From: "Dave McCarter" 
To: "ATM" 
Reply-To: "Dave McCarter" 


> For years, I was under
> the impression (from the Ingalls ATM books and
> Allyn Thompson's book) that a sphere was dead easy,
> practically automatic!  What they should have said
> is that testing a sphere is dead easy.  Actually making
> a good sphere is no easier than making a good paraboloid.

A sphere is indeed automatic. I friend of mine needed a good sphere, eight
inch diameter, f3.2. After the thing was ground and polished I had him hand
stroke on the turntable with instructions to go very slowly and keep the
stroke short. He had never polished before, and took to it well. After
about two hours with rouge he had a better than 1/20 wave sphere from edge
to edge. The slight random variation of his slow hand stroke and the slow
speed of the table naturally resulted in a sphere that completely nulled
out under knife edge, and showed perfectly straight Ronchi bands.

Dave

--- BBBS/NT v4.00 MP