Some interesting links:
http://www.abc.net.au/science/k2/moments/s64751.htm
http://www.math.iupui.edu/m261vis/LMirror/LMirror.html
http://www.astro.ubc.ca/LMT/lm/index.html
What you need is a liquid spinning mirror you can snap freeze without shattering or changing the shape of the material - hard asks. Imagine if you could spin say silver this way at say 400 degrees Kelvin (or whatever) - then span freeze it whilst its still rotating without loosing its perfect surface. If it was bonded to its spinning base - which itself was structurally extremely rigid - you could simply lift this entire structure and mount it like any normal large telescope - hey presto - large perfect mirror very, cheaply formed.
If the mirror device was inside a bessel oven - and continued to rotate at say 3 turns a minute whilst the parabolic arc formed, and could be instantly snap frozen (there is the trick) without rippling the surface - way to go. You might be able to do this if the film of liquid metal was very, very thin - e.g, < 100 microns thick. If you heated and spun say 600 grams of silver in a 3 metre wide circular block that transfered heat very well (e.g. copper) and was already roughly parabolic in shape - you might be able to run collant liquid through the block and instantly snap freeze the silver.
Be really tricky but if you find a way to do it that works - be fantastic!