The best way to troubleshoot elongation in RA is to record unguided tracking error over a number of worm cycles. PEMPro, if you use it, can do that easily, or you can set PHD2 to guide on one or multiple stars, but not send corrections to the mount. The data can then show clearly if the error is random, sudden, repeatable/periodic, or some sort of drift. And if this data shows no obvious error that could explain the elongation, then it may be some differential or other type of flexure. Could also be due to guiding on a star that's far removed from the main imager center FOV in the presence of some polar misalignment. This can result in slight arcs around the guide star, stretching the stars a bit.?
Regards,
? ? -Paul
Paul Kanevsky