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Here is one way of visualizing what's going on. Defocus a bit and you will see the shadow of the diagonal in the defocussed image. Now watch what happens as you slowly focus. The star disc shrinks to a point and the shadow of the diagonal shrinks to a point within a point and you don't see it.
Every part of the mirror that gathers light from a star focuses it to the same point. The diagonal merely removes some of the potential light, but doesn't change the image.
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