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hmm that sounds interesting, I'd try using the 6mm and 9mm without the barlow and see if you get the same effect.
Usually if the image is sharp in the middle and blurry at the edges, it's a result of either (a) field curvature or (b) coma or (c) bad correction at the edges for use in a fast focal ratio scope.
but i've you've tried refocusing I can't think of what it would be except something on the lens?
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