I haven't seen this topic discussed in existing threads. The problem I am seeing is that, shot-to-shot, I see huge variations in the tracking accuracy. In the summary image attached, I'm taking consequetive 20 sec exposures with a ZenithStar61 360mm refractor. Each image of the 36 I took (of an area near M-51, which is currently near the zenith) shows a 200% crop of a region near the center of each image.
Because of a peculiarity in the way that photoshop brings in a group of images, the image start in the second column going down, then 3 column and so on. The final 6 exposures are in the first column.
As you can see, the tracking errors change drastically shot to shot.
Maybe this happens all the time, and is only noticible at longer FLs? I've gotten good results at 135 mm.
The focus seems to change, but I doubt that is what is actually happening, If I shoot the region about Polaris (astrotracer turned off), the star shapes a perfect and the smallest stars are about 1 pixel across.