Both the camera, UR-E20 and the scope adapter are ok.
I do not have a photo of it now. (Getting dark here now) But I modified a photo in Adobe Elements so it looks the same. (Scope at 20x and no camera zoom)
As you see in the image it is a shading from the aperture. When I halfpress the shutter it will go away and then come back when I release it. I hear that is is the aperture mowing. And at aperture at 3.0 and higher the shade is always there.
If you have a long tube ( toilet roll core will do or roll up some paper ) hold it over the lens and zoom the lens in and out. If there is a problem with the lens then the vignetting circle will be off-centre. Otherwise is it possible that the scope eyepiece is not centered properly?
Neil.