Saturnian Challenge II - Encke’s division

[Wavytone]

Now as most of you have seen Saturn’s ring are a fine sight this year, and Cassini’s division - the black gap between the A and B rings - is easily seen with the rings nicely tilted.

But there’s another division which can be seen in good seeing - Encke’s division within the A ring. At 325km wide it subtends 0.8 arc seconds from Terra.

Visible in the MK91 at 300X, it will be a tough challenge for 8” - 12” scopes and will test your seeing.

There are other divisions too within the grasp of larger scopes but excellent seeing will be your real challenge.

So which divisions can you spot ? And with what scope ?

[Atmos]

I assume the A ring is the inner and not the outer?

It’ll give me something to look for with the Mewlon if the seeing gets nice enough down here in Melbourne.

[AstralTraveller]

It can be seen in a C11 under very good seeing. We had a couple of great nights a few weeks ago and once Saturn neared the zenith I could see what must be the Encke Gap. I'd never seen it before so I was quite chuffed but also doubted myself a bit. However, if you can see it in a 9.1" then it's reasonable that I saw it in an 11". It came and went a bit with the seeing but was there often enough. It was seen with two eyepieces: a 10mm CZJ ortho (280x) and a 9mm UO HD ortho (311x).

[mental4astro]

Here's a diagram of the Saturnian Ring System as a quick reference:

230927

The C ring is also sometimes called the Crepe ring.

There are other named rings, the D, E, F & G rings, but these are mostly invisible to us.

230925

230926

Seeing was poor here last night - image was "swimming" at just 80X :( Very warm night for Sydney last night.

Try again another night... again... :rolleyes:

[AstralTraveller]

Yep, even the Cassini Division was difficult last night. Only two indistinct bands on Jupiter. Mars looked like an interstellar jellyfish. But there was no dew :lol:.

[ngcles]

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.

[Wavytone]

Argh. L. You’re right. Silly me left off a zero.

Encke’s division in the A ring is 0.05 arcsec which has me wondering if i was imagining it. Might have to wait till the weekend to try again.

[AstralTraveller]

Thanks for the information Les. I'll stand corrected. I knew about the Encke Minima and the Encke gap but wasn't sure of their relative observability - wide low contrast vs narrow high contrast. I also though the feature I saw may have been near the outside of the A ring, suggesting the Encke Gap. However if the Minima is so much more easily seen that must have been it. I assume that both times you saw the Encke Gap you could also see the minima?

[The_bluester]

Andy01 and I had the same discussion last year with Saturn through my C925. The Cassini division was rediculously easy to see that night but we concluded eventually that we must have been seeing the Encke minima.

[mental4astro]

Resolvable vs Observable.

Well, here's a sketch done in 1855 by Sidney Phillip Coolidge of Saturn using the 15" Great Refractor of the Harvard College Observatory. The Encke division is certainly visually observable, as well as the Minima:

230975

You should note that the orientation of the Rings in the above sketch is also very similar to their current orientation.

VERY important to note in the notes Coolidge made in 1857 about his observations of the B-Ring as being "streaky". This wasn't much picked up on until the 1970's and then only the confirmation of the the spokes in the B-Ring by Voyager. Coolidge had both a keen eye and one heck of a scope at his disposal!

This should ruffle some feathers! :lol:

You can find some more info on Coolidge and this sketch in these two links:

https://cseligman.com/text/atlas/sidneycoolidge.htm

http://ejamison.net/bond.html

In many photos posted here there is also a huge variation on image detail with the same scope but in different hands. There are currently two images of Saturn posted, both using a C14, one clearly shows the F-ring, and the other just, just barely visible. Yet both show a remarkable resemblence to the sketch above with the amount of detail Coolidge was able to pull visually. I wonder now if the F-ring is visible directly too?

[Wavytone]

Ok so I wasn’t dreaming, must have been seeing the Encke minima as what I saw is identical to this sketch, in moments of perfect seeing with it just hanging there clear as anything.

http://ejamison.net/encke.html

Observers with a TEC 180 have seen similar.

[Merlin66]

Hmmm
in the texts I have available, I can't find any references to the "Encke minima"
Plenty of details on the various minor variations and the better defined "Encke Division", about 1/3 in from the outer edge of Ring A. Backed up by numerous detailed Voyager images.

