artifacts - Where does this reflection come from in a long exposure shot?

Using a long (10 second) exposure I found I was getting an odd "reflection" in the picture, where the brightest object leaves a residue in the opposite part of the frame.

You can see it here:
_{(source: alastairc.ac)}

I was still getting it in 4/5 second exposures as well. NB: Using Canon 550d with a 11-16mm F2.8 Tokina lens, at 16mm.

I assume it is some physical thing like an internal mirror or lens thing, but does anyone know for sure?

Answer

This isn't uncommon to see when you have a UV filter attached to the lens which, generally, a lot of people do because it gets recommended by the camera store as lens protection. If you want a really detailed explanation, there's one on Luminous Landscape showing and explaining the issue.

My take, and it's a personal opinion, is to lose the UV filter if you have one. The "protection" offered is usually minimal, at best, and the impact to certain images is not. So, to my way of thinking, the loss outweighs the gain.

Now, all of that assumes you have a filter on your lens... Looks like it, but you didn't say.