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I was recently listening to a talk about JWST and other future astrophysics missions when the speaker talked about many of the size limitations associated with mirrors etc. I then thought perhaps one could solve this problem similar to how JWST approached their limitation, i.e., by "folding up" the mirror. However, weight is also an issue so then I started to wonder if there were any materials similar to a fabric (not necessarily a fabric) that could used for a mirror.

Let's assume there is or are materials that are fabric-like (i.e., can be folded without kinking/wrinkling) and reflective like glass or polished metal.

Could one stretch such a material accurately and precisely enough to form a useable mirror with competitive resolution and clarity (i.e., good enough for new research, not blurry results by today's standards)?

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    $\begingroup$ If a fabric should be used, its mesh widths should be much smaller than the used wavelength for the mirror. But a fabric with a mesh width smaller than the wavelength of visible light is very difficult to produce. $\endgroup$
    – Uwe
    Commented Oct 9, 2017 at 20:16
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    $\begingroup$ Perhaps an idea for a radio telescope where wavelengths to be observed are much longer than the scale of the fabric's roughness. The Arecibo observatory main dish was originally wire mesh. $\endgroup$
    – Anthony X
    Commented Oct 9, 2017 at 22:20
  • $\begingroup$ Only slightly related; youtu.be/7n-rByfZfUE $\endgroup$
    – uhoh
    Commented Nov 16, 2017 at 7:43

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Mirrors have to have a very smooth surface. The JWST mirrors have surface imperfections on the order of 10 nm. They also have to be very rigid, in order to stay accurate through the rigors of launch. Despite this, the JWST mirror segments weigh only 20 kg each (for a diameter of 1.3 m).

It's possible to use a very thin mirror, and use electric actuators to bend the mirror to the required shape. But that just shifts the weight to the supporting structure.

The Hubble telescope used flexible solar arrays initially. The thermal shock when entering/leaving Earth's shadow meant the arrays would flex, and the vibrations would disturb the observation. Without an effective active control mechanism, flexible mirrors would have similar behavior.

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  • $\begingroup$ Isn't the vacuum and microgravity of space ideal for shaping a perfect parabolic or spherical mirror out of liquid material, smoothing out the surface of an unfolded fabric structure? $\endgroup$
    – LocalFluff
    Commented Oct 10, 2017 at 15:57
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    $\begingroup$ vacuum = your liquid evaporates away... $\endgroup$
    – Hobbes
    Commented Oct 10, 2017 at 16:11
  • $\begingroup$ All liquids, even heavy ones like molten silver for example? There are these ideas about melting the Lunar surface to glass, can't they work outdoors? $\endgroup$
    – LocalFluff
    Commented Oct 10, 2017 at 16:25
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    $\begingroup$ yes, all liquids (but to varying degrees, this is measured as 'vapor pressure'). Melting the lunar surface to glass can work, depending on the goal of the operation. If you just want to trap the dust, you don't care if you evaporate half the dust while fusing the rest into a sheet of "glass". But the resulting glass won't be a mirror-smooth surface. $\endgroup$
    – Hobbes
    Commented Oct 10, 2017 at 16:44
  • $\begingroup$ A spherical mirror may be shaped in gravity using a rotating horizontal cylindrical container filled with quicksilver. But this would not work in microgravity. Only the interaction of the earth gravity and the centrifugal force would shape the mirror. Without gravity, the quicksilver would be shaped into cylinder. By the way, the density of quicksilver is higher then that of silver. Of course quicksilver would evaporate away in a vacuum. $\endgroup$
    – Uwe
    Commented Oct 10, 2017 at 18:28
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The STS-77 mission had a related experiment, the Inflatable Antenna Experiment.

enter image description here

enter image description here

While intended for RF wavelengths, conceivably something could be done for other wavelengths.

That said, the experiment was not much of a success. It was quickly punctured by micro-meteroids and lost its shape. It was made of reflective mylar.

More information here.

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Mirrors can be made of thin, flexible metal-coated foil, and they work very well in space... except don't expect any considerable precision from them. Certainly not something like a telescope; maybe in a pinch a mirror for beaming power (sun light) to specific area of a planet or a central target not too far. Problems of wrinkling, creasing etc also cause extra engineering headaches.

Regardless, the mirror finish, reflecting photons, reacts to light pressure twice as efficiently as absorptive material, so the lightweight, thin foil mirrors are the primary material for solar sails.

solar sail craft

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  • $\begingroup$ "...twice as efficiently as absorptive material..." is good for normal incidence, but if the light is coming at an angle, an absorptive material will still absorb the full momentum vector of the photons, while a reflective surface will tend to ignore the tangential component of the photon's momentum. A reflective sphere would not accelerate twice as fast as a black sphere at all for example. $\endgroup$
    – uhoh
    Commented Oct 10, 2017 at 14:13
  • $\begingroup$ @Uhoh: Correct; also a black sail would make tacking against the solar wind impossible. $\endgroup$
    – SF.
    Commented Oct 10, 2017 at 21:19
  • $\begingroup$ space.stackexchange.com/a/691/12102 $\endgroup$
    – uhoh
    Commented Oct 11, 2017 at 1:58
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    $\begingroup$ @uhoh: space.stackexchange.com/a/377/103 $\endgroup$
    – SF.
    Commented Oct 11, 2017 at 5:39
  • $\begingroup$ OK it's not tacking proper, but it certainly is moving towards the sun, thanks! $\endgroup$
    – uhoh
    Commented Oct 11, 2017 at 5:47
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It's hard to fold things without leaving creases.

Mayak (Маяк) looks crinkly:

The July 14, 2017 NASA Spaceflight article Soyuz 2-1A launches with Kanopus-V-IK and over 70 satellites says:

Mayak is a three-unit CubeSat which was built by Tvoii Sektor Kosmosa – or “Your Sector of Space” – an independent, crowd-funded team of engineers in conjunction with the Moscow State University of Mechanical Engineering. Mayak – meaning Lighthouse – will deploy a highly reflective tetrahedral structure.

Each side of this structure has an area of four square meters, or 43 square feet. To ground observers, the satellite is expected to have an apparent magnitude of up to -10, making it one of the brightest objects in the night sky. The structure will double as a deorbit mechanism, hastening the decay of the satellite’s orbit.

below: Mayak Reflector – Photo: CosmoMayak, From Spaceflight 101

enter image description here

Echo 2 looks smoother but diffuse

(but it was for VHF radio signals, not visible light to begin with)

enter image description here

above: Echo 2, from https://en.wikipedia.org/wiki/File:Echo_II.jpg Click for larger size

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