Thank you for all you comments and helpful suggestions, they are much appreciated. The M22 image was a means to an end, in my search for the holy grail of round stars at longer focal lengths, so at this stage, I’m not too concerned about its shortcomings.
The M180 F12 is a Dall-Kirkham Cassegrain and one of the characteristics of this design is that it produces off axis coma. I think that it is generally accepted that D-K’s are best for Lunar, Planetary and double star work. Even with the x0.8 Reducer/Flattener, it remains a painfully slow F9.6 system. With my ST7 chip at 6.9x4.5mm (765x512 pixels) off axis coma is not a problem; it only becomes noticeable with the much larger 22x15mm chip (3888x2592) of the 40D.
Mating the 40D to the Mewlon has certainly provided me with a real insight into the astonishing difference in Quantum Efficiency between my SBIG ST7 and the non-modified Canon 40D, especially on faint objects. The ST7 produces a very recognisable image of the M16 pillars in less than 10 seconds whereas the 40D struggles even at exposures of 180 seconds, a huge difference!
My experiments have also given me a much greater appreciation and insight into the quality of the recent work posted by Barb and Dave using their C9.25 at F6.3 and standard Canon 40D. I have a long way to go!
I am happy to be making progress and each challenge that I overcome, leads to another one to grapple with. I can see that gradients and stellar profiles are fast working their way up the list! One day, I might even post some images I am happy with!
Cheers
Dennis