Well here are the results of my tests - the round one is the sun with a 200mm zoom, and a 2x magnifier and a solar filter. The second is the moon with the saame configuration apart from the solar filter.
You can see the focus isn't ideal, but provided it's sunny, I should be able to get a good enough shot of the diamond ring! I've got all my settings worked out, so no farting around at the crucial moment!
