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Another cause of `blackouts' is using a large exit pupil and the secondary mirror obstruction is actually blocking your eye pupil. If you use an 8mm pupil in a light polluted sky and your real eye pupil is only 4mm and the secondary obstruction is 33% ( the dark disc being 3.75mm diameter ) then you are only getting to see a sliver of your mirror as it is mostly obstructed by your eye pupil and the inner part by the central obstruction.
Just a reminder for the beginners that exit pupil is calculated by the aperture in mm divided by the magnification.
Most appraisals of off axis performance of eyepieces used that deliver exit pupils above what your eye can handle are actually apprasials of performance at slower ratios. If your eye can only take in half the delivered pupil then your scope is effectively stopped down to twice the F # . ie a an 8" F4 scope with half the pupil admitted is actually behaving as an 4" F8 - so you would expect some increase in edge sharpness despite the very large pupil coming from the eyepiece.
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