Improving Tracking
Welcome to the imperfect world of autoguiding and periodic error correction.
As telescopes get longer and as exposure times lengthen too (often as a result of slower optics) the key to quality imaging is improved tracking. Even with a perfect mechanical mount (which almost exist but are ludicrously expensive), perfect tracking is not guaranteed. Atmospheric refraction, minute errors in polar alignment and general flexure in the metalwork conspire to introduce small tracking errors.
In the short exposures we have been using up to now, these tracking errors have not been intrusive. The higher magnifications and longer exposure times make the effect of tracking errors more obvious in the final image. To some extent, many error ...
