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Let me throw up a question I have thought about before, that maybe the mirror makers can answer. Presuming the mirror is a pretty good circle, is the bottom or lowest point (for want of the right word) of the parabola absolutely going to be in the physical centre of the mirror. In theory, it should be and that's where the paper template method and the ruler method expects it to be, but what is the chance (say in these machine manufactured mirrors) that it is actually a bit/lot off centre from the physical centre of the circle?
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