Hard Boyled

Hi all, First post on here so I'll try and keep it brief. I recently purchased the SW 72ED scope and bought the recommended OVL field flattener with it. Unfortunately when I take images with my dslr (canon eos 800d) there is some distortion in the stars at all edges of the frame, they look elongated like they're stretched away from the centre towards the corners. I was wondering if others had this problem and how they fixed it. My guess is if I purchase the adapter to screw the field flattener into the focussing train it may help and also adding a few mm to the spacing between the camera and field flattener might fix it, but I'm not sure. Does anyone know what the official back focus of the OVL field flattener is? I can't find it anywhere online. Any help would be greatly appreciated, Cheers