James Webb Telescope

BowlOfRed said:
Because JWST requires fuel for stationkeeping. It requires consumables to be useful, and the design lifetime for them is 10 years.

Admittedly, they would be long odds (Hubble has had several issues and eventually one of the failures will kill it). But the long tail of survivability for HST does reach to around 2040. I suspect that JWST fuel usage will end up being below the estimates, but reaching 2040 would be a stretch as well.
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I see.

Although if recent experience is any guide, some NASA missions go on well past their planned duration. So we'll see. If they try to be as efficient as possible with the fuel use, maybe they can make it stretch longer.
 
Isn’t it normally coolant that is the limiting factor? As far as I remember coolant tends to run out faster than expected.
 
steenkh said:
Isn’t it normally coolant that is the limiting factor? As far as I remember coolant tends to run out faster than expected.
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Depends on the telescope and the design. Spitzer had two instruments that needed very cold temperatures and used evaporative helium cooling. Simple in concept, but makes it a consumable. 350l of helium lasted ~5.5 years (which was beyond the 5 year design). Also, most telescopes in earth orbit need some means to control attitude, but don't need propellent to hold station.

JWST has a design temperature of 40K, but is supposed to achieve that with passive cooling thanks to the big sun shield. That will be sufficient for 3 of the instruments.

There is a fourth instrument that needs 7K to operate. It will come with (what I think is) a newish cooler. It uses helium as the working fluid, but is a closed system, not evaporative. Unless something breaks, there's no expectation that the coolant will reduce over time.
 
steenkh said:
Isn’t it normally coolant that is the limiting factor? As far as I remember coolant tends to run out faster than expected.
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The primary mirror is cooled by being exposed to space, with an incredibly efficient five-layer heat shield between it and the Sun/Earth system. Being at the L2 Lagrange point, the telescope, the Earth, and the Sun are all in a line.

One instrument on the JWST, the MIRI (Mid-InfraRed Instrument) uses a a helium gas mechanical cooler (see Cryocooler on Wikipedia.) I don't know the expected lifespan on this piece of equipment.


ETA: Ninja'd by Bowl of Red!
 
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I knew NASA was responsible for the design, but I had some doubts they would think it through and do it right, until someone on the internet told me a plausible tale of potential competence. "Sounds reassuring" my ass.
 
theprestige said:
I knew NASA was responsible for the design, but I had some doubts they would think it through and do it right, until someone on the internet told me a plausible tale of potential competence. "Sounds reassuring" my ass.
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"I thought the lifetime of this telescope might be limited by the need to supply coolant that could be used up over time until someone on the internet explained to me that this isn't actually a design constraint."

Not seeing anything unreasonable there at all. There's no implied incompetence if coolant was going to be used up over time, so why you think there was any implication of NASA's incompetence in the worry escapes me.
 
Roboramma said:
"I thought the lifetime of this telescope might be limited by the need to supply coolant that could be used up over time until someone on the internet explained to me that this isn't actually a design constraint."

Not seeing anything unreasonable there at all. There's no implied incompetence if coolant was going to be used up over time, so why you think there was any implication of NASA's incompetence in the worry escapes me.
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Thanks, Roboramma. You are right, I didn’t think of the coolant restraint as being a sign of incompetence, but merely as a design constraint. Now I know that the design has removed this constraint entirely, and that is reassuring.

I do have other worries about the JWT. Some years ago there was an article in Sky & Telescope about how every major telescope had had design flaws that needed to be fixed afterwards, and that included the HST. I note that the HST is not the only major space telescope, so the article was definitely wrong, but I do see how so much more can go wrong with the JWT than with any previous telescope.

But if anybody can get it right, NASA can.
 
steenkh said:
I do have other worries about the JWT. Some years ago there was an article in Sky & Telescope about how every major telescope had had design flaws that needed to be fixed afterwards, and that included the HST. I note that the HST is not the only major space telescope, so the article was definitely wrong, but I do see how so much more can go wrong with the JWT than with any previous telescope.

But if anybody can get it right, NASA can.
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The HST absolutely had a major design flaw. Or perhaps it was a manufacturing flaw.

https://en.wikipedia.org/wiki/Hubble_Space_Telescope#Flawed_mirror

Analysis of the flawed images revealed that the primary mirror had been polished to the wrong shape. Although it was believed to be one of the most precisely figured optical mirrors ever made, smooth to about 10 nanometers,[27] the outer perimeter was too flat by about 2200 nanometers (about 1⁄450 mm or 1⁄11000 inch).[64] This difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its center.[65]
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A commission headed by Lew Allen, director of the Jet Propulsion Laboratory, was established to determine how the error could have arisen. The Allen Commission found that a reflective null corrector, a testing device used to achieve a properly shaped non-spherical mirror, had been incorrectly assembled—one lens was out of position by 1.3 mm (0.051 in).[70] During the initial grinding and polishing of the mirror, Perkin-Elmer analyzed its surface with two conventional refractive null correctors. However, for the final manufacturing step (figuring), they switched to the custom-built reflective null corrector, designed explicitly to meet very strict tolerances. The incorrect assembly of this device resulted in the mirror being ground very precisely but to the wrong shape. A few final tests, using the conventional null correctors, correctly reported spherical aberration. But these results were dismissed, thus missing the opportunity to catch the error, because the reflective null corrector was considered more accurate.[71]

The commission blamed the failings primarily on Perkin-Elmer. Relations between NASA and the optics company had been severely strained during the telescope construction, due to frequent schedule slippage and cost overruns. NASA found that Perkin-Elmer did not review or supervise the mirror construction adequately, did not assign its best optical scientists to the project (as it had for the prototype), and in particular did not involve the optical designers in the construction and verification of the mirror. While the commission heavily criticized Perkin-Elmer for these managerial failings, NASA was also criticized for not picking up on the quality control shortcomings, such as relying totally on test results from a single instrument.[72]
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Hopefully the lessons from that episode were learned and won't be repeated with the JWST.
 
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steenkh said:
I note that the HST is not the only major space telescope, so the article was definitely wrong, but I do see how so much more can go wrong with the JWT than with any previous telescope.
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This sentence confused me and maybe the poster ahead of me. It doesn't seem to follow or explain what you wrote before it.
 
RecoveringYuppy said:
This sentence confused me and maybe the poster ahead of me. It doesn't seem to follow or explain what you wrote before it.
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Sorry about that. The original article had a list of big telescopes, and how they had to be repaired after they were declared ready. The problems were not just about optics, but also the mount and the foundation.

When I look at at the JWT, I see a complicated process of launching, unfurling, and so on, and I worried about each of these steps that could end a disaster.

That was all I meant.
 
Gosh, I hope they didn't damage it.

https://arstechnica.com/science/202...-the-james-webb-space-telescope-has-occurred/

"Technicians were preparing to attach Webb to the launch vehicle adapter, which is used to integrate the observatory with the upper stage of the Ariane 5 rocket," NASA said in a blog post. "A sudden, unplanned release of a clamp band—which secures Webb to the launch vehicle adapter—caused a vibration throughout the observatory."

Let's be honest, words like "incident," "sudden," and "vibration" are not the kinds of expressions one wants to hear about the handling of a delicate and virtually irreplaceable instrument like the Webb telescope. However, NASA, the European Space Agency, and the rocket's operator, Arianespace, have a plan for moving forward.
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