The back focus distance for the Celestron 0.63x reducer/corrector was probably intended to provide the correct working distance for a typical DSLR having a back focus distance of 45mm, a specific camera adapter length of 10mm, and a universal T-adapter length of 50 mm. For imaging purposes, having the correct working distance within a mm or 2 will not only provide the intended focal reduction, but also avoid image aberrations that the flattener is there for. For visual use, the working distance is not so critical.
The mount base interference issue is most difficult when trying to maintain a large light cone with a minimum of glass. This would entail using neither a star diagonal nor a 1.25-inch nosepiece for the camera. Using the Celestron T-adapter plus sufficient spacers can maintain that larger light cone all the way to the camera. For example, for the ASI 294MC Pro camera, I use the Celestron T-Adapter (50 mm length) + 21mm extender + 16.5 mm extender + 11 mm T2 extender ring + camera's 6.5 mm back focus to achieve the 105 mm working distance from the Celestron 0.63x reducer/flattener to the camera sensor. (All of the extenders come packaged with the camera. Only the T-adapter needs to be purchased separately.) Used with my Celestron Evolution 8, it will contact the mount at altitudes greater than about 60 degrees.
For imaging at higher altitudes, I have to use a star diagonal. That means the visual back has to be used also, and since I only have a 1.25-inch visual back, that immediately constricts the light cone. I can, however, completely mitigate the mount clearance issue if I mount the visual back to the reducer, then the star diagonal, then the necessary extenders, and then the camera. The star diagonal body itself is the lowest part of that imaging train, and if I move the telescope tube nearly all the way forward on the dovetail, I get unrestricted clearance. I use a Baader star diagonal from with the eyepiece mount can be removed such that my extenders and camera can be directly screwed onto the diagonal without the need to further reduce the light path by using the camera's nose piece. To achieve the correct spacing (or at least as close as I think I can get it since measuring the light path travel through the diagonal is not easy), I just use the 16.5 mm extension coupled to the 11 mm extension ring between the star diagonal and the camera.
Edit: I looked up the length of the light path through the Baader diagonal on the Baader website. It is 35 mm just for the diagonal (i.e., no nosepiece or eyepiece holder). The length of the visual back to the start of the diagonal is also 35 mm. Combined with the 16.5 mm spacer and the 11 mm extension ring, I get 104 mm working distance. Close enough!
Alternatively, if it's an object I don't really need the reducer for, I can mount the camera (with the 11 mm T-extender ring) directly to the T-adapter and get very close to the zenith before hitting the mount.
I don't know how much shorter the uncooled version of the 294 camera is, so I don't know what impact that would have on the mount obstruction issue. I do have an uncooled 224MC camera. While I have only used it without the reducer (either with a barlow or nothing) for planetary imaging, if I were to use it with the reducer, I can get almost to the zenith without hitting the mount even at the 105 mm working distance from the reducer. The 224MC uncooled camera back focus is 12.5 mm behind the front of the camera, so with the Celestron T-adapter, it needs 105 - 50 - 12.5 = 42.5 mm of additional extension. I can use the 21mm and 16.5 mm from the 294MC Pro kit plus a 5 mm extension I bought separately.
Hope this helps. Yes, mount obstruction can be an issue, but there are potential mitigations without buying a different mount or optical tube assembly.
Edited by donstim, 13 September 2019 - 12:29 AM.