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

[Mike Lockwood <melockwo@uiuc.edu>]

Tom,

Polk, Tom wrote:
> The "high" zone centered about 1.0" from the edge
> looks bad. Is my interpretation correct that it is a high zone near the
> edge?

I believe so, except that the mirror is not high at the EDGE - it 
rolls off there.  Working inward toward the center, there is a high 
zone, then a bit of a dip, and then a high central area.

> Please help me determine the cause of this defect:
> The lap was made using a "perfect pitch" lap mold (a la Kevin McCarthy,
> which is ~12" square) using #64 Gugolz. The stroke was predominately 1/3
> amplitude 1/4 W side motion. Superimposing a Ronchigram onto a picture
> of the lap show the center line of the zone coincides with the missing
> squares. Did the missing squares on the lap cause this defect (please
> see pics)? When starting out, my thoughts were the 1/4 W motion would
> address this. After all, larger blanks are polished out with smaller
> laps.

No, this was probably not caused by your lap if you did the stroke as 
you described, provided that you randomized the stroke a bit.

Your mirror has the classic (OK, it's classic to me) problem of being 
an oblate sphere, that is, the outer zones (with the exception of the 
part near the edge) have a shorter ROC than the center.  This is the 
most common mirror making problem I see.

I am guessing you were working tool-on-top?

For my cure for oblate spheres, I will yet again refer to my page 
about oblate spheres:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/oblate/oblate.html

I just updated that text to better express my thoughts on that topic.

	Mike L.

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