Joe S. wrote:

20-inch? Where did that come from -- should say "10- or 12-inch
mirror."


It would probably prove beneficial if you initially attempt something
smaller knowing that the experience gained will result in fewer "surprises"
on the larger mirror you envision.

From the various effects of different strokes used in the grinding and
polishing procedure, to interpreting foucault results and parabolizing the
smaller mirror, to just plain getting the feel for the whole process, you
can't go wrong starting small. I believe that a 150mm is an excellent place
to begin. . .even after you have finished your larger scope, you'll still
have a nice sized scope to use when you just don't have the time or energy
to drag the larger one out. I have a 12.5 inch dob. I also have a 114mm
newtonian. Believe me, the smaller scope doesn't gather dust.

Almost 40 years ago, I did a 114mm mirror before subsequently progressing to
producing a couple of other 200mm ones and believe the introductory
experience of a smaller mirror to be indispensible.

--
Martin