Semicircles in photos

[batema (Mark)]

Hi all,
Last night on the SunshineCoast was fine. I thought I would post these images for feedback because it is not the first time I have seen this semicircular thing in photos taken using my Canon 400D and only a week ago it cropped up in my mates 350D as well. It may be a simple explanation that you have come across.
Also I thought I would try to do the horsehead even though Orion was very low on the western horizon. The second shot is cropped off the first and even though the result looks a bit ordinary I'm thrilled. I added a1 flat(white?) to the processing taking a 1 sec exposure in the middle of the day with a white shirt over ED80?????? 4X7min exp 400 with 2X7min darks plus one flat. Sombrero was 6X7min exp400 with 2X7min darks. Any feedback about getting rid of those edges would be appreciated.

Mark B

[[1ponders] (Paul)]

That's an internal reflection Mark. Don't know what it's off though. It could be simply from the bright stars in the image. What other shots has it come up in?

[batema (Mark)]

Hi Paul, They have also cropped up in some Eta shots that have been stacked and processed. Given taken from my back yard could it be a stray light????? I have included the shots. Mark