One thing I have noticed with scopes that experience mirror shift is collimation changes when the focus is adjusted and the primary shifts position. Major pain in the bum.
Geoff White told me of a simple technique that deals with mirror shift when focusing, is to always wind out in the one direction and then adjust focus in the opposite direction. This restores the primary mirror back into the position that had the target centered.
This same technique applies for collimation - you wind out a bit further than you would for the focusing proceedure to get a larger doughnut. You then wind in to reduce the size of the doughnut and this also sets the primary mirror back into its optimal focused position.this way when you focus, the mirror shift both recenters the object and collimation is also at its optimal.
Checking collimation also allows you to check for astigmatism in the optics. Rather than explain everything again, have a read through this following IIS link:
WARNING: Scope assembly practices may be vexing many SCT's and Maks
Alex.