See: The New Solar System, p 154-170

[Wavytone]

Agreed “Encke Minima” seems a bit vague.

See however https://www.cloudynights.com/topic/4658-encke-minimum-vs-encke-gap-whos-got-the-gap/

... the Encke Gap has been seen and photographed with amateur gear...

[mental4astro]

Encke Division - TICK :D

With an exceptional C8, AND in a Skywatcher 180 Mak last night from home. There's a lot to be said for these mass produced scopes, WHEN you happen upon a very good unit!

Easier to spot in the C8. In those brief moments of spectacular clarity, there it was cutting through the A Ring.

Totally unexpected. I was not out to see it, and trying for it did not cross my mind, but it was so clear and distinct my jaw just dropped when I recognized it

:jawdrop: :cool: :astron:

[Atmos]

I was out looking at Saturn last week with my 4" and didn't see it ;)
Cassini Division was a very very clean split but I think it may be a bit out of the range of a 4" at 225x :)

[AstralTraveller]

Alex,


I can't help but think that there is a fair bit of observer skill behind this remarkable sighting. All that drawing must sharpen your observing skills. You actually have to think about what you are seeing before you can draw it. I'm quiet confident that less experienced observers would have missed that detail, or at least not seen it unless someone else mentioned that it is there.

[mental4astro]

Thank you David, :)

I suppose that there does have to be some sort of spin-off to spending hundreds of hours at the eyepiece with pencil in hand, DSO's, planets and Moon. Another part for me is also following up a sketch with a bit of research into what I sketched. This helps not only identify features, but also understand what I've laid down. It pushes my own visual acuity. AND, it also helps me to recognize similar features in other objects/locations. Understand tale-tell signs, and you will actually see more. Reading this thread started by ol' Wavy stuck in the back of my mind. Then, when I saw the degree of detail on Saturn and knowing a bit more about its morphology (disc and rings), BANG! There was the Encke Division! I didn't set out to see it. I gave myself no reason to even think I would. Dave's C8 is one mighty Cat, as is Chris's Mak!

Sketching alone won't do this for you. But it does all come down to experience, as you said David, is key. Ol' Wavy can't draw a closed circuit diagram without short circuits... But there's nothing wrong with his visual acuity.

This thread really is a gem for observers. It teaches about Saturn's features, and helps push one's own visual acuity and optical quality of one's scopes. All these three ducks line up with the forth duck being exceptional seeing, and a surprise lies in wait! :)

Alex.

[wayne anderson]

Encke’s division

I find that when the conditions are VERY, VERY good Encke’s division can seen quite well in my 12inch LX200 if you are an experienced observer, but occasionally even 1st timers can see it.

Last Month i had a few people around for an astro night a few young children and adults all were 1st time viewer's through a telescope.

Viewing was of Venus, Jupiter, Saturn, Mars, Omega Centauri, Jewel Box, 47 Tucana, Swan Nebula, Triffid Nebula, Lagoon Nebula, Centaurus A Galaxy .........and many more open clusters and stars

With Saturn near the zenith and VERY STILL CLEAR conditions Cassini’s division was large and obvious i could also see Encke’s division in the rings at both 234x and 358x magnification using a 2x Powermate and 26mm Nagler and 17mm Ethos eyepiece.

I asked the first adult to look through the scope to describe in detail what he was seeing, he said.........

WOW its Saturn, i can see its rings and a big gap running around the middle of the rings, the rings have different bands of shading and the planet has bands of white, very faded brownish yellow and grey, there is a round grey area at the pole and i can see the rings going behind the top of the pole and a shadow on the rings from the planet.

I said Very good and explained about what he was seeing, then i said can you see anything between the big black gap in the rings and the outer edge of the rings like a "colour or shading or something" ( not mentioning anything about Encke’s division as i was throwing a curve ball to see if he could accually see it)

A few minutes of viewing he said OH... there is another very thin gap in the rings its very hard to see but its there

So on that night with 4 children and 3 adult first time viewers.... Encke’s division TICK for 2 children and 2 adults

[mental4astro]

:D :D :D

:thumbsup: Great post, Wayne :)

[Wavytone]

Congratulations, Wayne, great post - and all the more so considering how wacky the weather has been this year.

And to Alex, too, grasping what is possible with smaller scopes than his big dob ;)