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Astigmatism On Rough Grinding

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Posted by Rasim Ramadan on November 6, 2006 07:31:37 UTC

Good morning good people,
1st of all I would like to thank you for your wonderful ATM site, which is indeed one of the best on the net, I have read many articles from different websites it was good but yours truly excellent in details that make the novice ATMr find how to start (like me).
I have one Q. if you have time to help me get an answer:
• I start hogging out my 15 inches mirror and 19mm thickness, using Chordal stroke, my target seggita is 3.985 mm with f/6, now I’ve reached 3.10 mm depth, and the curve spreads out to cover 75% of the mirror diameter.
• The problem: when I measure (using dial indicator) the curve depth 10 cm far from mirror center all around I found that the depth varies from point to point along this circle which has a radii of 10 cm inside the mirror curve, the variation is about 0.10 mm in average, I think that this is Astigmatism while I am still on the rough grinding and I didn’t spread my curve yet to cover 100% of my mirror diameter which is about 38.5 cm (approx 15”).
• Possible causes, my tiles tool back is not 100% flat when I use the level but the mirror is flat, when I measure the tool thickness from west > east by 1 cm, the level reads the air bobble to the edge off its vertical two lines, small but can be clear, (note I didn’t work TOT yet)
• Q. How I can correct this problem before it is too late?

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