I've a problem in getting round star even in 20 seconds of unguided exposure with my SkyWatcher AZ-EQ6 GT mount. What I've observed is that the stars do not form very long trails but they trail short distance on either sides (left side and right side). This happens even if tripod is bubble leveled, polar alignment is done precisely with QHYCCD Polemaster, balancing is done proper, no wind is present and multi-star alignment is done with EQMOD and Cartes du Ciel. That is to say that if the star if found round in first 20 second sub, then in the next 20 second sub it'll trail to left, then in the next 20 second sub it'll trail to right, then in the next 20 second sub it'll go back to almost a round star. After an hour all the stars will remain in the field of view but 2/3rd of the subs will have star trailing captured. This trailing does not happen in up and down directions. The scope I use is not too big and heavy. Its Explore Scientific ED80 Carbon Fiber Apo Triplet refractor and the camera is ZWO ASI 224MC. The field of view is 0.55° x 0.41° and the image scale is 1.51"/px.
Could this be a result of periodic error in the mount? If so then is it recommended to record and save the periodic error with EQMOD or use PemPro? I haven't tried PHD2 autoguiding because I'm still studying the native behavior of the mount. Do you think that autoguiding will be a better solution over periodic error correction?
Any input will be highly appreciated. Many thanks for your time in advance.