Pressure casting??

In the book 'Unusual Telescopes' there are descriptions of mirrors made from glass or plastic membranes held in figure by pressure differentials (in the examples negative pressure applied behind the mirrored membrane). The figures are not quite parabolic though (though I don't remember the explanation if one was supplied).

Can one take a membrane and place it in a heated liquid environment, where the liquid on one side of the membrane has a slightly different density than liquid on the other, so that the membrane dimples to a paraboloid, the heat annealing the membrane so that no stresses remain?

Unlike normal pressing, because one is using liquids there might be no distortion from separation once everything is done. Also, one might be able to make quite large membranes (difficult if one uses a press).

Has anyone done anything like this?

Jess Tauber

jess tauber said:
In the book 'Unusual Telescopes' there are descriptions of mirrors made from glass or plastic membranes held in figure by pressure differentials (in the examples negative pressure applied behind the mirrored membrane). The figures are not quite parabolic though (though I don't remember the explanation if one was supplied).

Can one take a membrane and place it in a heated liquid environment, where the liquid on one side of the membrane has a slightly different density than liquid on the other, so that the membrane dimples to a paraboloid, the heat annealing the membrane so that no stresses remain?

If the pressure is uniform, and so is the tension on the membrane, you get a sphere, not a paraboloid. You can work things out so that you get a paraboloid, but it's not trivial.

I don't know if anyone's done anything like this on a large scale. I suspect it's been done on a small scale, but I'm not aware of anything specific.