[stephen2615 (Stephen)]

Thanks for the response.

I have the correct back focus for both cameras. I have to have 55 mm.

The 16200A CFW7 with the OAG M = 55 mm (54.5 to 55.5 mm from QHY). I have to screw the OAG directly onto the flattener/reducer. A filter in the filterwheel should give me about one mm gain in the back focus. I measured the distance from the 16200A filter wheel cover plate and because of the thread distance on the reducer, my back focus distance is 56.7 mm. I think with the filter, it takes it down to about 56 so it is one mm too long. I am pretty sure that one mm out with back focus is not going to stop the camera from focusing.

The 163C has 55 mm back focus by way of spacers and the OAG. So both have (almost) the correct back focus.

As far as the focuser goes, 92,000 has a huge blob of Acrux, while 100 has a moon sized blob of Acrux. The focuser travel distance is about 90 mm.

I am somewhat gobsmacked at this problem.