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The James Webb Space Telescope and stuff


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On 1/17/2022 at 5:56 AM, cubinator said:

The mirror is made of beryllium metal coated in gold, not glass. It won't break.

i believe the failure modes here are either permanent deformation and/or cracking. idk what the properties of beryllium are, but according to wikipedia it is brittle at room temperature (and its going to run much much colder than that) has a high modulus of elasticity (i believe that means it its harder to deform non-permanently). gold is very malleable, so i doubt were going to have problems with layer separation. but metals tend to get more brittle at colder temperatures so i think were leaning towards cracking. the mirrors were designed to flex a bit to enable them to be finely calibrated, so the engineers took those properties into consideration. the actuators are likely selected such that they are incapable of causing either of these failure modes. i guess thats why they get paid the big bucks. 

Edited by Nuke
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15 minutes ago, Nuke said:

i believe the failure modes here are either permanent deformation and/or cracking. idk what the properties of beryllium are, but according to wikipedia it is brittle at room temperature (and its going to run much much colder than that) has a high modulus of elasticity (i believe that means it its harder to deform non-permanently). gold is very malleable, so i doubt were going to have problems with layer separation. but metals tend to get more brittle at colder temperatures so i think were leaning towards cracking. the mirrors were designed to flex a bit to enable them to be finely calibrated, so the engineers took those properties into consideration. the actuators are likely selected such that they are incapable of causing either of these failure modes. i guess thats why they get paid the big bucks. 

Googled beryllium and it looks like an good material light, thermal stable and stiff. Its common for fast moving optics like seekers. Now it has an 4-3-3 on the fire safety diamond giving it an impressive 10 who is one less than Hydrazine

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On 1/9/2022 at 10:57 AM, JoeSchmuckatelli said:

Question - with the requirement of keeping the sun shield between the observatory and the sun, are there parts of the sky in the axis of the orbit that cannot be imaged? 

Doh. Those parts they'll image at night, obviously.

On 1/8/2022 at 7:35 PM, Gargamel said:

Let’s just hope they ground the mirror correctly......   they’ve been known to mess that up.    

Looking at the picture @StrandedonEarthposted I'd be more worried about bird droppings on the mirror!

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So... according with the lastest Scott Manley video, the two mirrors that aren't in position yet are flawled... each motor has a couple sensor, one in each motor isn't working, there is a work around, but they laft these two for last... Even worst, they knew it before launch

 

EDIT: They knew since the vacuum chamber test, they just thought it to be much work to do... no words

Edited by VaPaL
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1 hour ago, VaPaL said:

So... according with the lastest Scott Manley video, the two mirrors that aren't in position yet are flawled... each motor has a couple sensor, one in each motor isn't working, there is a work around, but they laft these two for last... Even worst, they knew it before launch

 

EDIT: They knew since the vacuum chamber test, they just thought it to be much work to do... no words

Let’s see, after years and billions of dollars and you’re on some of the final tests and two sensors out of hundreds go bad (that we heard of). Tear it apart, put it back together, and retest, costing how many more billions and taking how many more years?  Yeah, we can work around that 

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23 minutes ago, StrandedonEarth said:

Let’s see, after years and billions of dollars and you’re on some of the final tests and two sensors out of hundreds go bad (that we heard of). Tear it apart, put it back together, and retest, costing how many more billions and taking how many more years?  Yeah, we can work around that 

Yeah... maybe... or let's risk throwing billions and decades away... I know I know, it would work "just fine" with two last mirror, just some loss in "apperture" and some aberrations to be compensated...

But still... I would not be ok given green light to a flawed equipment of this importance, but maybe that's just me... I really trust their decision, but I don't know... it irks me

 

EDIT: looks like they are moving fine, already 4,5 out of the 12,5mm for both of them

Edited by VaPaL
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38 minutes ago, VaPaL said:

Yeah... maybe... or let's risk throwing billions and decades away... I know I know, it would work "just fine" with two last mirror, just some loss in "apperture" and some aberrations to be compensated...

