Hi,
I just got a Canon 50D and last night I tried a bit of astrophotography. It was too cold so I finally packed up. Afterwards, when I attempted to take terrestrial photographs I noted that they had a dark circle. I tried taking a couple of shots and the circle usually remains in the same place in one or photos is slightly to the bottom.
I'm not sure why this is any ideas, I might technical support on Monday.
See the following example
The black line points to the circle