The spinning equations are now available right here:
w = sqr(g/2a) where w is the angular velocity in radians/sec g is the gravitational constant and a is the focal length
More practicallly:
r = 211.3/sqr(a) where r is in rpm and a is in cm.
I actually made some f/1 off-axis paraboloids this way many years ago, as it was orders of magnitude cheaper than anything else, and they worked! I got a 100 micron blur circle at the 7 inch focus, which was fine for what I wanted.
However, for true *optical* quality, ie. diffraction limited imaging, DON'T EVEN THINK ABOUT IT! Resins of almost any type are exothermic and will distort to a grossly erroneous surface. The stuff that I used was an epoxy casting resin mixed with an "adduct" of pre-reacted hardener/resin. It requires expert polymer chemists to get that right.
---------------------------------------------------------------------- A. David Beach MNZIP MRNZS................Industrial Research Limited Scientist....(d.beach@irl.cri.nz).........24 Balfour Road Machine Vision Team.(http://161.29.40.7/).Parnell Production, Automation and Control........PO Box 2225, Auckland Ph +64-9-303-4116....Fax +64-9-302-8106...New Zealand ----------------------------------------------------------------------