If your guiding seems to be behaving a little worse than you expect, consider this embarrassing anecdote:
For the last few months, I have been struggling with pretty bad guiding from my EQ8-R mount. When I first got it, I was able to get about 0.7″ RMS from it, give or take the weather. Recently, I was lucky to get under maybe 1.3″ RMS. It would regularly oscillate between 1″ and 2″, no matter what I did with the gear mesh, balancing, or PHD2 settings. I would watch the close-up of the guide star, and note how the calculated centroid would tend to move around the star seemingly randomly. This would convince PHD2 that the guide star was farther away from the lock point than it might have actually been.
The other night, I took the guide camera off to use for some planetary work. When I put it back on the guide scope, I decided to try mounting it using the C-threads instead of the 1.25″ nosepiece. Doing this required me to refocus. I used SharpCap to get a good focus on the guide scope, and then I recalibrated PHD2 and started guiding. To my surprise, guiding immediately settled at a consistent 0.6″-0.7″ RMS.
So, the moral of the story is ensure your guide scope is well focused!
