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You could always stop down your telscope's aperture to increase the f-ratio in exactly the same way as is done in a camera lens. But you very rarely want to do that because normally you want all the light you can get.
But the same goes really for camera lenses when it comes to astrophotography: you normally want the aperture wide open to collect as much light as possible (unless your lens performs very poorly at full aperture; much like a badly figured telescope mirror that can typically also be improved by masking its edges).
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