Until now I have been imaging at ~0.40X using the diagonal between the camera and the focal reducer or at ~ 0.72x using the adapters and spacers supplied with the ASI294MC-Pro along with the SCT threaded coupler from the diagonal. To get to actual 0.63x, presumably the optimal focal reduction for best field flattening the spacing between the focal reducer and the sensor needs to be about 106mm based on the calculation found here.
Luck would have it I happened to have an eyepiece projection camera adapter tube of just the right length and threading for a back plate opening. Unfortunately, because clouds were moving in I only had time for one round of 35 x 15s shots with M42 still partly blocked by tree branches. Nonetheless the exercise was valuable. There's plenty vignetting that could be largely eliminated via flat frame correction. Also, because this was shot from my terrace tracking was sub-optimal despite the use of the auto-tracker.
Update: The camera adapter configuration in this above posting provides no provision for using a filter. Luckily, after a little experimentation I found a combination of adapters that can include a filter about halfway between the focal reducer and the sensor. In this configuration the sensor focal reducer spacing is about 110mm resulting in a 0.62x focal reduction. This setup still needs to be tested. Another M42 anyone? ;-)
