C8 collimation issues

[moriniboy]

Whilst carrying out a star test I have noticed with a friends C8 that one side of focus appears to be collimated and the other side of focus its off, is this a common problem?

Is his best option to collimate as an average so its slightly off either side?

 

 

[Peter Drew]

This is uually caused by movement in the focusing system. To get round this, defocus in the direction that "lifts" the mirror, collimate in this position and then subsequently always bring the telescope to focus from this direction. 

[faulksy]

or fit a external focuser then colimate and leave the mirror focuser alone