Am I experiencing field distortion while astro photographing when stars on the edges give trails?

Question

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)?

Thanks!

Answer 1

Field distortion would be an overall blur nearer the edges rather than anything you could conceivably call a trail; the stars would simply be fuzzier round blobs.

You are using the equivalent of a 600mm lens (200mm times 1.5 (crop factor) times 2 (doubler)). That means that an arc-second is about half a pixel on your 24MP sensor. (The vertical field of view of a 600mm lens is 2°20', and you have 4000 vertical pixels.) The tracking unit you are using claims a typical peak-to-peak tracking error of 5 arc-seconds over a five minute period. That, in turn, means that you should expect about a 2-3 pixel tangential smearing with exposures in the range you're working at. Given that stars are still awfully small at 600mm (35mm equivalent), 2-3 pixels represents considerable distortion. To get better than that, you'd need to either shorten your exposures (by raising your ISO) or move up to a more sophisticated tracking system, perhaps one that uses real-time imaging of the pole star for self-correction (which will not be cheap).