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I think another way of saying it may be that the secondary and spider are so completely out of focus that you don't see them in the image. Also, the main mirror easily sees light from a star around the both sides of the secondary mirror, as it is so much bigger.
I'm no optics expert, but this sounds like a fair analogy:
Hold a pen in front of your face, then focus on the house over the road, you can see all of the house, and the pen becomes much less visible.
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