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Here is a flat with the Canon 300mm F2.8L at f/3.6 with external aperture. I have also extended (x2.5) the dew shield or lens hood with an aperture further out. The lens then only 'sees' the area of sky that is being imaged.
The shadow of the mirror is still there but far less pronounced. This image was stretched in contrast as well.
Hope this helps you realize you do not have a dud 5D MII.
Welcome to full frame astrophotography it will test your optics as far as vignetting and flat fields or lack thereof not to mention astigmatism and coma.
Bert
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