Originally posted by cam_abyss It is a rectilinear lens... and distortion is supposedly corrected for pretty well... though I'm guessing there still probably is some
The thing with wide rectilinear lenses are that they do distort, allot. They have to in order to be rectilinear. Everything gets enlarged at the sides and corners of the picture compared to the center. Take a picture of a group of people and those standing at the very edge of the frame will be partially enlarged and look, well weird.
This distorsion will make stars that move across the frame appear to move at different speeds, The stars in the corners of the lens will move faster then the stars in the center of the lens. Astro tracer can't compensate for this, the image sensor can only rotate/move and compensate for one speed, and that is the speed in the center of the frame.
To compensate fully the whole camera and lens will have to move so that the stars don't appear to move, so the stars in the corner stays in the corner of the lens.