Hi Greg,
Tonight we got a clear sky for some hours and I could do some test with a shim to get rid of the tilt. Not so easy because it isn't stable enough when the camera is balancing on a shim.
This is the data I got from my measurements:
http://www.astrofriend.eu/astronomy/...645-300ed.html
Now I know something about the error, the thickness and direction too I think. But clouds stopped me to proceed this night.
The center star is Mizar, the double star in Big Bear.
/Lars