The journals kept by the likes of Galileo, Kepler and Herschel provide details about what objects they discovered and what kinds of optical instruments they used to discover them.

Taken together, this should allow placing a lower bound on the visual acuity of the discoverer, perhaps even the deterioration of the same over their lifetime.

Of course, we can take for granted that most astronomy pioneers were individuals with exceptionally sharp eyesight, but has any systematic study been conducted with results of the type "to see Jupiter's moons using his 20x telescope, Galilei must have had a VA of at least 20/16"?

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    $\begingroup$ I am not aware of such a historical study but astronomical markers for visual acuity have been studied. Jupiter, when closest to Earth, takes 50 arc-seconds on the sky, and is seen as a disc by people slightly above average (20/16 vision), but not average (20/20). When it is farthest it only takes up 29.8 arc-seconds, so someone with 20/9 vision can see it as a disc, but with 20/10 or worse only as a point. See also Berlucchi on astronomical acuity $\endgroup$ – Conifold Mar 14 '17 at 19:38

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