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I am guessing I understand what you are saying.
The question is why there is astigmatism... Assuming you rotate lense and direction of stroke with regards to the tool then wedge in the tool should not matter. It is important that the tool does not flex because of an uneven work surface. Also the mirror is too thin to block. If you do block it then only with a small backing as the glass could be drawn into astigmatism.
The anneal could also be bad but if the problem orginated with an uneven blank then it should grind out fine. Working around a barrel I would:
push push push rotate work cw to the right push push push step cw to the left and repeat. This way the tool/lense rotated about twice + for each trip around the barrel.
-Rick |