Differential flexure?

[boz_m (Alexander)]

I have a Celestron 8” EdgeHD and I am having trouble with my guiding. In PHD2 I get pretty good results (under 1 RMS error usually) but my subs move slightly frame to frame. Eg each 1 min sub moves consistently to the left. It does t rotate just slowly moves one direction.

The problem seems to be that my guide scope locks on fine but that doesn’t exactly align to the OTA. Is this differential flexure? I have heard long FL setups and SCTs can have this issue.

My guide scope has a pretty flimsy connection to the OTA and I can easily see movement if I give it a wiggle (not loose movement as the screws are tight, just not fully secure). If this is the issue would beefing up the connection fix the issue?

I want to avoid using an OAG. I tried it but Have issues then with the correct backfocus (I posted yesterday about this), interrupts the FOV too much and harder to get guide stars. Also my Guide camera has issues with spacing to get the same focus as the OTA. So if possible want to try to keep the guide scope rig

[Paul Haese]

It sounds more like mirror flop to me. Something I thought the edge scopes were immune from with the rear locks.

To test differential flexure take a series of images 1,2,5 and 10 minutes. If your stars get progressively elongated then that is differential flexure. Is the star are still round but the frame moves that is mirror flop.