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I use the 1.4x occasionally with my 200mm f2.8L lens, but I've given up using the 2x converter at all. A really good quality lens can do ok but even a decent lens with that same higher native focal length will almost always perform better.
I have (on the moon) used the 1.4x with my 530mm f3.3 Tak Epsilon. It does yield greater resolution than without it, but you'd have to be desperate to use a TC with a fast Newt.. it's like the worst of both worlds. Your f4 scope would become f5.6. You'd be better off with a cheap 80mm refractor and reducer/flattener.
In short, I also really favour longer native focal length.
Phil
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