On Thu, 28 May 2009 18:38:14 -0700 (PDT), Frogwatch
wrote:

On May 28, 5:56*pm, Richard Casady
wrote:
On Tue, 26 Aug 2008 22:52:24 -0400, 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?

There are optical tests with simple equipment.

What do use as a blank?

Two discs of pyrex from a mail order house. One is the mirror, one the
tool. You put grit on them and rub them together.

Casady

The action of grinding naturally produces a spherical mirror. You can
measure its focal length by wetting it and seeing where it fills with
light from a small bulb.
The proper shape for a telescope is a paraboloid and for a very long
focal length, say f10, the diff tween a sphere and paraboloid is too
small to notice. For a short focal length one like what I am making,
you have to "parabolize" it. This is done as part of the polishing
process using a very simple device called a Foucault tester. The test
can literally be done with a candle flame and knife edge. In this
test, variations in shape from a paraboloid are enormously magnified
so you know where to concentrate polishing. This test (or a variation
on it) is normally done on ALL telescope mirrors and this is what
Perkin -Elmer neglected to originally do with Hubble necessitating the
corrective optics.
The entire mirror making process is a beautiful excercise in applied
trigonometry and patience.

Very interesting. Thanks.

I'd have said 'geometry' instead of 'trigonometry', but you're the
expert at lens grinding here.
--

John H