[Jason D (Jason)]

Originally Posted by MrB

While I had the primary+cell removed, thought I would check the centre spot.... yes, It's a long way out

How far off? This could definitely impact the quality of your view.

Originally Posted by clive milne

Zero effect on collimation but a significant effect on field illumination.

Clive, I disagree. Moving the secondary mirror by 5mm towards or away from the focuser has little to no impact on the quality of the image including field illumination. This can’t be the source of the OP’s issue. There are three potential impacts when moving the secondary mirror towards or away from the focuser:
1- Moving the focal plane within the focuser. Some EPs might not reach focus.
2- Accuracy of DSC (if present) will be impacted as the angle between the optical and OTA axes change
3- If the OTA opening is tight or if the OTA has baffles then vignetting might be a possibility

Originally Posted by MrB

I think it's the focuser now as I still can't nail collimation. I can get it very close, much closer than before, but no amount of fiddling will nail it. Still got some sleuthing to do.

What makes you believe you have a collimation issue? It is the quality of your image or is it based on the reflections you see via your Catseye collimation tools? Which Catseye tools do you have?

Jason