Pete, it's a hard one for sure, and hard to get folks to believe such a difficult observation in an 8" aperture without staking your credibility on it. Otherwise folks would have reported it more often, it'd be more generally accepted. I forgot about the Harvard study, that's a good data point. I also did not consider the A ring brightness, not sure how that plays a role. Lower contrast? If so, then yea, that makes it more difficult, especially for the eye. Plus, it borders a slightly brighter ringlet at the edge of the A ring, that may play a role, too.
Here's an example of what we might perceive or what to look for.
Browsing some 8" images of Saturn, so far none have managed to image it. But, the conditions where it might be seen are so very rare. Less rare when you stack a bunch of lucky snapshots, but still not an easy target. Maybe Damien Peach can do it.
In the image above, notice the Enke minimum which is interesting in itself. It kind of speaks to the resolution we see in the rings. For example, we normally see a bright ring in the B ring bordering the Cassini division, but there is really some dimmer rings between them. The same seems true of the Enke minimum, we see a broader stretch of rings as a thinner dark area bordered by two brighter ringlets. I'd love to resolve the bright rings in the B ring for what they really are.
In order to image it, all we need is a slight drop off in each pixel along its length. The enhanced contrast beyond the visual level, which probably does not bode well.
More from that discussion.
"For other image forms, resolution limit also can and does deviate significantly, both, above and below the conventional limit. One example is a dark line on light background, whose diffraction image is defined with the images of the two bright edges enclosing it. These images are defined with the Edge Spread Function (ESF), whose configuration differs significantly from the PSF (FIG. 14). With its intensity drop within the main sequence being, on the other hand, quite similar to that of the PSF, resolution of this kind of detail is more likely to be limited by detector sensitivity, than by diffraction (in the sense that the intensity differential for the mid point between Gaussian images of the edges vs. intensity peaks, forms a non-zero contrast differential for any finite edge separation)."
"FIGURE 14: Limit to diffraction resolution vary significantly with the object/detail form. Image of a dark line on bright background is a conjunction of diffraction images of the two bright edges, described by Edge Spread Function (ESF). As the illustration shows, the gap between two intensity profiles at λ/D separation is much larger for the ESF than PSF (which is nearly identical to the Line Spread Function, determining the limiting MTF resolution). It implicates limiting resolution considerably better than λ/D, which agrees with practical observations (Cassini division, Moon rilles, etc.). Gradual intensity falloff at the top of the intensity curve around the edges can produce very subtle low-contrast features, even if the separation itself remains invisible."
Unfortunately, no mathematical approximation is mentioned to guide us. If it is possible, visually or imagery, it needs to be reported from high in the sky in the southern hemisphere with the wide ring tilt. So, we still have to just go look and believe our lying eyes. Or someone else's.
I thought I saw it in my 6", but I am glad I was open minded despite the excitement of thinking I may have seen it years ago when Saturn was better placed closer to the zenith in very good seeing. I'm glad I recanted my claim to it and wrote it off as a diffraction or seeing effect. I'm convinced a 6" cannot do it, but open minded enough that maybe once in a lifetime...or two. Effectively impossible.
Edited by Asbytec, 01 September 2019 - 04:07 PM.