Re: ATM mirror grinding mechanics

[Cary Chleborad <bennu@ns.net>]

Steve Bell wrote:
> 
> 
> To quote from "Making Your Own Telescope" by A.J. Thompson: "By using
> diametric strokes of a length that will bring the edge of the mirror
> almost to the center of the tool (approximately the full diameter
> stroke), then at the end of the stroke, the pressure per unit area of
> surface, and hence the abrasion, is greatly increased at the edge of the
> tool and the center of the mirror. Thus, a greater amount of grinding
> takes place in these regions, with the result that the upper disk
> becomes concave, the the lower one convex."

OK...Imagine that you have a mirror and tool setup in front of you.  You
begin by pushing the mirror COC (the same should hold true for
diametrical strokes, though) away from you until the center of the
mirror is at the far edge of the tool.

Now, as in the example given above, the pressure per unit area of
surface is increased at the center of the mirror and the edge of the
tool.  BUT!!!!, given that the force is equally distributed upon the
mirror, isn't the presure at the center of the tool and the edge of the
mirror closest to you also elevated?
Since the force is evenly distributed upon the mirror, the pressure at
both points, and all in between, should be the same and even wear should
occur on both the mirror and tool.

There has to be some subtle action in how the force is applied that
makes the top piece go concave and the bottom convex.


> 
> Since it's the differential pressure per unit area produced by the
> stroking that causes the concave/convex result, if you would grind in a
> vertical position and applied the same amount of pressure, the results
> would be the same.

Wish piece would go concave and which convex?


-Cary Chleborad