Alternative mirror technologies, WAS ATM Liquid-mirror telescopes

[Chuck Knight <cknight@flash.net>]

> I could probably build a mylar
> mirror for even less!  (Say, have you actually tried that vacuum
> technique, or is it just theory?  I have a 14" embroidery hoop
> that's just "screaming" for an experiment!  Why would it form a
> parabola, specifically, instead of a spherical mirror?)

I've got a question.  I've seen occasional references to mylar 
film mirrors...pulled into a parabola with a vacuum.  What
makes it take on a parabolic shape?  

I understand how a spinning liquid is forced into a parabola,
but how does the membrane mirror work?

If nothing else, it sounds like a fun experiment!  Now where
did I put those mylar balloons?  :-)

     -- Chuck Knight