[The_bluester (Paul)]

Having dipped my toe in to imaging with the C925, I have naturally arrived at a bit of a quandry. From what little I have been able to do so far the scope seems to perform fairly well. My biggest issue to date is sorting out spacing between the .63 reducer I have been using and the camera, at the moment the spacing is too great and stars at the edge of the field are not great, though they are better than they were previously as the original spacers I had set the distance even greater and severe coma was visible at the edge of the field.

Now, this is an SCT, and that means mirror flop so I was considering adding an external focusser to the train to at least prevent that from happening during focus changes, not much helping it under gravity as the pointing changes through the night, it is vanilla C925 so it does not have mirror locks. The problem is that if I fit an external focusser by just screwing it to the rear of the corrector, I may not be able to get the spacing right any more, and even if I can, changing focus will change the spacing and so the reduction.

Is this a non issue (Provided I am able to get the spacing right to begin with) in that focus movements should be so small as not to materially change the reduction offered by the reducer/corrector?