To: Kevin and Danielle McCarthy ,atm{at}shore.net
From: Dennis Rech 
Reply-To: Dennis Rech 


At 08:35 AM 3/2/03 -0500, Kevin and Danielle McCarthy wrote:

>I recently found the Mirror-Matic site
>(http://www.mirror-o-matic.com/Mirror-Operations-5-2-5.html), and decided
>to give his polishing method a try.  I'm polishing an 11" sphere (the
>sphere is my target figure), and have made an 8" polishing lap per that
>site's recommendations.  The mirror faces up, and is centered to a
>fraction of a millimeter.  I've set my machine to mostly rotation (about
>25 rpm), and set a slow (~ 6 rpm) overarm motion that produces about 1" of
>left to right motion.  At one extreme, the lap edge overhangs about 1", at
>the other, the edge of the lap is about flush with the mirror. The mirror
>has polished out, and the edge is pretty good.  In the center, though, are
>some significant ring zones.  I would be interested to hear if others have
>sucessfully used this technique, as opposed to more conventional very slow
>rotation with mostly back and forth polishing motion.  Are central zones
>inevitable, and would it be better to leave the overarm fixed?  I think
>the central zone width may have been increased due the slow
>"blending"moves of the overarm.  Comments?
>

Hello Kevin,
The method in the Operations Manual are pretty much the ones that are used
by the professionals that live down the street from me. They have had good
results over the years, so I have suggested using it. I think that the
first time that I heard of using the stationary arm method was from John
Gregory, a professional speaking at the Bellingham Optics Workshop.
The method is also discussed in Advanced Telescope Making Techniques Volume
1, The Spinning Method.
Since the tool is not crossing the edge with a back and forth motion, there
is less tendency to turn the edge. Having the tool off to the side allows
cerium to be added easily. And finally, the method does a pretty good job
of removing astigmatism if it is present.

The downside is that it often leaves the mirror center a little high or low
which is usually not a problem since it will be figured to some new shape
later.

Generally, the turntable is spun faster than yours, 50 to 60 rpm.

There is a possibility that the zones that you see are still artifacts from
fine grinding. Its not uncommon to move the overarm a small amount during
fine grinding and overlay a slightly different curve partially over an
older steeper one.  Unless you have checked the mirror with a small
sub-diameter spherometer over several zones on the mirror, its easy to have
 zones that cannot be seen until polished.

If the pits are all fully polished out and the edge is good, you may want
to proceed to figuring rather than spend additional time trying to get the
total mirror to a sphere.
Hope this helps,
Dennis

--- BBBS/NT v4.00 MP