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On CN I got this reply which reinforces that everything should go back exactly the way it was originally set out by the scope maker for each scope. Tilt I can measure. I quote:
I've done a fair amount of analysis of the C14HD with regard to centering the corrector/secondary assembly. It turns out that the system is fairly insensitive to centering. On the C14HD systems getting the whole assembly centered to within +/-2 mm is plenty good enough. I don't have any data on the 8" systems but I suspect that they behave in a similar way and if you have a non-HD system, the centering tolerance is sure to be even less restrictive. That 0.5 mm centering offset will have absolutely no effect on your imaging performance. The key parameter to pay attention to is secondary tilt. The system is VERY sensitive to small alignment errors in the secondary and that's where you should spend all of your effort to make it as perfect as you can get it. Even a tiny error can have a significant impact on system Strehl and MTF performance.
Given the way that the secondary is mounted, it's a bit of a tricky calculation but I've been meaning to compute the sensitivity of the system to secondary tilt one of these days. I'll post the results if (and when) I ever get around to it.
John
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