Joshua, Greg - answers to your questions and more information:
Attached are snaps of my imaging rig. We have:
* FSQ106 -> Feather Touch Focuser -> mechanical adapters -> QSI683
Note: no reducer in image train
* Orion 80mm F5 guide scope -> ASI120 mini guide camera
* Parallax tube rings on both OTA and guide scope
* Skywatcher EQ8 equatorial mount
"So, all you did between these two image is a meridian flip?"
Yes
"To me, image ....23456 is much softer than the other, almost out of focus I would say, but I see there is only minutes between the images, same temperature and the same focus position."
I noticed this as well.
"Without shifting anything, can you take 2 or 3 images on a row of, say 60 sec exposure or less, pointing more or less east about 45 deg altitude, then flip the meridian and do the same pointing west, don't rotate the camera."
Worth a shot - will do.
"In your last post, you rotated 90 deg, is the rotator after the focuser?"
No, it isn't. See attached image of the focuser showing where the manual rotator (captains wheel) is on the Feather Touch focuser. The arrow is indicating the point of rotation. The only thing between there and the OTA is the adapter that screws on to the OTA tube.
"Are you using the silly Tak mounting clamshell?"
Yuk - no
. Parallax tube rings all round.
So the summary of all this is that the consensus seems to be that the issue is flexure and is not with the OTA. I still have an open mind on this and will continue to explore further. I would rather have the issue being flexure as this is something that is more under my control. However, the logic behind the camera rotation test indicates that the problem lies on the OTA side of the rotation point and I cannot ignore this.
Being new moon now of course the weather forecast for the next three days is overcast
so it may be a few days before I can get back on to this.
Thanks for all the contributions so far.