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judging from the size of your camera, a 72mm achromat is overkill. your lens diameter is like 40something mm's. so no, you'd waste your money on a 72mm achromat.
are you positive the fl is 77mm? that's a high powered achromat.
thats' called barrel distortion when the outer edges are all blury. that's just the type of achromat being used. you can have cheaply made glass as well making it difficult to focus.
i wouldn't use an achromat bigger than 50mm for you camera. stick with something around that size. to determine the best distance from you achromat to your GG is some math. for instance.
some of your problems sound like it could boil down to a bad lens. lens can create trouble as well.
An achromat at 77mm focal length would be 1000/77 equals power. Divide your Fl by 25.4. Having a 77fl achromat would equal to you having it at or around 3inches away.
1000/Focal length=Power
Focal length/25.4=equals distance.
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