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OK, here is what I am on about - but we'd better start from the premise that I don't know nothin!
I often see mention of poor results if a laser collimator is hitting the primary even a small amount (less than a mm) off mirror centre (not barlowed laser). I have seen mention of finding that the spotted centre was not the centre on some mirrors. Recently I was washing a primary and wondered what to do if the centre "doughnut" came off in the process (It didn't).
First image (sorry - scanner to server to mailbox has died? so back to trusty mobile phone!) is meant to be the centre cross section of a super accurate mirror - axis of parabola = physical axis of mirror. Second image is a wonky mirror - axis of parabola is offset compared to the physical axis of the mirror - but the parabolic surface is all present. Third image is a wonky mirror where the parobola is deeper so there is no top flat surface and the LHS height is less than the RHS height - so part of the parabolic surface is missing.
All spoken by someone who clearly knows nothing about grinding mirrors! Feel free to set me right.
Now the images are a gross exaggeration and that may the only issue - such exaggeration is just unreal - the axis of the parabola will always be close enough to the axis of the mirror.
I guess both wonky versions would produce readily seen gross problems optically?
So is worrying whether the axis of the parabola is coincident with the axis of the physical mirror an issue? Can we happily use the brown paper folded into quadrants to locate the centre spot?
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