Greetings and Salutations.

On 13 Nov 2003 01:33:58 -0500, Allan Adler
wrote:


I've only seen the skill of grinding glass discussed in the context
of grinding lenses. In particular, the glass is supposed to acquire
a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly
flat? Does this involve essentially different techniques, both in grinding
and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one
can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one
read about how to hand grind glass perfectly flat?

Ignorantly,
Allan Adler


There is a wonderful trilogy of books titled "Amateur
Telescope Making", edited by a fellow named Ingalls that covers
all aspects of optics - including how to make optical flats.
Go to your public library and see if they have a copy. The
articles there will tell you more about the process than you
want to know.
However, in short...take three thickish pieces of glass
and a series of grits, ranging from fairly coarse, to jeweler's
rouge. Starting with the coarsest grit, grind all three plates
on each other, until all three have an even scratch pattern on the
ground face. Clean everything off. Goto the next finest grit.
Repeat above steps for each grit until polished.
It helps if you have a monochromatic light source, say,
a sodium arc lamp, or, a candle burning salt for that bright,
yellow flame. That is how one tests the surfaces to see how
"flat" they are getting. By putting two of the plates together,
face to face, and illuminating with the monochromatic light,
one gets a series of interference fringes. The shape of the
fringes and the number of them give an idea of how parallel
the surfaces are. In a "perfect" world, you would have one,
wide fringe that covers the entire contact surface...that
would mean the plates are "perfectly" parallel, and, if you
have used three plates, perfectly flat.
(remember that it takes three points to define a plane,
mathematically...works with glass too).
Needless to say it is a TAD more tedious than this
explanation, but, this hits the high points. (haw, haw, haw).
Regards
Dave Mundt