[HN Gopher] Flexible membrane mirrors for large space-based tele...
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Flexible membrane mirrors for large space-based telescopes
Author : geox
Score : 46 points
Date : 2023-04-09 11:02 UTC (11 hours ago)
(HTM) web link (www.optica.org)
(TXT) w3m dump (www.optica.org)
| ck2 wrote:
| Two wild thoughts:
|
| Could a technique evolve someday where you don't even need the
| mirror but just the sensor and the collected data is then
| processed as if it was focused by a mirror?
|
| Could a liquid be developed based on a molecule with near perfect
| reflectivity, a few liters of it is taken into space and then
| pulled out like a soap bubble at molecule-level thinness?
| icegreentea2 wrote:
| For your first thought - if we could capture the phase
| information of the incoming light, then yes. We actually
| already do this for radio waves- you may have heard of
| synthetic aperature radar, or AESA/PESA radars in a military
| context, or the event horizon telescope network (the thing that
| got us the 'first picture of a blackhole').
|
| For your second thought - as literally described, such a thing
| would probably be badly impacted by solar wind.
| swayvil wrote:
| 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
| klyrs wrote:
| > How to make it parabolic?
|
| Constant acceleration?
| swayvil wrote:
| Hmm. Good idea.
|
| Or put it in orbit. Let gravity deform the bubble. Maybe.
| klyrs wrote:
| 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
| eesmith wrote:
| I think the link you want is
| 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 .
| klyrs wrote:
| 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 wrote:
| Would spinning it while cutting make it parabolic?
| ajb wrote:
| Not quite, you would get an ellipsoid, I think. Not sure if
| that is close enough.
| LeifCarrotson wrote:
| 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.
| postalrat wrote:
| Maybe a combination of spinning, accelerating forward,
| curing, etc etc.
| Gravityloss wrote:
| Since there are no gravity loads in space, one could have
| humongous mirrors with this technique. Not an expert but I
| imagine thermal load design becomes an issue too.
|
| Current rocket fairings can be 5 m in diameter and 20 m in
| length. One could roll up a 20 m diameter mirror in there....
| dylan604 wrote:
| Or 40, 60, 80, depending on if you fold it couple of times
| before rolling it.
| DennisP wrote:
| And if we get anything like the launch prices SpaceX has
| talked about for Starship, we could launch a lot of them.
|
| There's still the cost of the telescope itself, but it's
| probably possible to save money on that when cheap launch
| lets you build heavier, _and_ mass-produce the telescope.
| klyrs wrote:
| Micrometeorites and solar wind seem like they'd be pesky
| obstacles to a membrane. JWST experienced a strike almost
| immediately after it was deployed, which is quite tragic. The
| bigger the target, the easier it is to hit.
| skykooler wrote:
| However, the bigger the mirror, the smaller the effect of an
| individual micrometeoroid strike.
| m3kw9 wrote:
| I'd like to see more than a dot that NASA regularly shows as
| exoplanets using the latest tech.
| ben_w wrote:
| I think a Jupiter sized planet, 20 light years away, imaged at
| 500nm wavelength, would need a 826 meter diameter mirror just
| to make it 2 pixels instead of 1.
|
| Space is big.
| jlokier wrote:
| Fortunately a large mirror doesn't have to be a continuous
| surface, and it doesn't need all of that area either.
|
| A constellation of optically linked telescopes each with
| small mirrors would be able to image the same exoplanet.
|
| Controlling the optical linkages or equivalently the
| trajectories precise and fast enough is difficult, but as far
| as I know this is not a fundamental limitation.
|
| What a fun project that would be to build!
| JoeAltmaier wrote:
| Hm. Flat mirrors take less space that hollow rolled-up tubes of
| mirror. Maybe they could be rolled around something else in the
| rocket. Hm.
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