Ckr

I don't believe the 55mm comes into play here as I'M NOT USING A REDUCER/FLATTENER/CORRECTOR. If back focus is the distance from the last optical component to the image plane, then my objective lens back focus is 420mm and my telescope length is about 305mm at mid focuser travel, leaving 115mm between my image plane and the end of the draw tube. Subtract the camera ffd (6.5mm) and add for insertion into eyepiece holder (20ish mm)

I just got the same scope and couldn't find focus with my ASI294mc pro last night. It's my understanding that "back focus" applies only when using reducers/flatteners/correctors, and remains fixed for that reducer/camera combination. Please correct me if I'm wrong. What I'm missing is "out" focus. I'm going to set up (in daylight) and add extensions 'til I reach focus in the mid range of drawtube travel. I have found that it's easier in daylight, after finding a distant target, to assemble, adjust, and note (WRITE DOWN!) distances/extensions required to reach focus for all combinations of camera, scope, reducer, diagonal etc. that I expect to use. So, with the SW 72ED focal length of 420mm and the distance from the lens to back of drawtube (at center of drawtube travel) being around 305mm, 420 - 305 - 6.5mm (ASI294 flange focal distance) = 108.5mm approx. extension required, plus insertion distance, call it 130mm (deja vu). At least on paper, with a broken measuring tape, in dim light with bad eyes, no glasses, and a headache. We'll see. Good luck!