Fiddling with the scope after an interesting evening playing with different tracking rates. I used PHD2 to track at .5 seconds and made a PEC graph. Then I proceeded to use the comet tracking rate to trim to eliminate RA drift. The final rate that I used was 448925. This rate mate the RA curve nearly flat (aside from the remaining PE) over a long period of time.
I've eliminated polar alignment as the issue, as I was able to use PHD2 to get polar alignment to less than 30-40 arc seconds. When stacking the pictures in DSS, I see no rotation of the field. Nearly all movement is in Y (RA).
Was looking at the dovetail plates and considering what I'd learned from the spreadsheet of flexure. It showed that the sag was occurring strongest at vertical. Thus, it had to be from the rings sagging. Wen I consider the earlier tests, I think that things may have been complicated by other factors. Again this is like peeling an onion.
So I looked again at the rings. I remember that I'd considered putting the rings as far apart as I could. I noticed that the wide d plate was actually standing off from the backside ring because of an impact with a finder base. I figured that maybe I'd try putting a narrow dovetail on the ring set.
When I removed the wide dovetail, I was surprised that the rear ring was loose! It felt tight so long as the wide dovetail was in place, but as soon as it was removed, the clamp wasn't finger tight and I could swivel the ring on the mounting bolt. This was something surprising to me and convinced that I needed to pull the ring set and use the proper holes, even though they're close together. The camera remains supported by its own dovetail, so focuser sag should not be an issue.
Several minutes later and the rings are back in place, firmly seated in their original holes. These will not move now.
I'll check again tonight and see what I learn. Of course, I'll have to recenter the guide and camera. I may need to rebuild a model, but that's pretty easy.
Hopefully this will fix the remaining issues. Sure would be nice to have the problem solved.
If this finally works, then I can get back to testing if the system will function OK without the camera dovetail in place. That will be important when I go to a CCD based system. Also, I'm interested in testing the reducer/flattener for the SV70ED.
Finally, there's a power supply annoyance. I'm finding that the power splitters I have been depending on to provide a distribution of 12V are flakey. Getting some intermittent connection issues. I believe that over time they'll get better, but it's still annoying that the device is not connecting as good as it should. I could dedicate a 2 way or 3 way plug to do the job if needed.