But still... I would not be ok given green light to a flawed equipment of this importance, but maybe that's just me... I really trust their decision, but I don't know... it irks me

 

EDIT: looks like they are moving fine, already 4,5 out of the 12,5mm for both of them

they'd be throwing billions and years away by wasting time replacing two redundant sensors

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41 minutes ago, VaPaL said:

Yeah... maybe... or let's risk throwing billions and decades away... I know I know, it would work "just fine" with two last mirror, just some loss in "apperture" and some aberrations to be compensated...

But still... I would not be ok given green light to a flawed equipment of this importance, but maybe that's just me... I really trust their decision, but I don't know... it irks me

 

EDIT: looks like they are moving fine, already 4,5 out of the 12,5mm for both of them

There is also the risk of messing things up or damaging other things while doing the repair, while they already had a solution to the issue

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44 minutes ago, VaPaL said:

Yeah... maybe... or let's risk throwing billions and decades away... I know I know, it would work "just fine" with two last mirror, just some loss in "apperture" and some aberrations to be compensated...

But still... I would not be ok given green light to a flawed equipment of this importance, but maybe that's just me... I really trust their decision, but I don't know... it irks me

 

EDIT: looks like they are moving fine, already 4,5 out of the 12,5mm for both of them

It was a calculated gamble, I'm sure. NASA is big on risk analysis, and the analysis probably showed a greater chance of messing things up by trying to fix it, instead working around it.

Perfect is the enemy of "good enough", which can be a tough lesson to learn when building something.

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26 minutes ago, insert_name said:

There is also the risk of messing things up or damaging other things while doing the repair

 

23 minutes ago, StrandedonEarth said:

Perfect is the enemy of "good enough"

Yeah yeah... perhaps I was just a little... overcautious with all of this. It's a already risky mission, losing redudance pre-launch knowing how much redundant stuff already saved a lot of mission or launching a possible flawed spacecraft seemed a little too much.

But in the end, as both of you said, it was deemed not worth repairing the sensor

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7 hours ago, HebaruSan said:

Makes me want to insert the "it's happening" meme!


Nah, the first pic published by Webb will be the “it’s working!” meme: “So powerful it can see something happening a long time ago in a galaxy far, far away”

Edit: Ok, I couldn't resist tweeting.... 

Spoiler

 

 

Edited by StrandedonEarth
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15 hours ago, StrandedonEarth said:


Nah, the first pic published by Webb will be the “it’s working!” meme: “So powerful it can see something happening a long time ago in a galaxy far, far away”

Edit: Ok, I couldn't resist tweeting.... 

  Reveal hidden contents

 

 

Almost want it to do a pale blue dot pic, with the caption “Hello World!”    
 

But, ya know, don’t stare into the sun and all.  

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On 1/15/2022 at 3:25 PM, Elthy said:

Does anyone have technical details on those acctuators? How do they work, how are they so precise?

This

https://authors.library.caltech.edu/91580/1/106983S.pdf

 

3 hours ago, Gargamel said:

Almost want it to do a pale blue dot pic, with the caption “Hello World!”    
 

But, ya know, don’t stare into the sun and all.  

The JWST have a device for looking at planets in other solar systems.

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7 hours ago, TheFlyingKerman said:

This

https://authors.library.caltech.edu/91580/1/106983S.pdf

 

The JWST have a device for looking at planets in other solar systems.

damn that's a cool design. i like how you can drive both the coarse stage (ball screw) and fine stage (cam driven flexure) with a single stepper motor. those things can be heavy so mass saved (especially considering that there are 7 actuators per mirror). they use a coupler with a lot of backlash, so you can operate the whole range of motion from the flexure without actuating the ball screw. this also explains why it takes so long to jack the mirror out of its retention system. its geared down 24:1 before it even gets to the ball screw (with a 2mm pitch). the stepper has to rotate 12 times to get a mm of motion, and the stepper is geared down 60:1 (and i doubt its fast because that would mean more heat to get rid of). it also explains why they didn't change the sensors, as you can still get fairly reliable open loop control out of this thing even without feedback. this kind of thing makes me wish i was a machinist.

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20 minutes ago, Kerbart said:

I'm pretty sure that was the plan all along. Any talk about "5 years operating" is mainly to get congress approve the budget.

Why would Congress prefer to get less bang for their buck?

On the other hand, I'm guessing promising 20 years when there's a chance you might only get 5 would have been a risky career move.

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