Building telescopes on the Moon is becoming an achievable goal | Hacker News Reader

Building telescopes on the Moon is becoming an achievable goal

theconversation.com

Many years ago a colleague once told me about how one could take an arbitrarily large vat of mercury on the moon, and set it spinning. Gravity plus centrifugal force causes the surface to become a more or less perfect paraboloid, and boom, now you have a giant, geometrically perfect primary mirror for a telescope. You can vary the focal length by changing the rotation speed.

Looking back, I've forgotten why this seemed like a good idea for the moon specifically. I guess maybe it wouldn't work on earth because the gravity is too strong, so you wouldn't get a noticable paraboloid curvature from the spinning bucket?

Oh it works on earth, my college had one for about a decade.

https://en.m.wikipedia.org/wiki/Large_Zenith_Telescope

Thanks for the link, that's is so cool! Wikipedia says it was decommissioned "due to the test site's bad weather" (PNW). I guess one benefit of doing this on the moon is no weather, or atmosphere at all to contend with. You essentially build an optical space telescope that only gets to look wherever the moon happens to be pointing. I suppose lower gravity also means that the mirror could theoretically be made much larger, since it weighs less and therefore conventional support structures and materials can go a lot farther.

Dumb question: would mercury evaporate on the moon?

For some reason I intuitively expect the lack of atmosphere to mean that any liquid would quickly bubble off. But I'm not sure that makes any sense as I think about it.

Not a dumb question! It definitely has nonzero vapor pressure (quick search says 2e-6 atm at room temp), so by definition it would boil at any pressure lower than that. The moon's trace atmosphere is something like 1e-15 atm. I guess you'd have to put it in a clear pressure container.

But also in this google search, I learned that Hg freezes at a relatively moderate temperature of -39C. The moon surface is far hotter than this during the day, and far colder at night.

Maybe this was the whole point of doing it on the moon?? You could spin the Hg at daytime, and then remove it from the spinning mould at night time when it freezes. If you move the fabricated mirror to a permanently shadowed crater (you've got about 2 weeks to get it there before the sun rises again), it would remain as a perfect optical surface more or less indefinitely. You could fill up a whole crater with big mirrors and make a giant multi-reflector aperture!

Best use case for the Moon I have ever heard. It shouldn't be too complicated to install smaller radio telescopes one at a time to eventually use interferometry without risking astronauts. And if we can put Webb in space, we can drop an identical telescope in an eternally dark crater, again, without risking lives.

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