that was the source of my problems that i had for close on 6 months. i thought it was the miss alignment of focuser/primary axis, a chat to a few members saw my problem as astigmatism. I found the problem last weekend...
I set the mirror clips just above the mirror.. all good.
installed the mirror to the OTA, went on my business to collimation.
collimation complete then I'd go for a star test/view... everything was dead. and i know how to collimate a newtonian, done it more times than i could poke a pointy stick at successfully....
what was happening as i would tighten the lock nuts to hold collimation the lock nuts would press onto the mirror cell, from that the mirror cell warped just enough to push the floating mirror into the mirror clips and causing the primary to pinch, of coarse when i pulled it apart to check! no problems as it returned to what it was previously.
hence ill get a decent mirror cell to hold the mirror better, and have increased accuracy in collimation after all with photography you need a whole new level of accuracy
hope that helps troy