I am getting through the learning curve of a used C14 edge HD (with Celestron 0.7 reducer) that I picked up recently. So Celestron publishes this 'Nominal Back focus' (NBF) for the C14 (and same for C11) of 146mm from the rear flange of the OTA. General interpretation of this is that at this focus position the movable primary is in the optimum position (least aberrations) with resp. to all the other optical components (of which in the edge hd there are plenty).
So I cobbled together extension tubes and eyepiece to put the EP field stop right at these 146mm then focused the scope and admired the sharp image.
I unmounted the extension without changing it, added the reducer which (interestingly) Celestron says has the same NBF of 146mm, added my eyepiece holder and looked. The focus was way of. (about 1-2inch). So I am adjusting the scope focuser until I have a sharp image in the EP again (now with reducer and my extensions in the optical train).
Asking myself this. When w/o the reducer the optical parts where at their perfect design position, then they can't have been with the reducer installed. So the OTA will have introduced aberrations. Now should that lead to the conclusion that the reducer not only reduces the FL but also un-does the just introduced aberrations? Or does Celestron simply ignore that as the refocus is assumed small (is 2inches small?) and the change in aberrations can be neglected?
What's the verdict of the experts?