On Aug 26, 10:52 pm, Wayne.B wrote:
On Tue, 26 Aug 2008 19:31:21 -0700 (PDT),
wrote:

Progress while waiting for Fay to go away. Finished rough grinding
yielding a focal length of 24" (this is a 100 mm diameter f 6 mirror)
and finished the #120 grit and have started on the #180. My
magnifications will be a low of 15 X an a high of about 30X.

How do you know when you have the shape just right? What do use as a
blank?

At this stage, one simply uses a depth micrometer (or more simply a
feeler guage under a straight edge) to measure the depth of the
"bowl". The one grinds the "tool" (another glass blank) against the
mirror in such a way that they naturally form a concave and convex
spherical surface. Later, to change from spherical to paraboloidal,
one uses an amazingly simply shadow test called the Focault test to
measure the figure errors. It is so amazing that this actually works
and was figured out over a hundred years ago.