Joe, done.. the central obstruction is umm .. interesting. What’s more there is an additional disk that fits on to increase the CO if desired near the moon (when did your hear of a cat that needs MORE CO ?!?). Focal length is 3200 - a tad longer than the MK91. Will be experimenting with the thermal aspects but I expect an insulated dewcap extending back to the rings should do well. The corrector is thick - quite a huge hunk of glass - so there is a considerable thermal mass right there. I have photos of it in pieces on the bench for final assembly.
Morton one has to understand the “use case scenarios”
The M615 is for the occasions when I don’t have room in the car for the cheesegrater, ie weekends away with the family. Wife likes to travel... often, and it works for me