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Andrew,
Possibly a long shot idea, but have you measured the focuser trueness to the tube. I only suggest the possibility because I had issues collimating a vixen 5" f5, turned out it came collimated with a 5mm positive offset partly on account of the focuser being a touch skewiff, I shimmed under it and that made life a lot easier. Your f4 is going to be a lot harder on the mechanics of an ota, and although the machined focuser should more or less fit perfectly angled onto the tube. I wonder how little a misalignment it would take to throw everything else out at a fast f4? Apologies if you have covered this idea.
Steve.
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