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The optical quality of the later 4" scopes is variable. I have one that is superb, and delivers images nearly as good as my Tak. The other had almost no anti-reflective coatings between the crown and the flint, causing intolerable ghosting on anything brighter than 2nd mag stars - sounds a bit like what you're describing. I spent a fortune getting it recoated and got rid of the ghosting, but it still has a bit more spherical abberation than it should. Unitrons have adjustable collimation, and as such, it also goes out of collimation relatively easily, so you need to ensure your focuser is square to the objective with a laser or sight tube, and check the collimation using a cheshire eyepiece. Still, at f15 they're fairly tolerant of collimation errors.
Cheers,
Andrew.
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