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Atmospheric dispersion/refraction
Bad Bayer matrix in the camera
Problems with the orthogonality of the reducer/gflattener (which would not surprise me judging by the bodgy fitting that was supplied to you)
If that's the corners, you either have a spacing issue, the reducer/flattener is not orthogonal as discussed, or your tracking is not working (but that would show in all corners etc)
I think it will be somewhere in the focuser side of things for sure
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