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Author Topic: Is it possible to use NaK rotating liquid mirrors as telescopes on Earth?  (Read 474 times)

Offline chiralSPO

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If you don't know what this type of telescope is, start here: https://en.wikipedia.org/wiki/Liquid_mirror_telescope

If you don't know what NaK is, start here: https://en.wikipedia.org/wiki/NaK and here (without a space between / and /) https:/ /www.youtube.com/watch?v=Nn3M1hfjxMU


Compared to NaK is mercury is less shiny, almost 16 times denser (so it takes a greater mass of liquid and more energy to spin up a mirror of the same size) and poses greater environmental risk. The main problem is that NaK is extremely reactive. In addition to posing a fire hazard, the mirror would tarnish within seconds on exposure to air, and be totally useless. But if it were protected from H2O, O2, CO2 and N2) either by an inert gas or vacuum this would not be a problem.

My question is which strategy would interfere least with the optics: A long chamber sealed at the far end with a thin glass window and filled with inert gas, a similar chamber that is sealed by a window that is very close to the mirror and filled with a inert gas (let's assume that the pressure of the gas can be raised or lowered for small changes in refractive index, if needed), or the same chamber without a window that is filled with a very dense inert gas like xenon (4.5 times denser than air, with a refractive index about 1% greater than air at the same pressure.) See attached pdf for schematics.

This last option is certainly the most expensive option and provides protection for the shortest periods, but I would guess that the optics would be significantly better with a smooth transition between air and slightly greater than air rather than having a pane of glass (no matter how thin) which would cause significant refraction. Is this a reasonable conclusion?


 

Offline Bored chemist

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If you put mercury in a similar container you wouldn't have a toxic risk.
Even the tiny changes in refractive index of air caused by temperature changes are enough to seriously distort images (that's why stars "twinkle").
The distortion caused by a vortex of mixing xenon and air would be a lot worse.

Try gallium.
 

Offline evan_au

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As noted in the first Wikipedia reference, I have the same problem with all 3 designs - the rotating NaK liquid only forms an ideal parabolic reflector when the telescope is pointing straight up. As soon as you try to tilt it to look at other parts of the sky, or to follow an object across the sky for a long exposure, it no longer forms a parabola with its central axis pointing along the telescope axis.

This really makes it unusable as a conventional astronomical telescope.

Over a 12-month period, the Earth's axial tilt will let you scan a region of sky, but you can't point it where you want it, when you want it!

On the decision about whether to install a window or not: Any condensing moisture in the air (eg wisps of fog) contain tiny droplets of water. Water is denser than Xenon, and so these droplets will drift down through the Xenon, and react with the NaK mirror, leaving tiny pieces of oxide floating on the surface.

The geometrical effect generating the parabolic shape is used to produce large glass mirrors. In this case, the molten glass is spun in a rotating furnace until it solidifies. Then the parabolic mirror can be tilted to any desired angle as part of a conventional telescope.
 

Offline alancalverd

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The density and viscosity of liquid are actually helpful. It doesn't really matter how long or how much energy it takes to form a parabola, but the heavier and stickier it is, the fewer surface ripples will form from acoustic noise and seismic tremors (not to mention gravity waves).

As Evan says, you can spin a parabola from molten glass or pure silica. You can anneal it as often as you like, then use the solid parabola at any angle you want. If you are worried about stress bands in a thick mirror, make a thin one by spinning the glass on top of a bed of molten tin (the float glass process).
 

Offline chiralSPO

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Thanks guys! I figured it was a non-starter, but wanted to be sure.
 

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