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It your example the scopes have different focal lengths, 150mm aperture at f5 is 750mm while the 100mm at f5 is 500mm thus the different images. If instead the 150 was reduced to f3.33 then the focal length would match the smaller scope and have a brighter image. In terms of contrast, at the same focal length the larger scope would also have an edge, assuming both are the same type of telescope.
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