[ngcles]

Hi Alex & All,

Originally Posted by mental4astro

Four conditions need to be met to see it:
* Excellent optics
* Outstanding seeing conditions
* Sufficient aperture
* Acute vision

.

Here mate we're in complete agreement

Originally Posted by mental4astro

There are also other features that are in the same realm of the Encke Division. On the Moon there are the two wee riles that run down the centre of the Vallis Alpes & Vallis Schroteri. The angular size of each wee rile is the same as that of the Encke Division. Yet seeing or photographing these two features is neither questioned or denied.

This is an important statement -- and I'm sorry, it's not correct. The angular width of these features are not in the same "realm". Here are the angular widths given Saturn at opposition at 10AU distance from Earth and given the Moon's mean orbital distance of approx 380,000km and the formula:

Angular Diameter in arc-seconds = 206265 X (Actual diameter (km) / Distance km).

https://en.wikipedia.org/wiki/Angular_diameter

The two rilles mentioned are about 600m across:

Cassini Division (4,800km): 0.67 arc-seconds
Vallis Schröteri rille and Valles Alpes rille: (~600m): 0.325 arc-seconds
Encke Division: (325km): 0.045 arc-seconds.

Dawes limit various apertures:
Dawes limit (approximation) formula 11.6/D (where D is the aperture in cm)
Dawes limit 50mm: 2.32”
Dawes limit 15cm: 0.77”
Dawes limit 18cm: 0.64”
Dawes limit 23.5cm: 0.49”
Dawes limit 25cm: 0.46”
Dawes limit 30cm: 0.38”
Dawes Limit 46cm: 0.25”
Dawes Limit 63.5cm: 0.18”

I should also add that in one of the other threads, regarding these lunar features you wrote: "At this distance, that 500m detail is 0.0476" in size. Oh, and guess what, the Encke is 0.045"... Hmmm."

You didn't show any working for that calculation, but it appears to me to be incorrect.

Using the formula I cite above: 206,265 x (Actual diameter (km) / Distance km)

206265 x (0.5 / 363766) = 0.28 arc seconds -- ie more than six times the apparent angular width of the Encke division from 10A.U.


In other words, these two rilles are about half the apparent angular width of the Cassini Division but, they are about seven times the apparent angular width of the Encke Division. These two rilles are in the same realm as the Cassini Division, but not in the same realm as Encke.

Yes, (as I have pointed out myself several times during this debate over the past year or so via several threads), the Dawes Limit isn't the be-all and end-all in terms of visually detecting (though not "resolving") high contrast albedo features. Yes, certainly, high albedo contrast features somewhat smaller in angular width than the Dawes limit are detectable in high quality telescopes in superb seeing. I have little doubt that features less than half, maybe even down to a quarter or a fifth of the Dawes limit might be detectable. Beyond that ...? Hmmm ...

The observational circumstances on the Moon with rilles of this sort are extremely high albedo contrast difference. Absolutely black -v- near brilliant white (when the shadow is created by the sun-angle).

The contrast at the Cassini division is high, but not as good as a rille on the moon. Further, the "B" ring is the brightest of the Saturnian rings and is at its highest surface brightest at the inner edge of the Cassini division.

The Encke division contrast could be described as "good-ish" but not nearly as high as Cassini because the "A" ring is somewhat fainter than "B" and tends (generally) to fade toward its outer edge -- where the Encke division is located. It's a black -v- grey situation, not black -v- white.

What should be noted is that for a 7" (18cm) telescope, the Dawes limit is 0.64" and the angular width of the Encke division (0.045") more than fourteen times smaller. Even for a 9" (23.5cm), Encke is over eleven times smaller than the Dawes limit.

Adding complications are the significant, approaching large central obstructions (~30%) native to all Maksutov-Cassegrainian (and Schmidt-Cassegrainan -- even larger) telescopes further interfere with and detract from visual contrast compared to unobstructed telescopes or those with a genuinely small (<20%) obstruction.

Originally Posted by mental4astro

To argue that it took a 32" refractor to first show the Encke Division is a mute (sic) point. Refractors of that time were all achromats with questionable eyepieces (all excellent for their time though) and it wasn't until the that particular scope that an instrument of sufficient quality was available to make out the Encke.

To be candid, this argument is absurd. Between the time of Encke observing with a 9.6" refractor at Berlin in the 1820s and the first observation of the division in 1888 using the 36" Lick refractor, there were 17 "great refractors" in excess of 30cm aperture built and commissioned around the world by makers such as the Clarks, Merz & Mahler, Zeiss and Cooke. Nearly all of them made by renowned telescope/lens makers and were (still are) of exceptional visual-use quality. Sure, they will have chromatic aberration but that will not affect their capacity to resolve or "see" fine detail. Telescopes and the technology behind making lenses was vastly higher than the days of Gallileo & Huygens etc. They not only were but still are pretty much state of the art visual-use telescopes. All of them, were long (if not very long) focal-length instruments that simple eyepiece designs worked perfectly well with. Most of the advances in eyepiece design (particularly in the last 50 years) has revolved around making eyepieces cope with optically much "faster" telescopes and to have wider, well-corrected fields (and better anti-reflection coatings). Almost irrelevant to planetary observing where all you need is "sharp" at the centre of the FOV -- the width of field is a much less important consideration.

