Kevin, must have been the random sub I looked at then
(I picked only one)
If you're seeing elongation towards the edge I'd try varying the spacing from your flattener to the camera, that's usually the cause. Refractors don't usually suffer with coma.
The Tarantula is another target with quite high dynamic range, but is a bright target. Pick your poison
It's well placed at the moment which makes it popular.
Adding an extension tube can be enough to upset the balance unfortunately, as it pushes the weight of the camera further out. It might not be much, but it's worth checking.
Sounds like you've got some interesting cameras to experiment with
