Re: ATM Another Question from a Newbie--Mirror Figuring in the Cell

["Dan Chaffee" <dchaffee@gvi.net>]

> Since a small amount of deformation always occurs when using a large thin
> mirror, even when it is mounted in the best of cells, why not take
> advantage of this fact, and perform the final stages of polishing with the
> mirror mounted on its cell?  That way, deformations can be smoothed out.
> At the final polishing (after the figure tests okay), one could mount the
> mirror, and polish lightly to remove the high spots caused by the cell
\
I wish things were that simple<G>
In figuring mirrors, we want the least number of variables 
possible. Recall that a cell is designed with the minimum
number of points to support _the weight of the mirror_ for
maximum deformation allowable. When polishing and figuring, 
the force on the points is the weight of the mirror + the 
pressure of the polishing strokes, which will deform the mirror 
in excess of the calculated tolerences for the given support.
Localized areas of pressure on the back of the mirror tend
to result in _low_ spots being polished into the front
surface---not the kind of thing I'd want to deal with
in the final stage of rounding out a big paraboliod.

D Chaffee