Thanks for the comments Roland and Howard.
What do you think I am seeing when the position of my images is shifting over an extended period ? Friday night I was taking images of M51 over a period of about 5 hours. I took the first image at about 10:30 pm when it was .just on the west side of the meridian. The last image was at 3:30 am. I plate solved the first and last images and measured the difference in the coordinates at the center of the two images.
The RA had shifted 7.91" and the Dec had shifted 25.5"
Can this be explained by atmospheric refraction ?