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Well, whatever it is, it's not going to be the secondary mirror shadow - when you're in focus there is no "shadow" cast from the secondary. If there was then you'd see it with a normal eyepiece.
Remember, every point in the field of view is generated by light hitting *all* of the visible portion of the primary. The secondary obstruction is present everywhere, equally. There's no reason why the centre of the field would get any less light than the edge.
It's only when you defocus that you can see the secondary shadow, and even then you can see easily enough that the shadow is present everywhere in the field of view. Look at a defocussed star as you move it around - the shadow is visible just as much when the star is at the edge of field as in the centre.
I'd suggest that the real culprit is some optical mismatch between your camera + zoom and the light cone coming out of the eyepiece. Perhaps the light cone is diverging or converging a bit, and your camera's optics can't handle it.
regards, Bird
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