Thank you for your nice words. I have been quite surprised at how (relatively) flat the field of view is on such a large image sensor, 22.2mm x 14.8 mm according to the Canon manual.
The Tak Mewlon 180 F12 is a
Dall-Kirkham design and it is written that this design of Cassegrain suffers from noticeable
off-axis coma, with only a small central region of the FOV being corrected.
Although I have noticed comatic stars at the edge of the field of view, and softness at the edges on the full frame Moon image, it is not as bad as I had expected, although the Canon 40D is not a full frame sensor like the 5D and 35mm film cameras.
Cheers
Dennis