Hi Wouter,
Yes, I know, and the fact that this mount works so well for you was a big factor for me getting it to replace my iOptron SmartEQ mount that I had been using with the same WhiteCat 250mm and 120mm/30mm ZWO guide scope. The weight of that scope is well within the limits of the SmartEQ (11lbs) and very easily within those of the CEM25P (27 lbs).
The guide scope is not the factor in the DEC deviation. If it were, it should show the same deviation in RA, however there the guiding is nearly perfect.
I suspect this is a mechanical problem in the DEC array only. When you look at the guide graph, these excessive deflections happen suddenly, then the guider sends repeated guide pulses to the mount that, however, have no effect whatsoever, until suddenly again, the scope swings over into the other direction. It looks to me that there is a lot of backlash, while a tension builds that, when passing a threshold, sends the mount into the opposite direction, where the same then plays out again.
I tightened the drive belt in DEC to the iOptron specifications (~3 mm play), but that had no effect at all. There is no other play I can detect when tightening the tension screw. The only thing left is the little gear wheel on the shaft of the stepper. If that is not centered and tightened, I think that could slip just enough to explain this behavior.
If anyone else has experience with that, please post here, before I tear the mount apart in vein.
Greatly appreciate any suggestions.
Jo
PS: I see the exact same behavior whether the mount is perfectly balanced or with an (intentional) slight imbalance in DEC to preload the gears and take out slack that way. Doesn't seem to make any difference whatsoever and I would not have expected it, since there is no discernible movement when I manually try to move the mount in DEC. No mechanical play whatsoever. That's why I think the problem is at the level of the force transfer from the stepper to the belt.