James Webb Telescope

acbytesla said:
I have to wonder if they are using Engineer Scotty estimating. You know. Where he says it will be done in three months when they can actually get it done in far less.:D
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You mean...?

Kirk: Mr. Scott. Have you always multiplied your repair estimates by a factor of four?
Scotty: Certainly, sir. How else can I keep my reputation as a miracle worker?
 
smartcooky said:
You mean...?

Kirk: Mr. Scott. Have you always multiplied your repair estimates by a factor of four?
Scotty: Certainly, sir. How else can I keep my reputation as a miracle worker?
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Yep.

I know real life engineers who believe this is the most important rule for dealing with the boss.
 
Mike Helland said:
Have I missed any other pictures or is it still just the one?
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I don't think so. At least, they haven't released any new ones to the public yet.

We're going to have to be patient for a little bit longer, but I think it's pretty close. All of the major adjustments have been made already and the instruments are at their operating temperatures. It's now just what they call "commissioning" of the instruments to calibrate them and make sure that they are ready for use. I guess that could take another month, according to the timeline that has been communicated to the public.
 
I'm pretty sure that as soon as they have something for us they will be extremely enthusiastic about sharing it as soon as possible.
 
Another reminder that the original schedule was six months from launch to first science. That means we should expect to see proper images towards the end of June. Given how well it's all gone they might manage to shave a week or two off that, and we might get another bonus like that calibration image before it, but I haven't seen anything that suggests the original schedule has changed significantly.
 
https://blogs.nasa.gov/webb/

New blog post.

The 18 primary mirror segments range in temperature from 34.4 kelvins to 54.5 kelvins. An advantage of beryllium mirrors is that they don’t change shape with temperature the way glass mirrors would at these temperatures, so the temperature range does not affect the telescope alignment process.

Currently, four of the 18 mirror segments are above 50 kelvins: at 52.6, 54.2, 54.4, and 54.5. These four mirror segments emit some mid-infrared light that reaches the MIRI detectors. Since all the mirror temperatures are now below 55 kelvins, it is expected that MIRI will be sensitive enough to perform its planned science, but any additional cooling of these mirrors will only enhance its performance. The Webb team hopes to see the mirrors cool by an additional 0.5 to 2 kelvins.
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That's a bit of a pity. But, at least it should be cold enough, although barely so in the case of one of the mirror segments. Let's hope it does cool down more.

I didn't quote the rest of it, but apparently the attitude of the telescope does likely affect the temperature of the mirror segments. They are going to test this during the commissioning process to see how much the attitude affects the temperature, by putting the telescope in a hot attitude for several days and then in a cold attitude for several days and see what happens to the temperature of the primary mirror segments. They only cool down very slowly though.
 
Puppycow said:
https://blogs.nasa.gov/webb/

New blog post.



That's a bit of a pity. But, at least it should be cold enough, although barely so in the case of one of the mirror segments. Let's hope it does cool down more.

I didn't quote the rest of it, but apparently the attitude of the telescope does likely affect the temperature of the mirror segments. They are going to test this during the commissioning process to see how much the attitude affects the temperature, by putting the telescope in a hot attitude for several days and then in a cold attitude for several days and see what happens to the temperature of the primary mirror segments. They only cool down very slowly though.
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I doubt the mirrors will get much colder since they are not actively cooled. They are merely shaded from the sun and the Earth. I would imagine that each segment is close to the ambient temperature of space not affected by sunlight.


The MIRI instrument is what is actively cooled by the cryo-cooler.
 
Puppycow said:
https://blogs.nasa.gov/webb/

New blog post.



That's a bit of a pity. But, at least it should be cold enough, although barely so in the case of one of the mirror segments. Let's hope it does cool down more.

I didn't quote the rest of it, but apparently the attitude of the telescope does likely affect the temperature of the mirror segments. They are going to test this during the commissioning process to see how much the attitude affects the temperature, by putting the telescope in a hot attitude for several days and then in a cold attitude for several days and see what happens to the temperature of the primary mirror segments. They only cool down very slowly though.
Click to expand...

acbytesla said:
I doubt the mirrors will get much colder since they are not actively cooled. They are merely shaded from the sun and the Earth. I would imagine that each segment is close to the ambient temperature of space not affected by sunlight.


The MIRI instrument is what is actively cooled by the cryo-cooler.
Click to expand...


If you look at the Hot/Cold Side Temperatures plot on the Where is Webb page, you can see that the average primary mirror temperature is still slowly falling. Hover your mouse over the 100 day mark on the Primary Mirror line, and you will see it was 42.6K while today (day 115) it is 42.3K.
 
smartcooky said:
If you look at the Hot/Cold Side Temperatures plot on the Where is Webb page, you can see that the average primary mirror temperature is still slowly falling. Hover your mouse over the 100 day mark on the Primary Mirror line, and you will see it was 42.6K while today (day 115) it is 42.3K.
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You can also see that MIRI is actively cooled by helium which is how it is under 7 degrees Kelvin. Do you know what temperature the mirror is likely to stop cooling?
 
acbytesla said:
You can also see that MIRI is actively cooled by helium which is how it is under 7 degrees Kelvin. Do you know what temperature the mirror is likely to stop cooling?
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If you look at the blog post Puppycow links to it says

The 18 primary mirror segments range in temperature from 34.4 kelvins to 54.5 kelvins.

Currently, four of the 18 mirror segments are above 50 kelvins: at 52.6, 54.2, 54.4, and 54.5.
​

I can't find a listing for the individual temperatures for all 18 segments but if I were to guess, I would say that the "bottom" ones nearest the sun-shield are likely to be the ones with the highest temperatures because some heat does make to through the shield.

I can't see any reason why all the segments should not be able to get down to under 40K, although the blog also says this...

The Webb team hopes to see the mirrors cool by an additional 0.5 to 2 kelvins.​

What do they mean by "mirrors". Do they mean the primary and secondary mirrors, or are they talking about the mirror segments? Usually, when they refer to the "mirror" they are talking about primary as a whole.

ETA:

So I found this from Feb 9 but I have been unable to find an updated version.


JWSTMirrorTempFeb9.png


In the JWST co-ordinate system, the axis V3 is "ante-spacecraft", so away from the sun-shield, which means in this diagram the sun-shield is at the bottom. The four segments nearest the sun-shield were the warmest back then, and I see no reason why that should have changed.

It is worth noting that from the blog...

The secondary mirror, hanging out on the end of its support structure a long way from any heat sources, is the coldest mirror, currently at 29.4 kelvins.​

... and if you look at the diagram, the secondary temperature was 30.8 K back on February 9, so it has only fallen 1.4° in 73 days, while the primary mirror segments seem to have fallen about 6 to 9° over the same period. This would seem to indicate there will be a limit of about 30K beyond which they will no longer cool.
 
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