Resolving a line is easier than resolving a point source. The Cassini division is only 0.65 arcsec wide but we can resolve it with scopes as small as 50-60mm. A 60mm can't resolve a 1-arcsec double.
The maximum resolution of the human eye on a non-linear source is 28 arcsec.
Yep, our resolution of lines is way better than for dots (both presumed to be black features on brightish white surround). The Snellen Standard is kind of a blend of those two extrema...with the letters on the chart comprising short line segments that are one arc-min thick, for nominally-acceptable "20/20 vision" And even there, as most of you have probably noted... certain letters are more apt to get confused: O vs Q vs C vs G, M vs H, P vs F vs E, etc. The retinal-limit acuity is around 20/10 = twice as resolved vs meh 20/20. Note that power lines are not good discriminators of astigmatism. Even with objectionable astigmatism, you may be able to resolve lines, it they happen to be oriented in the clocking direction of your astig. Nother thing that very few people realize >>> If you are myopic, and wear ~distance glasses~ for looking at the starry sky... then your magnitude limit is impaired, because your pupils are rendered smaller, as seen through your glasses, from the outside. e.g. (extreme example) if you are 10D myopic, and your eyeglass lenses are an inch in front of the pupil of your eye, as seen through the cornea... then your functional pupil is only 80% as big as without those glasses = stars 64% as bright. This is why some people get lens implants... naked eye stars look a lot brighter! Far-sighted goes the other way... with glasses rendering stars brighter.
I never see this kinda stuff ever discussed among amateur astronomers. Maybe it's just me, but I pretty much notice everything ~optical~ in every-day life... and (thankfully) have developed the habit (and obsessive skills) to derive pretty much everything... from first principles. Tom
Edited by TOMDEY, 18 September 2020 - 02:06 AM.