We have astro coming up for next month's photography club comp. One of our members shot the stars with his D810 Nikon 14-24 at f2.8 ISO 1600 20 secs. Every star has a black dot in the middle. Very strange and I've never seen anything like it. The stars are sharp so it doesn't appear to be a focus problem. I've only seen a crop of one section but every star has this black dot in the middle. Any ideas what would cause that?