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James Webb is fully deployed

nasa.gov

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Re: James Webb is fully deployed

#451

Post by joshvm »

bmitc wrote: post #411
bichiliad wrote: post #250

Earlier quoted context omitted.

One thing they mentioned that kinda sends it home to me is that the telescope is aimed at the dark and away from the sun, and the other side is especially reflective[0]. In other words, one side is too dark to see, and the other is too bright to see. [0]: https://twitter.com/NASAWebb/status/1479161991252131841

It seems to me that that isn't quite right. "Dark" just means no visible light, but an infrared or thermal camera could see stuff. The article above mentions this possibility and the problems, which are harnessing and operating at cryogenic temperatures.

It's probably not practical for JWST, but starlight is actually fairly bright. If you travel to somewhere with essentially zero light pollution, like a desert, if it's a clear sky you can see with starlight. At the South Pole on a clear night we would walk out to the dark sector (where the telescopes are) with our headlamps off because it just wasn't necessary. The only illumination sources outside were red safety lights[1] and we'd normally turn those off for long exposure photography. You do need a fast lens though, and that inevitably means lots of glass.

Whether the telescope is reflective enough to get a good photo is another matter - I would guess not, it's designed to minimise stray light.

As for IR, the telescope is probably bright compared to the background at least for now. The main issue though is whether you'd need to actively cool the engineering camera. Cooling stuff in space is difficult because you can only really dump heat via radiation. It's probably not worth the weight to carry a separate cryocooler just for that. The inability to conduct heat is partly why it takes so long to cool down, aside from minimising thermal stress on the infrastructure.

[1] The main reason they exist is so that you have a point of reference when walking about outside, particularly on foggy or cloud days. The meteo folks also use them as visibility markers in winter - e.g. IceCube is something like 1km from the station by line-of-sight. Turning them off for a brief period is usually fine and we'd notify the station before we did it. On a clear night though you don't need them at all.

Re: James Webb is fully deployed

#452

Post by rcxdude »

WalterBright wrote: post #435
sailingparrot wrote: post #355

Earlier quoted context omitted.

Your point is completely unfair. Sure if our goal was to mass manufacture a "cheap and reliable JWST" we could, with a lot of money, create an entire assembly line and benefit from economies of scale. And then what? What are we supposed to do with 100s of JWST? It's cool they only cost 200M$ each now, but we don't need them. What we need after JWST is a different telescope with different technologies to answer questi…

Is it unfair? In Spring 2020, in this forum, the vaccine experts here told me in no uncertain terms that a vaccine would take 18 months to develop. I said it could be done in 6 if all the slack was removed, and things done serially were done concurrently. I was told that was all completely unreasonable and impossible. 6 months later, the vaccine was released. The JWST has what, 390 single points of failure, and cost…

> Yes, it'll be heavier, and that will cost more.

It won't just cost more, it'll be impossible. Aircraft care a lot about weight, spacecraft are obsessed with it, because it's often an absolute limit based on what launch technology you have available and where you're trying to get. JWST doesn't have any spare weight to try to build more redundancy.

> But suppose for $1B one could build a less reliable JWST

Well, the calculus of many single failure points makes this questionable. Even if you're 99% confident in each point of failure, with 390 failure points you have a 2% chance of success. You need extremely reliable components. Also, it's not clear that the majority of the budget is being spent on increasing that reliability: certainly a lot goes into testing, but all of that testing is generally a lot cheaper than a launch and rebuild.

Re: James Webb is fully deployed

#453

Post by IAmYourDensity »

WalterBright wrote: post #337
tokai wrote: post #219

Earlier quoted context omitted.

Machine shop experience is not related project cost estimations.

Sorry, I'm not buying that. But that notion might be one reason why government projects cost so much. I've taken apart cars many times. I know when I'm being fed a baloney sandwich when taking my car in for service, and use that to pare down the estimate sometimes as much as 50%. (For all the women who rightly complain about being scammed by auto mechanics, I can vouch for them doing their best to scam me about 75% o…

I'm not buying it either. NASA wasted billions on the space shuttle and single-use rockets until a programmer from Paypal showed them how to build re-usable rockets. Musk regularly refers to the economics of airliners as inspiration.

Obviously ideas to improve the economics of space exploration can come from anywhere, but one of the first places I'd look would be the Boeing machine shop.

And we haven't touched on human spaceflight, where economy must be balanced with safety. Hmm, I wonder what industry has the most experience with such engineering tradeoffs?

Re: James Webb is fully deployed

#454

Post by nullc »

rmk wrote: post #426

How does this compare to the 'man on the moon' projects of China? It appears that from a purely scientific standpoint, this telescope will have a bigger impact than those of other space programs. Is my thinking on the right track here? Also, is this complementary in function (and mission) to the Hubble? It appears to me that the spectrum is split up between Hubble and James Webb. As an aside, it is interesting that t…

Programs like man on the moon make their contribution to mans knowledge through the engineering required to achieve the end, rather than in what they discover when they get there.

