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I agree with the field rotation diagnosis, but also agree it's likely the reducer would be introducing some distortion, but i'd expect it to be making stars ilongated from the centre out, as other have said.
If you try with & without the focal reducer I would expect even if it's field rotation it will look less noticable without the focal reducer because you'll be looking at a narrower FOV. I might be wrong, but that's what I'd expect.
Roger.
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