View Single Post
Old September 22 2012, 09:40 PM   #170
gturner
Admiral
 
Location: Kentucky
Re: Envisioning the world of 2100

sojourner wrote: View Post
You did see where I said "huge single mirror" right? Of course a multiple mirror telescope could be sent up on MLV's.

Frankly, NASA doesn't have the budget to build SLS and payloads to put on it.
Oh, I did see that, and I'm not suggesting multiple-mirror telescopes with conventional mirrors launched from Earth, I'm thinking of giant single mirror (or multi mirror) telescopes where you fabricate the mirror in space.

Regardless of whether your mirror is single or segmented, in space it can be extremely thin, light and structurally weak, barely more than a backing for a reflective layer, far beyond the point where it couldn't survive 1G, much less 3G's in and the high shock and vibration environment of a launch. The payoff in mirror area per pound would certainly be ten-fold, and probably be a hundred-fold or more as the techniques mature.

Getting there would take a lot of engineering and experimentation, much of it in orbit, perhaps exploring ways to make rigid aerogel substrates and vacuum coat them to provide a reflecting surface, then a way to smooth and polish the desposited surface to optical quality. If nothing else, it would be learning how to frit-bond very thin thin webs of pre-machined ULE glass to a lightweight face. We'd be pushing the bounds, figuring out what we can do, and then how to do it, with lots of failures and mis-steps along the way.

Does an aerogel (the world's most rigid substance by weight) make a good backing? How much vacuum deposition does it take to leave a polishable surface of glass, silica, or metal? How do you accurately form a parabola or hyperbola (especially the convex secondary) in free fall? Can you make an inflatable blanchard grinder? Do you have to? If you use frit-bonding or other methods of assembly, will you have to re-anneal the the assemblies?
Can you build mirrors in a Bigelow habitat, perhaps launching extremely thin, ?

Small, cheap launches, especially to a commercial space station like a Bigelow module, or even to the ISS, could make progress on such questions, whereas the SLS launches are so expensive that they'd only conceivably launch a fully assembled and tested space telescope with a mirror built on the ground in the conventional fashion, which means the mirror and its mounting have to survive high shock, vibration, and G loads. Even if they built it, we'd still be in the 10-meter class, and we'd only launch one, and the technology and techniques would never lead to something bigger, or cheaper. Basically, whatever space telescope the SLS launches will be a technological dead-end, because the future of space telescopes is making them huge, thin, and light.
gturner is offline   Reply With Quote