he can confirm all that cooling stuff you can jump in after 2 or 3 days right?'
Previous scientists. 'Er PM I said it would be 5 or 6..'
Me. 'Yeah got that Doc. So (to all) we can get an early image if we hit all those targets yes? CAN WE!?'
NASA guys. 'Yes...'

Like the Titanic getting to New York a day early if they just ignore all those peripheral issues.

That's a headline.

 

[anorlunda]

He says NASA will stick the plan, no short cuts even if everything has ran like clockwork so far.

You seem to be treating it as a single unique event. But the JWST will be repeatedly adjusted and calibrated in the future. Any statement of the form, "Adjustments are done, the project is finished," can be mooted if they decide to improve on adjustments.

When scientists begin examining the images in search of scientific evidence, we might say "The JWST is in service." But there is nothing to prevent the same scientists from also suggesting that the quality of the images could be better with more adjustments.

I see it as a continuous process.

 

[pinball1970]

You seem to be treating it as a single unique event. But the JWST will be repeatedly adjusted and calibrated in the future. Any statement of the form, "Adjustments are done, the project is finished," can be mooted if they decide to improve on adjustments.

When scientists begin examining the images in search of scientific evidence, we might say "The JWST is in service." But there is nothing to prevent the same scientists from also suggesting that the quality of the images could be better with more adjustments.

I see it as a continuous process.

I was being a tad facetious @anorlunda I was commenting that cooling and final correction seemed to have concluded or are overlapping with final commissioning.
I could smell an early spell bounding image. Here is what that 10 billion dollars got you. More to come and better.
Noted that is an evolving and iterative journey.

 

[Oldman too]

Here is what that 10 billion dollars got you.

Jeez... that would launch, umm, a little over two SLS Rockets. JWST's price tag might not be so out of line when adjusted for inflation.

 

[pinball1970]

I remember seeing this a few days after launch and thinking wow, that is a lot of stuff to get through!
Webb is at 115 days so approaching 6 O clock on the ellipse.

 

[mfb]

Telescope alignment has been completed and all instruments are in focus already, so we are a bit ahead of schedule.

 

[Oldman too]

so we are a bit ahead of schedule.

That is great news, I understand that JWST is a bit overbooked for observing time.

 

[pinball1970]

The first 12 months are already booked in according to the NASA site. It is keeping exactly what will be the first targets secret.

@Oldman too

 

[Oldman too]

The first 12 months are already booked in according to the NASA site. It is keeping exactly what will be the first targets secret.

@Oldman too

They do seem to be leaking hints, I'll see if I can find links to what I've seen so far.

 

[Oldman too]

Hi @pinball1970 , It looks like there isn't a particular observation schedule because of the Time Constrained Feature. I could be wrong but it looks like https://www.stsci.edu/jwst/science-execution/approved-ers-programs is the early favorite list, contingent on this feature, https://jwst-docs.stsci.edu/jwst-opportunities-and-policies/jwst-call-for-proposals-for-cycle-1/jwst-cycle-1-observation-types-and-restrictions/time-constrained-observations This is in addition to the cycle 1 GTO and GO as well as the all important
https://www.stsci.edu/jwst/science-execution/approved-programs/cycle-1-calibration Researchers still have to learn the science systems since it's a new to everyone.