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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

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?

andromeduck 3y ago
Oh it works on earth, my college had one for about a decade. https://en.m.wikipedia.org/wiki/Large_Zenith_Telescope https://en.m.wikipedia.org/wiki/Large_Zenith_Telescope
- cshimmin 3y ago
  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.
timmg 3y ago
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.
- cshimmin 3y ago
  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!

andromeduck 3y ago

Oh it works on earth, my college had one for about a decade. https://en.m.wikipedia.org/wiki/Large_Zenith_Telescope https://en.m.wikipedia.org/wiki/Large_Zenith_Telescope

cshimmin 3y ago
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.

timmg 3y ago

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.

cshimmin 3y ago
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!