[ATM] Ronchi interpretation and corrective action for high zone atedge

["Polk, Tom" <TPolk@decatur-al.gov>]

Hello,

I'm getting close to polishing out a 12.5" (317 mm), f/L 68" (1727mm),
0.73" (18 mm) plate glass mirror. Friday I received a 133 lpi Ronchi
grating from Willmann-Bell and this weekend I had some time to capture
some Ronchi-grams. 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?

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.

Corrective actions:
Since this is my second lap and 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?

Given some of the ideas discussed on this list, I feel sheepish
submitting this. I don't want to compound my problem with my
inexperience and need some advice and this is the best place for
experience and knowledge. Thanks for your time.

I have uploaded some images of my lap:
http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/Pi
tch%20Lap%20GS%20.jpg

Close-up, squares are ~0.75":
http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/La
p%20Close-up%20.jpg

Ronchi-grams start outside of focus and move toward mirror:
http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/ou
tside%20focus%201%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/ou
tside%20focus%202%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/ou
tside%20focus%203%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/At
%20Focus%204%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/In
side%20Focus%205%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/In
side%20Focus%206%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/In
side%20Focus%207%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/In
side%20Focus%208%20GS%20.jpg 

http://www.atmlist.net/contrib/tpolk-at-decatur-al-dot-gov/Tom%20Polk/In
side%20Focus%209%20GS%20.jpg 


Thanks!!
Tom Polk
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