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One thing worth noting in the quest for perfection is that the centre of the paraboloid is unlikely to be the exact geometric centre of the mirror. When I picked up my 12.5" conical mirror from Mark last year he mentioned that the centre of the paraboloid will "wander" around by a few millimetres in the final stages of figuring. I would suppose that is why final collimation on a star is recommended (although seeing conditions usually make it difficult to achieve this!).
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