What's the deal with that bullseye in the middle of the picture? I went to shoot the northern lights and all of my images have that mess in them to some degree or another. I used the XTi's kit lens and a really good Canon 24-70 lens and it happened in both. I was shooting RAW, at a few different ISO levels, f-stops, and exposures and it occurred in every one. It's well above freezing outside so I don't think it's the cold. What gives?