I recently purchased a used Celestron 80mm guidescope with a focal length of 600mm. Normally I’d probably have gone for another ST80, but in fact I wanted this one to use with a slightly longer focal length OTA anyhow, so the extra focal length of the guidescope suited me.
I really couldn’t get it to work though. The guiding was utterly unreliable. It was only when I looked at the images from the guidescope in detail that I think I might have uncovered why:
That really doesn’t look good to me. I’m going to put it to one side for the time being and see what I can do to sort it out later.