You are seeing exaggerated errors I believe when you move the camera away from the SCP. As a test I mounted my 450D using the 18-55 lens at 20mm unguided on a tripod with 30sec exposures the other night. At the SCP the stars are pretty round, but on Orion setting in the west the stars are 1cm long lines!
This page I have found to be an excellent description
http://www.astrophotoinsight.com/pub...d-drift-method
but swap south for north.
Mark.