To the OP, I would say that one thing that can cause this condition is having a secondary mirror that is not centered or if there is tilt in the system.
To check this condition, use a star at high power and defocus using counterclockwise. You want to defocus only about 3 or 4 waves and note the position of the tiny point of light at the center of the secondary shadow.
If the scope looks to be in collimation and this spot is not exactly centered inside the secondary shadow when it is defocused, this usually means a bit of decentering.
Now this important message. Generally, this does not matter if the scope is not used with a lot of different accessories with different amounts of flange to focal plane distance. If you are always using the same diagonal, then a tiny amount of decentring is not going to have any effect as long as the collimation is good when the scope is at best focus! See, in normal use, the scope is not used out of focus, and if the scope is in perfect collimation when it is in perfect focus, then just don't worry about it.
Defocused testing is only useful for rough collmation. It is not really precise to try to collimate using lots of defocus. Once the rough collimation is complete, the fine collmation is done on an in-focus star. As long as you show good collimation when in focus with the eyepeices you use, then you can ignore a slightly decentered secondary.
If on the other hand, you have configurations that change the back spacing (going from binoviewers to mono-viewing), if any change in collimation on an in focus star is noted, then this condition probably should be addressed.
Mostly though, it can be ignored as long as when in focus, you see a Spurious disk with an even first diffraction ring. If you can see that, ignore the fact that there is a slight asymmetry to the pattern inside and outside of focus (not centered Poission Point) and fine collimation should always be done with a perfectly focused star.
Edited by Eddgie, 03 November 2019 - 09:49 AM.