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 th

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!