Thanks guys, it's such a balancing act between exposure and freezing the shutter. I could use a faster shutter if I took the 2x barlow out of the equation, and probably get more frames with the ISS in it as well, but I want the image scale too!
Vincent, you need to use the fastest framerate you can, with a fast shutter. I was using 60fps and 1/1250s exposure. The 900nc is definitely capable. Keep trying! Go to 30fps and use 1/1000s exposure or faster to stop the blurring.