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It is the shadow of the mirror in the 'up' position, all DSLR's have it. It seems worse with faster lenses or scopes.
It is made more pronounced by light scattering inside the lens or telescope that does not contribute to the image. This is light from outside the area you are imaging that is scattered inside the optic and so lowers contrast.
Below is a flat with my Canon 5D and a poorly baffled 100ED.This was from 2007. I have stretched the image to enhance the contrast.
Bert
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