Re: ATM 8" refractor project

["Dan Chaffee" <dchaffee@blitz-it.net>]

>You will want to build several spherometers as Dave indicates.  Difference
> in them is the radius you measure to.  Also several dial indicators are
> helpful.

I don't understand why. I made a very successful 7" achromat with
one and only one spherometer and NO dial indicator. It consists of
a micrometer attached to the end of a lever arm that yields a movement
33% of what the micrometer indicates. The accuracy is easily under
.001" and the two 'feet' balls contact very nearly to the edges of the
lenses
giving the maximum depression that can be measured (and resulting in
better accuracy  than if measuring a smaller segment of the curve).
By reversing the direction of the motion of the lever arm and placing
a bolt near the pivot, it doubles as a very sensitive wedge checker.
There is no lateral color that I can see in this lens, which means wedge
is as good as it needs to be. I had the micrometer and I think it set
me back $10 for the aluminum to make the lever arms and $4 or so for
several ball bearings for the points and to calibrate with.

Dan Chaffee