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Most likely the main mirror is not at right distance, try to move it a bit by adjusting colimation screws (of course, you will have to colimate the scope after this, pages 14 & 15 of user manual, attached).
You did not indicate in which direction you are moving focuser... knowing that will tell you in which direction the main mirror should be moved.
BYW, this barlow is most likely really bad, I think it is single negative lens (and probably made of plastic), so perhaps you should just forget about it.
Last edited by bojan; 19-09-2018 at 12:45 PM.
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