Corina, you beat me to it. I think the 30 sec. exposure is the problem. The sensor will start to get warm and noise will rear it's ugly head. (even at minus 21 degrees) I would try an aperture more like f/5.6. This will make your exposure more like 8 sec. This should be well within the XT's capabilities. Also, try bracketing your exposure, like 4-8-16 sec. This will negate the need to lighten up the image in post. Hope this helps.