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Nigel it helps to have a bit of an understanding of what alignment steps are the more critical ones.
Errors in centering the secondary under the focuser will result in uneven edge illumination at the focal place. Not critical and you may not notice any roll off in brightness at the edges.
Error in centering sight tube cross hairs over primary centre spot (secondary tilt and aiming). This results in a tilted focal plane at the eyepiece. Small errors within a few mm are ok.
Error in aligning primary centre spot to illuminated cheshire ring. This is the one you want to get as close as possible for good results.
For astrophotography all of the tolerances become a lot tighter for best results.
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