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As well as all the other advice in here, have a look at how the secondary is mounted. If it is held on using silicone I would suspect your ‘stig may be caused by a strained secondary. Also, what mount have you got it on, and have you tested how stable the collimation is as you point the scope in different parts of the sky?
Lots of things to try, and although f4 is fast, a barlowed laser collimator should be perfectly adequate in your scope.
Cheers
Andrew.
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