Hi Wavytone & All,
First up, I wonder if everyone is talking about the actual Encke Division, or, the Encke Minima? The former is a 325km wide gap almost at the very edge of the “A” ring, whereas the latter is a wider but less well defined dimming at around the centre distance between the outer edge of the Cassini Division and the outer edge of the “A” ring. The minima is very much easier to see than the gap but sometimes people speak of the gap when they mean the minima or assume they are one and the same thing.
I also wonder if you could check how you came to the angular diameter figure for the Encke Division? You say above it is about 0.8 arc seconds given the current distance of Saturn of approximately 1,362,000,000km and a width of the Encke division of about 325km. The angular diameter of the (considerably larger) Cassini Division (4,600km wide) is about 0.8 arc seconds and the angular diameter of the Encke division accordingly is about 0.05 arc seconds.
The formula I use for calculating angular diameter is A=D/a x (180/π) where:
A is the Angular diameter is expressed in degrees.
D is the diameter of the object, a is the distance to the object (D & a have to be in the same units) and π = 3.1415926.
The Dawes limit for resolving detail: R = 116/D D in millimetres, R in arc seconds.
But that’s not quite the end of the story. As we are (probably … hopefully …) all aware, the Dawes limit is the practical application (estimate or approximation) of how close two stars of equal magnitude can be seen in a given aperture as resolved. For 23cm in aperture, the Dawes limit is just on 0.5 arc seconds. But, the rule needs some modification for rendering observable (but not necessarily “resolved”) a dark feature between two high surface brightness features. The Cassini Division and the Encke Division are archetypical example of such a situation. The Cassini Division is sometimes observable in a 75mm refractor, more easily in 90mm, despite the fact that the Dawes Limit for such a telescope is about 1.5 arc seconds and the division itself about half that size. In fact experience has shown that such features are “observable” but not “resolved” down to about ¼ of the Dawes limit. In order for the Cassini Division to be properly “resolved” (ie has a measurable width) in near perfect conditions is about 20cm, but as I said, it can be “seen” in considerably smaller apertures.
Therefore, I would expect that the Encke Division could possibly be barely and fleetingly observable (not resolved), only in practically perfect observing conditions using an exceptional 23cm telescope of extremely high contrast at very high magnification. 20cm is likely beyond it. I might be proved wrong but …
From my own experience, I can say I have seen the Encke Minima a multitude of times in apertures of 25cm and above. It needs good seeing but not necessarily perfect. The Encke Division is an altogether different bucket of fish. I can only say I have seen it twice in almost 40 years of observing Saturn. The first time was with a very fine 307mm f/5.3 Newtonian at x367 in 2000. I spent about 90 minutes that night observing Saturn continuously and definitely saw it three or four times.
The second time was two years ago on a night of exceptional, if not singular seeing at Mudgee with my 46cm Newtonian at x462. Again, over an extended period, I think I saw it definitely several times.
I wait with bated breath for the arrival in the next few days of my latest telescope to see what it will reveal on Saturn. One thing I can tell you is that the view through the 24” Clark refractor at Lowell Observatory in the U.S was something I’ll never forget!
See also:
http://ejamison.net/encke.html
Best,
L.