Giusto per dare la mia opinione: credo che le ipotesi non siano ben definite, pertanto le conclusioni difficili da esporre. Ritengo che Knisely sia quello che abbia provato a rendere le ipotesi il più precise possibili, ma il limite inferiore sulla "detection" della divisione di Encke rimane opinabile visto che non si riesce ad inquadrare bene come essa si configuri in paragone alla divisione di Cassini.
L'eterna lotta tra esperimento e scienza, qualcuno potrebbe pensare. Ma qui c'è Knisely che non solo ha provato ad inquadrare teoricamente il problema, ha invece anche provato direttamente sul campo con risultati in accordo con la teoria. Dall'altra c'è invece qualche persona di sicura e indubbia esperienza, ad esempio Steve Coe, che però mette in campo la sola esperienza. Ci si deve credere...oppure no insomma
Ecco dove Knisely mi convince di più:
OK, there are a few numbers here that need to be brought out. First, the Cassini division isn't totally dark or well defined, so there will be some variations in the figures given for its actual physical width (4700 km to 3500 km figures are often cited). Right now, the current best "official" figure for its width is 4700 km (2920 miles), so we can use that number. Now at a mean opposition distance for Saturn (about 8.533 A.U.), 4700 km is an angular span of only about 0.76 arc seconds across. The division was first discovered in a 2.5 inch refractor (discovered by Cassini in 1675) and has been observed regularly with telescopes as small as 50mm. The Dawes limit resolution of a 50mm aperture is only about 2.28 arc seconds, but the division does *not* have to be resolved in order to be detected. Careful studies have shown that certain linear features when placed on a white background with no other detail anywhere near them can be detected at sizes considerably smaller than the resolution limit of the instrument (see p. 49-50 of AMATEUR ASTRONOMER'S HANDBOOK, by J. B. Sidgwick, c. 1971 Dover). This is the case for the Cassini division. As long as the aperture is large enough to separate the division from the diffraction "diffusion" effects of the outer edge of the A-ring, the division can be successfully seen.
Now the Encke Division is considerably smaller. Best measurements put it about 325 km (202 miles) wide, which at a mean opposition distance would be only 0.053 arc seconds of angular width. However, again, to merely detect the division would be possible with an aperture that does not have nearly that level of resolution, so we can' just rule out being able to see it in any scope smaller than a meter or so in aperture. However, the true Encke Division lies so close to the outer edge of the A-ring that diffraction effects from the darkness of the outer edge of the A-ring will cause a problem with telescopes being able to separate it from the outer darkness at the edge of the A-ring. Resolution limits argue that an aperture of at least nine inches will be required to do that. My experiments with using stopped-down apertures to see where the Cassini division begins to show up have supported this idea. There are some seeing and contrast effects that may mimic a true division in the rings, so one must be careful here. Again, I remain very skeptical of any reports that the true Encke division is visible in apertures significantly smaller than nine inches. Clear skies to you.
