From: "Tom Stokes" 
To: 
Cc: "atm" 
Reply-To: "Tom Stokes" 


First, I need to explain how I got here.

I pitched an 8"x3/4" plate glass disk to a 12"x3/4" ply
disk. I then bolted the ply disk to my MOM 20 turntable which is also
3/4" ply. I used 3 bolts spaced 120 degrees apart and used a washer on
each bolt to keep the ply disks separated. This 3 point support slowly
failed as the washers became recessed into the wood and the wood itself
became slightly warped. Every time I tighten or loosen the bolts I'm
putting a bending stress into the glass. Now I have astigmatism. Using a
knife edge near the focus, I can see a Ying Yang pattern. Inside or outside
of focus, the knife edge is strongly tilted and not straight.

The fix is simply to use more (or thicker) washers to get back to the 3
point support system. The pitch supporting the glass should take care of
the rest. Will the astigmatism ever polish out or do I go back to fine
grinding?

Tom Stokes
http://home.pacbell.net/tstokes shows my MOM 20.

--- BBBS/NT v4.00 MP