> Also, is this complementary in function (and mission) to the Hubble? It appears to me that the spectrum is split up between Hubble and James Webb.

Hubble's IR compliment was spitzer until it ran out of liquid helium. Webb is spitzer's replacement, with orders of magnitude more resolution.

Spitzer has qualified a lot of interesting mid IR targets to go look at in more detail.

In general non-vis space telescopes are more interesting than visible spectrum ones: Our vision only covers one octave so the odds that some random important physical process will best be observed in the visible spectrum isn't that great.

The visible spectrum is also the same spectrum that is well transmitted by the atmosphere so many vis observations can be conducted from earth with much larger and less costly instruments. Adaptive optics can mitigate atmospheric distortion at least somewhat, but there is no solution to non-transmission but space.

The atmospheric transmission has a two fold impact too: To study the atmosphere of extrasolar planets we need to study wavelengths that their atmospheres block. ... which, of course, are also wavelengths that our atmosphere blocks.

Re: James Webb is fully deployed

#455

Post by [deleted] »

dreamcompiler wrote: post #410

To me the second most exciting idea about JWST is that it might give us fairly definitive proof that other intelligent life exists in the galaxy. The most exciting idea is if that happens we'll realize they might have built their own version of JWST and proven we exist.

[deleted]

Re: James Webb is fully deployed

#457

Post by causi »

cycomanic wrote: post #400
causi wrote: post #398

I can't help but be a little sad. JWST will be an incredible tool for scientific discovery, but its sensitivity range means it will never produce the kind of compelling imagery that Hubble did. Those pictures played a role in inspiring many of the people in the field today.

I would not be so sure about that. Many (most) astronomy photos are false color photos from quite different wavelength regions (often several different overlayed ones). So I do expect some fascinating images to come out of jwb as well.

Fascinating, sure, but Hubble has almost six times the frequency range of Webb. In terms of magnification and dust penetration Webb is leagues ahead, but for a given magnification Hubble just sees a wider variety of stuff.

Re: James Webb is fully deployed

#458

Post by terramex »

dev_tty01 wrote: post #409
terramex wrote: post #364

Earlier quoted context omitted.

Momentum flap will bring average CoP closer to CoM but CoP will shift as JWST is rotated to point at different targets. Unwanted torque can be minimised by carefully choosing observations targets so it mostly cancels out but it cannot be done perfectly.

I wonder if the target scheduling will take that into account to minimize average drift? Interesting set of calculations...

> As an additional challenge, the JWST observation schedule in the next 21-day period will not be known at the time of SK [station keeping] maneuver planning. A planned observation schedule one week ahead will be available, but the actual observation schedule will be event-driven. If a ‘target of opportunity’ arises then the schedule can be changed within 48 hours to point at the new target.

https://ntrs.nasa.gov/api/citations/20140007519/downloads/20...

Re: James Webb is fully deployed

#459

Post by intricatedetail »

JohnBooty wrote: post #208
daveslash wrote: post #204

Earlier quoted context omitted.

RE>> "I understand the reasons for not putting a camera on or near the JWST" What are the reasons? I'm sorry ~ I'm ignorant on the topic, but I'd love to learn more. Why/what are the reasons?

Great blog post from NASA themselves explaining it: https://blogs.nasa.gov/webb/2022/01/06/why-doesnt-webb-have-... the post is really not very long and I would suggest getting it straight from the horse's mouth, but for those not willing to devote a click here are arguably the most relevant bits! "deployment surveillance cameras would not add significant information of value for engineering teams commanding the spac…

Wow this sound so patronising. I am sure there will be a moment when they say oh wish we had installed those cameras...

Re: James Webb is fully deployed

#460

Post by tux3 »

dev_tty01 wrote: post #394
JohnBooty wrote: post #349

Earlier quoted context omitted.

I just want to make sure I understand. Your position is that they should have added significant complexity to a maneuver that was already the most complex of its kind. A maneuver with $10 billion and two decades of work on the line. In order to make the first 30 days of a 10+ year mission marginally more entertaining. You propose this while fully aware that the "entertainment" in question consists of things unfolding…

NASA has often added video or image capability with low scientific value to increase public engagement, despite the associated risk and cost. They made a call not to do it this time. My only point is that it would have been a nice PR opportunity if they could have found a way to make it work within their mission parameters. I apologize if making such a simple and obvious point has offended you in some way.

This is a good PR opportunity in the same sense that you giving all your earthly possesions away to charity is a good PR opportunity :)

It's only good if you ignore the cost, and look only at the benefits.

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