I have started astrophotgraphing. My primary lense I use is Sigma 70-200mm F2.8 APO with Nikon D7100 and Sigma 2X TC. I also use Astrotrack without guidance.
I have gotten rid of the coma by making the A 3-4 steps bigger ie from 5.6 --> 9.0. But I am still getting the stars on the side of the picture with short trails. Or in the other words they are not round anymore. But that starts allready quite near the center.
I know the Astrotack is perfectly aligned to the polar star as the stars in the middle of the picture are still round even with 3 mins exposure. I have also taken many pictures trying to exclude the wind factor and other things that might 'shake' the camera. Also using remote shutter and 'Mup'.
Am I just experiencing a field distortion? Or could it be something else? And any idea how to get rid of it (using DSS to stack my pics)?