a lot of modern glass also performs better "stopped down" to use a photographic terms.
Camera lenses in general are sharp (for terrestrial work) wide open, but when stopped down a good lens can rival telescopes.
Telescopes tend to sit at these "stopped down" f/ratios as they're aiming to hit that perfect combination between sharpness and light gathering capability. Plus, longer exposures negate the absolute requirement for fast telescopes.
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