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Dumb! Did the aforesaid 3 hours smoothing on the 20" without rotating the mirror on the turntable. Even when testing I would pick it up, place it on the test frame, plonk it back on the turntable and carry on never assuming for a minute I was doing a bad thing, until it was too late. Now I have polished in some astigmatism! The book blames the turntable. I'll have to strengthen it a bit and put in a 4th jockey wheel. Probably the 380mm x 40mm concrete tool didn't help. It's already 3 layers of 12mm marine ply so you would think it was ok.
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