Re: [ATM] Ronchi interpretation and corrective action for high zoneat edge

["Jerry" <wa4guu@verizon.net>]

-----Original Message-----Trimmed down
From: Polk, Tom
 
polishing out a 12.5" 
"high" zone centered about 1.0" from the edge looks bad.
Is my interpretation correct that it is a high zone near the edge?

Yes. That is one way to look at it. There are other ways to look at it. That
is probably the best. 


Corrective actions:
lap .... is still "fresh",
I have cast some pitch square to pop into place.
Use a wider stroke (1/3 W) of the same amplitude.

Is this action correct for the problem?

Wider W would be one of the things I might try on that shape, or adding
about a 1/2 inch to the stroke length, or a few other things.
But first...

Until you are finished polishing, what you have is somewhat to be expected.
So it is not a problem until you are figuring.

I would not blame the missing partial facets for the figure. I might still
put some pitch there, but not for the reason you might think. Look at the
edge of the lap at the 11:30 and 2:30 clock positions in your picture where
you have left out several partial facets in a row. There is an increased
danger removing the lap from the mirror, or putting them back together, the
edge of the lap substrate at one of those locations could come in contact
with and scratch or chip the mirror. You might put partial facets at the two
or three central of those two locations. I would put pitch there even if was
not high enough to contact the mirror. It could be you do have pitch there
and I'm just not seeing it in the picture.
  

As for the corrective action, just wait until the mirror is fully polished
and then you will (should) go to shorter work sessions for figuring. The
shorter work session might straighten it out some. 5 to 10 minute sessions
will not heat the work as much as long polishing sessions. I am assuming
that your polishing sessions are longer than that by a large margin.

You don't have to worry so much about going awful wrong at this stage. When
you get close to a parabola, then you worry about messing up. A serious
mistake then not only is a waste of that work session, it can also be a
waste of much or all the figuring before that session. It is different when
you are nowhere near a parabola. If you mess up it will probably only waste
the one work session.

What are you using as a base for the lap?  How thick and heavy is it? I saw
your last post about 28 pounds of weight on the center of the lap. I have
never worried about uneven hand pressure except to try not to do it by
accident. The extra weight might be fine for polishing but for figuring it
could be too much weight. I haven't used weight like that figuring so I
can't say for sure, but I think you will do better without it when you
figure. Maybe one of the pro's will comment on this.



Jerry   


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