Jove, JA and Bojan,
Thanks very much for your replies! (Being a relative newcomer to this hobby, and only having a refractor 'scope, my apologies for using all the wrong terminology. These reflecting telescopes are all dark magic to me at the moment..)
This is what I love about this forum - so many talented people happy to share their knowledge and experience.
Jove - it was a crazy impulse buy that may cause me to regret it. We'll see. If I can't get the optics de-coma'd then it will be relegated to terrestrial imaging when I need a super-long focal length. (My partner loves whale watching, so I can see it being used for that.)
Bojan - thanks for the links to your thread about your Russian lenses. I'm off to read them now and see how well they translate to my "Gyoza Cannon" lens. (Well, it's Japanese, and Yum-Cha is Chinese, so I guess a Japanese dumpling is close enough for government work....)
I don't think the tube is being pinched because it uses a tripod foot which originally had a single 1/4-20 thread in it. I drilled and tapped a second 1/4-20 thread in it and then used two bolts to hold it to a dovetail. I didn't want it to suddenly pivot on that single bolt!
I had a look around for tube rings, but didn't find anything that was the right size. Also, there's no easy way to use two rings on it - the entire lens body (aside from the part with the lens collar on it) moves forward / backward as the focus changes.
Anyway, I'll have a poke around and see what I can find.
Thanks again for the information!
Cheers,
V