[Moon (James)]

I'm pretty sure the guide scope wasn't too far away from the main scope.

It's not the distance between them, it's where they are pointing that’s important. And even then, where the selected guide star is in the FOV.

I made sure there was no flex, but I didn't tighten the guide scope screws too tight. So that could have contributed.

There is always some flex, even if you can't see it. Thermal expansion is enough, for example. Everything can bend to some degree. You just might not be able to measure it.

If polar misalignment is unlikely to be the cause for field rotation, I'm back to square one.

Polar misalignment is the cause of field rotation. The images will look worse the further away the guide star is from the centre of your image.

can anyone explain how field rotation actually happens with an eq aligned mount?

Your mount is never 100% perfectly polar aligned, you want the alignment error small enough that it doesn't impact your images for the exposure duration you are using.


Hope this helps a bit...

James