I doubt any of these will solve the problem because I think the magnification on all counts is just too low. Assuming this 12.5" Discovery Dob is f/5 and given that you seem to favor lower powers than I would, I suggest an 8-9mm solution at the longest. Actually in my 10" f/5 my most used magnification for Jupiter is 244x, and there is an abundance of contrast at 181x, so much that I don't ever go lower than that on any planetary target. If your 2x Barlow is a high quality model, try using it with your 16mm T5 Nagler. It may vignette a little, but that is of no consequence.
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Mike
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Mike
