James Webb Telescope

Some temperature updates for those who haven't been keeping up

Red = Feb 21
Black = Feb 11

Frame and mirror
Primary Mirror -228° (46K) -223° (50K)
Instrument Radiator -231° (42K) -222° (51K)
Fine Steering Motor -235° (39K) -229° (44K)

Instruments
MIRI -155° (118K) -139° (134K)
NIRcam -228° (46K) -209° (64K)
NIRSpec -227° (45K) -201° (72K)
FGS/NIRISS -221° (56K)-196° (78K)
 
Webb Mirror Alignment Continues Successfully

Webb continues on its path to becoming a focused observatory. The team has successfully worked through the second and third out of seven total phases of mirror alignment. With the completion of these phases, called Segment Alignment and Image Stacking, the team will now begin making smaller adjustments to the positions of Webb’s mirrors.
Click to expand...

New images and more info at the link.

They are now on to step 4, coarse phasing. Apparently ahead of schedule.

https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html?units=metric

ETA: There's also a new feature on this page (or maybe I didn't notice it before) called "Webb in 3d solar system" that lets you look at the solar system, zoom in and out, and point the camera in any direction. It's pretty neat.
 
Last edited:
Puppycow said:
Webb Mirror Alignment Continues Successfully



New images and more info at the link.

They are now on to step 4, coarse phasing. Apparently ahead of schedule.

https://jwst.nasa.gov/content/webbLaunch/whereIsWebb.html?units=metric

ETA: There's also a new feature on this page (or maybe I didn't notice it before) called "Webb in 3d solar system" that lets you look at the solar system, zoom in and out, and point the camera in any direction. It's pretty neat.
Click to expand...

Probably added a week or two ago.
 
smartcooky said:
By a week. However it doesn't matter all that much because they will not be able to start the final phase, instrument calibration, until the mirror and all the instruments are down to their operating temperatures.
Click to expand...

I wish they put the target temperatures on that page along with the current temperatures. Then you could see how far it still has to go.
 
The primary and secondary mirrors, NirSpec, NirCam and FGS-Niriss all operate between 37K and 43K (Nominal 39K) and are passively cooled

Miri needs to be at 6.7K and is actively cooled using a helium cryocooler
 
And I'm guessing that the cryocooler hasn't actually been turned on yet, judging from MIRI's current temperature.

From what I understand, it's not being used right now anyway, so there is probably no hurry to do that. NirCam is the instrument being used to adjust the mirrors.
 
Last edited:
Puppycow said:
And I'm guessing that the cryocooler hasn't actually been turned on yet, judging from MIRI's current temperature.

From what I understand, it's not being used right now anyway, so there is probably no hurry to do that. NirCam is the instrument being used to adjust the mirrors.
Click to expand...

OK, so I don't actually know why that would be, but if I was to hazard a guess, I would say they want to control the cooling process - they do not want MIRI cooling down too quickly.

EDIT: OK, this is not quite right. Here is a pdf on the cryocooler (835kb)

http://ircamera.as.arizona.edu/MIRI/miricooler.pdf
.... the Mid-Infrared Instrument (MIRI) is unique among the four science instruments in that it operates around 7K as opposed to 40K like the other three near infrared instruments. Remote cooling of the MIRI is achieved through the use of a Joule-Thomson (J-T) Cooler, which is precooled by a multistage Pulse Tube Cooler. The MIRI Cooler systems engineering is elaborate because the Cooler spans a multitude of regions in the observatory that are thermally and mechanically unique with interfaces that encompass a number of different organizations. This paper will discuss how a significant change to the MIRI Cooling System from a solid hydrogen Dewar to a Cooler was achieved after the instrument Preliminary Design Review (PDR), and it will examine any system compromises or impacts that resulted from this change so late in the instrument design.​

After the Dewar-Cooler replan, the number of interfaces had increased by a large amount overall. The Cooler compressors and their electronics could not be supported within the Integrated Science Instrument Module like the Dewar because of the large amount of heat generated by the compressors, as well as exported vibration issues from the compressors. This meant that the compressors needed to be isolated from the instruments and remote cooling needed to be provided to MIRI.​
If I am understanding this correctly, the cooler compressors generate heat and that will be working against the cooling action (somewhat like the fact that the back of your fridge gets quite warm). Consequently, the MIRI instrument cools more slowly than the other instruments.
 
Last edited:
smartcooky said:
If I am understanding this correctly, the cooler compressors generate heat and that will be working against the cooling action (somewhat like the fact that the back of your fridge gets quite warm). Consequently, the MIRI instrument cools more slowly than the other instruments.
Click to expand...

Maybe, but I haven't seen any explicit statement or report as to whether the active cooling system in turned on or off right now. I would have imagined that the active cooling system would mean that it can be cooled down more rapidly than the other instruments, which rely on passive cooling. I figured that it's just not turned on right now because they currently don't need it to be on as they aren't using MIRI anyway. And when the time comes, they will turn it on and it will then cool down quite rapidly. Just my guess.
 
Puppycow said:
Maybe, but I haven't seen any explicit statement or report as to whether the active cooling system in turned on or off right now. I would have imagined that the active cooling system would mean that it can be cooled down more rapidly than the other instruments, which rely on passive cooling. I figured that it's just not turned on right now because they currently don't need it to be on as they aren't using MIRI anyway. And when the time comes, they will turn it on and it will then cool down quite rapidly. Just my guess.
Click to expand...

The thing is, MIRI is the one that has to go to the lowest temperature... 7K (the other three are all pretty close to reaching their target of 40K) but MIRI has a long way to go, and its not linear. Getting to single digit Kelvin temperatures is hard - the closer you get to 0K, the more energy has to be put into cooling.

Given that all the other instruments are cooling passively at about the same rate and have reached similar temperatures, but MIRI is about 60° higher and cooling more slowly, its cooling has to be being controlled actively.
 
Last edited:
Maybe it's because the others are designed to be passively cooled while MIRI is designed to be actively cooled (thus passive cooling is less efficient for it due to having less radiator area or something like that). Or maybe it's being actively temperature-controlled already, and this is the temperature they want it to be right now.

ETA: I think it's probably the former though, since it does seem to be cooling somewhat already.
 
Last edited:
Puppycow said:
Maybe it's because the others are designed to be passively cooled while MIRI is designed to be actively cooled (thus passive cooling is less efficient for it due to having less radiator area or something like that). Or maybe it's being actively temperature-controlled already, and this is the temperature they want it to be right now.

ETA: I think it's probably the former though, since it does seem to be cooling somewhat already.
Click to expand...

Yep... that's a fair assessment.
 
You must log in or register to reply here.

Back
Top Bottom