From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


Oblate is caused, IMHO, by the edge of the tool dwelling around the 70%
zone or wherever the tool/mirror makes a stop and changes direction.  It is
the edge of the tool that creates the greater polishing action.  Watch
where it stops during strokes.

If your tool edge is working too hard on the mirror, from, say, accented
pressure during the stroke, it will create a low zone there.

Is your pitch all right?  Is the pitch lap even thickness all over, or is
it poured on a flatter radius tool?  If the lap is thin at the edge
relative to the center, it will act like it's harder at the edge.

You might be right about the pvc flexing, too.  Can you beef it up with a
layer of plywood?  Most plywood tools for a 30 cm would need to be 50mm or
thicker, I'd think.

It's all polished now, right?  20 mm abberation will respond quickly.



Colin





Hi  all,

I was repolishing one lousy 30cm/ F6.5 mirror from a friend with a new tool
I made from 30mm  thick PVC.

No matter what stroke I put on my machine , the mirror  keeps going more
and more oblate. I have try TOT, MOT,  center to edge, edge to edge, 1/3 
and even something between 1/3 and 1/2 strokes without success. I pressed
the tool for 8 hours to  be sure about  contact, checked the  flatness of
the back and try all again. Now the mirror has a 20 mm oblate spherical
aberration and I ran out of my tricks.

My last possibility is that the PVC base is flexing too much and causing the error.

Any ideas on what is going on?

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