For a few years, I have been using the drift align method with a Canon DSLR, Bogen Tripod, Astrotrac, and Astrotrac wedge. Its always worked very well , but something has me a little confused. When I first line up close to the Celestial Equator, and Meridian, I typically run 8 minute exposures with a Canon 200MM. lens. Then after adjusting the wedge to the East or West to reduce Declination drift, I'm always left with a bit of East West trailing, which I had always been taught to ignore. Next, I adjust the mounts North South orientation, while still imaging on the Celestial equator, but now near the Eastern horizon. Strangely, after again correcting for Declination drift, I'm left with mostly round stars. The East West component of drift I was left with near the Meridian, has all but dissappeared near the horizon.
My question is. Since both adjustments are made along the celestial equator, why would the East West drift almost dissappear completely near the horizon, even before making any further adjustments, but remain obvious at or near the meridian? And this seems to happen every time I go through the procedure. My guess was some sort of flexure in the tripod, ballhead, or camera attachment, when lined up near the Meridian? Or am I missing something in the geometry, where East West trailing at the Meridian would typically be greater than near the Horizon? I didn't want to go through to much work finding the weak link if this is typical result?
Thank you in advance for any insights.