PHD guiding question

Don't know if anyone has had this happen, but when using PHD guiding, I have recently experienced a net "drift". PHD says everything is just fine, but there is a noticeable long term steady drift of the image on the camera. Probably less than an arc minute over an hour, but that is still a lot of arc seconds. As the set-up gets more settled, this is just one more thing to track down. Could this be flexing in the mount between the main and the guide scope? I guess I could check how well they are aligned from one side of the sky to the other? Do I just need to re-calibrate? Any other ideas?

Thanks,
Charlie

Charlie,
Is it the object image or the guide star image that gradually shifts? Assuming the guide star stays put and it is the object image shifting, I would say **dollars to donuts** that it is differential flexure between the 2 OTA's. Even if one of the scopes is an SCT, it is unlikely a mirror shift as these tend to be inconsistent and sudden.

My best results with diff flexure is to balance both OTA's for the center point of the ring or rail mounting system. This is different from DEC and RA balance on a GEM. Sure, you have to have your GEM balanced. But you can have a perfectly balanced GEM and your OTA's can be way out of balance to the center of the ring/rail mount. Be sure the balance includes cameras and make sure the focusers aren't flexing either.

my 2 cents worth.