Dan,
I've done some quick searching on the web and have found this website which talks about
imaging the moon. However the part that really interested me was point 7 which states:
7. When photographing a full, or near full (gibbous) Moon, the problem is one of abundance of light. Using filters to reduce light and enhance contrast can dramatically improve pictures. However, cheap filters (and sometimes expensive ones) can cause a double-image from Moon-light bouncing off the front of the lens, onto the inner side of the filter and back down the lens, so some experimentation is necessary. (Photographing the Moon off-centre will more easily show up these reflections.)
I'm going to retry (when I get some time!) imaging the Horsehead but try and keep Alnitak well and truly out of the frame, although I feel it pumps out so much light it probably will still have an influence.
Cheers,
Mario