Further comments on polishing Silicon for those interested:
Traditional polishing compound (cerium oxide) don't work with silicon With "chemo/mechanical" polishing, polishing silicon with a polyurathane pad is very easy, and it should be straight forward to produce a fairly good spherical surface, and with manipulating pad sizes, it may also be possible to 'zone' the sphere into a parabola.
Polishing silicon with pitch is definitely an art. The alkaline slurries used with pad polishing tend to soften the pitch causing downgrading of the polished surface. There are techniques to get past this problem.
Alpha Alumina (Al02), zirconium, and diamond (say a .05u finish) have all been used in pitch polishing of silicon, but they're all tricky...
Obviously, anyone with experience in pitch polishing of silicon who has any comments please do pass on, this is one area of polishing expertise I am looking for improvement in.
(This follows a line from sci.astro.amateur, but continued here for ATM intrests - hopefully not redundant
Bill Marriott Forest Knolls, Ca. btk@ix.netcom.com