Hi Mick. In my very limited time, ie; 1 day, as an 'Astrophotographer'?, I can only say that, when I was taking images last night & experimenting with aperture/shutter settings, I had the same 'shadow' effect when I didn't have enough aperture. I could actually see the 'shadow' disappearing when the shutter button was pressed 1/2 way, with diff. aperture settings.
When I got down to around 5.6/4?? etc, the 'shadow' was gone, & I got clear shots.
Does this sound right, Paul/Mike anyone?
L.