6 ms·

You could blow a bubble. Go to space. Take a blob of viscous polymeric whatever. Squirt a little air inside. Blows up automagically. Let the goo cure. Now you

You could blow a bubble. Go to space. Take a blob of viscous polymeric whatever. Squirt a little air inside. Blows up automagically. Let the goo cure. Now you have a ~10 kilometer bubble. Inject some vaporized silver, using electrostatic deposition or whatever. Cut the bubble in half. Now you've got 2 concave mirrors. How to make it parabolic? Hmm

postalrat 4y ago
Would spinning it while cutting make it parabolic?
- ajb 4y ago
  Not quite, you would get an ellipsoid, I think. Not sure if that is close enough.
  - LeifCarrotson 4y ago
    It's not close enough to just put a sensor out in front, but it's definitely useful. Lots of multi-mirror telescopes use several conic sections; if you needed maximum light you could probably build a telescope with a monstrous 10km ellipsoidal primary, and let that bring the light down to a <1km secondary, and then bring the light back to an aspheric or hyperbolic tertiary optical system built with conventional mechanisms to actually focus the light. However, I don't think you need to cook your optics with that many square kilometers of background radiation if you don't mind a longer exposure. Instead, you'd be better off installing a few small subsets of that hypothetical single mirror 10s of km away from each other, tracking the distance of each satellite through interferometry, and then putting it back together in software.
    - adgjlsfhk1 4y ago
      the data volumes are probably too high to make this work well. interferometry requires ridiculous amounts of data (hundreds of terabytes per image) and doing that processing in space will be tough within the thermal requirements.
  - postalrat 4y ago
    Maybe a combination of spinning, accelerating forward, curing, etc etc.
klyrs 4y ago
> How to make it parabolic? Constant acceleration?
- swayvil 4y ago
  Hmm. Good idea. Or put it in orbit. Let gravity deform the bubble. Maybe.
  - klyrs 4y ago
    After making that suggestion, I realize that Earth's gravity would make perfectly parabolic lenses if the suggestion was viable. That people aren't already getting perfect(-enough) parabolic lenses from this technique suggests to me that it's probably not actually viable. Oh well. Come to think of it, a film suspended from a ring would create a catenary[1] under constant gravity -- which is pretty darned close to a parabola but, I suspect, not good enough for astronomy. [1] https://en.wikipedia.org/wiki/Catenary https://en.wikipedia.org/wiki/Catenary
    - eesmith 4y ago
      I think the link you want is https://en.wikipedia.org/wiki/Liquid-mirror_telescope https://en.wikipedia.org/wiki/Liquid-mirror_telescope - "The liquid and its container are rotated at a constant speed around a vertical axis, which causes the surface of the liquid to assume a paraboloidal shape. This parabolic reflector can serve as the primary mirror of a reflecting telescope." This method in this article uses the same principle: “It has long been known that rotating liquids that are aligned with the local gravitational axis will naturally form a paraboloid surface shape,...“ See also https://en.wikipedia.org/wiki/Liquid-mirror_space_telescope https://en.wikipedia.org/wiki/Liquid-mirror_space_telescope .
      - klyrs 4y ago
        I'm familiar with liquid mirror telescopes, but I was responding to some theorizing about using an inflated film. Without gravity, the film would produce a sphere; suspended in gravity, it would produce a catenary.

postalrat 4y ago

Would spinning it while cutting make it parabolic?

ajb 4y ago
Not quite, you would get an ellipsoid, I think. Not sure if that is close enough.
- LeifCarrotson 4y ago
  It's not close enough to just put a sensor out in front, but it's definitely useful. Lots of multi-mirror telescopes use several conic sections; if you needed maximum light you could probably build a telescope with a monstrous 10km ellipsoidal primary, and let that bring the light down to a <1km secondary, and then bring the light back to an aspheric or hyperbolic tertiary optical system built with conventional mechanisms to actually focus the light. However, I don't think you need to cook your optics with that many square kilometers of background radiation if you don't mind a longer exposure. Instead, you'd be better off installing a few small subsets of that hypothetical single mirror 10s of km away from each other, tracking the distance of each satellite through interferometry, and then putting it back together in software.
  - adgjlsfhk1 4y ago
    the data volumes are probably too high to make this work well. interferometry requires ridiculous amounts of data (hundreds of terabytes per image) and doing that processing in space will be tough within the thermal requirements.
- postalrat 4y ago
  Maybe a combination of spinning, accelerating forward, curing, etc etc.

klyrs 4y ago

> How to make it parabolic? Constant acceleration?

swayvil 4y ago
Hmm. Good idea. Or put it in orbit. Let gravity deform the bubble. Maybe.
- klyrs 4y ago
  After making that suggestion, I realize that Earth's gravity would make perfectly parabolic lenses if the suggestion was viable. That people aren't already getting perfect(-enough) parabolic lenses from this technique suggests to me that it's probably not actually viable. Oh well. Come to think of it, a film suspended from a ring would create a catenary[1] under constant gravity -- which is pretty darned close to a parabola but, I suspect, not good enough for astronomy. [1] https://en.wikipedia.org/wiki/Catenary https://en.wikipedia.org/wiki/Catenary
  - eesmith 4y ago
    I think the link you want is https://en.wikipedia.org/wiki/Liquid-mirror_telescope https://en.wikipedia.org/wiki/Liquid-mirror_telescope - "The liquid and its container are rotated at a constant speed around a vertical axis, which causes the surface of the liquid to assume a paraboloidal shape. This parabolic reflector can serve as the primary mirror of a reflecting telescope." This method in this article uses the same principle: “It has long been known that rotating liquids that are aligned with the local gravitational axis will naturally form a paraboloid surface shape,...“ See also https://en.wikipedia.org/wiki/Liquid-mirror_space_telescope https://en.wikipedia.org/wiki/Liquid-mirror_space_telescope .
    - klyrs 4y ago
      I'm familiar with liquid mirror telescopes, but I was responding to some theorizing about using an inflated film. Without gravity, the film would produce a sphere; suspended in gravity, it would produce a catenary.