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I noticed this recently while imaging with a mate with a larger refractor, ~2x the objective area. My scope f/5.5, his f/7, same camera (within manufacturing tolerances), same gain, same exposure time to hit "sky limited".
f-ratio had me fooled there for a while. It may work well for camera lenses, but comparing telescopes with differing light collecting areas has to be the way of the world, or should that be, the sky?
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