Similarly, the astronomers that used them were no mugs behind the eyepiece either. Back in the 19th century, while photography was in its infancy and genuine astrophysics somewhat in the future, the career of a "professional astronomer" rose or failed on their visual acuity and skill at the eyepiece.

Originally Posted by mental4astro

No one quibbles when the Cassini Division is seen through a high quality 50mm scope. If one still insists on believing the Rayleigh and Dawe's limits are the smallest detail that a given apertre can show, your 8" scope shouldn't show you the Cassini Division either. So think about it.

I have thought about it. Here is the simple arithmetic: The Cassini division is somewhat larger than 1/4th the Dawes limit for a 50mm telescope and is likely detectable (though not "resolvable") in good conditions. The Cassini division has somewhat better contrast/albedo features compared to Encke.

Seeing the Encke division is all together in another league -- and not in the same realm (to use your words). Using 18cm it is worse than 1/14th the Dawes limit for that aperture and even in 23.5cm is about 1/11th the Dawes limit for that aperture. Then there is the relative lack of contrast inherent in Maksutov-Cassegrainian telescopes with their comparatively large central obstructions added to the fact that out at the near edge of the visible ring Saturnian ring structures, the albedo contrast while good-ish, is not in the same league as Cassini and a considerable distance behind the rilles you cite (that in reality have considerably larger (seven times) angular widths than the Encke division).

Originally Posted by mental4astro

I am not just only saying I've seen it. I am also giving tips on how to improve your chances.

Not for one moment do I doubt the sincerity of you claim Alex but I have to conclude your "detection" is a spurious observation. Many, many observers of very high repute in the past have made what are now termed "spurious" observations -- errors. I have very little doubt that what your telescope/eye/brain (interpretive) combination is "seeing" is the Encke minima that is somehow being interpreted as a narrow gap.

As for famous cases of "spurious observations" look no further than the Martian "cannali". Once "discovered" suddenly many observers "saw" them. After they were disproved, suddenly, no-one saw them anymore.

Also, look up the curious case of "Baxendell's unphotographable nebula" (NGC 7088).

Baxendell's Nebula (NGC 7088) is a nebula that apparently never was. The discovery of a large faint nebula near the globular cluster M2 in Aquarius was announced by the English amateur astronomer Joseph Baxendell in 1880 ("A New Nebula," MNRAS 41, 48). He found the object, designated as NGC 7088 by Dreyer, on September 28, 1880 at his private observatory in Birkdale, using a 6-inch refractor, and described it being of "irregular oval form, its longer axis lying in a nearly east and west direction". It was, he reported, 30' north of M2 and 75' × 52' in size. He writes "It seems to be similar in character to the large nebula near the Pleiades [found by Tempel], but is slightly less bright. NGC 7088 was visually seen by a number of other observers, including Dreyer (1885, 10-inch refractor), Bigourdan (1897, 12-inch refractor), Hagen (1915 and 1917, 16-inch refractor), Wolf (1927, 6-inch refractor), O'Connor (1929, 15-inch refractor), Becker (1930, 12-inch refractor) and Lehner (1930, 4-inch refractor). However, the object could never be photographed – hence its nickname, "Baxendell's Unphotographable Nebula". It was eventually concluded that the object wasn't real and that visual sightings of it were due to effects such as reflections of the nearby bright cluster M2.

This object, despite the "observations" of some great astronomers, simply does not exist.

People make mistakes. Our brains are natural, active interpreters of what our eyes apepar to be seeing at both conscious and sub-conscious levels. None of us are immune. We can do our best to have other people confirm sightings and trying our best to be objective, but it doesn't always work.

https://www.daviddarling.info/encycl...ls_Nebula.html

Originally Posted by mental4astro

The Encke Division is a very illusive (sic) feature. Not all scopes can show it. Not all eyes can see it. There are also lunar features that are the same angular size that provide a parallel test.

The Encke division is an extremely severe test, of telescope, observer and seeing, I agree. Yes, it takes functionally perfect seeing. Good, very good or excellent seeing are generally not enough. Conditions of this sort are exceptionally rare. Near or at sea-level maybe only one in a thousand days. The lunar rilles are in no way a parallel test as I have demonstrated.

Not all 'scopes can show it -- certainly correct and I'd respectfully suggest the minimum aperture for a very high quality telescope with a small or no central obstruction in functionally perfect seeing is about 30cm. The fact that it eluded a great many highly credentialled observers, using giant (30-70cm), very high quality refractors sited (often) at high-quality observing sites for over 70 years after Encke's heyday, in fact speaks volumes for the difficulty this feature presents.

Originally Posted by mental4astro

You can only try, and try often. If you don't, then one or more of the above four conditions are not being met. Seeing conditions for me this year have not been good enough. But I will keep trying too.

So shall I.

Best,

L.