A planetary gauge to test out your scope with - Saturn's Encke Division: Encke Test

[mental4astro]

Hi all,

With the trio of planets of Jupiter, Saturn and Mars approaching their opposition shortly, there is one planetary feature that will show the ultimate quality of your optics - Saturn's Encke Division. Also called the Encke Gap.

You will need to push the scope's magnification to close to its limit for this exercise.

But the Encke Division (0.045") is smaller than the angular resolution of my scope!

Actually, no it's not!

The quoted angular resolution given for scopes refers to the scope's ability to resolve two stars - but stars ARE NOT pinpoints of light, but actually disks, the Airy disk, a diffraction pattern in reality - we are not actually seeing the disk of the stars. And the resolution limit is the ability to distinguish between two similarly brilliant stars to be able to make out a pinch between the two Airy disks.

258068

When it comes to extended objects, such as the Moon and planets, the actual resolution capability of a scope can be 10 to 20X finer than the Rayleigh or Dawes limit. When it comes to extended objects, there is no diffraction pattern at play, no Airy disk - the possible diffraction pattern is totally disrupted, and the possible resolution limit is much, much finer.

Just remember, the Cassini Division (0.65") was discovered in 1675 by the French astronomer Giovanni Cassini using a 2.5" scope - the Cassini Division is much thinner than the "traditional" resolution limit for a 2.5" scope - 2.19" Rayleigh limit!!!!

I have seen the Encke (0.045") division in 7" Maks. I have also not even come close to resolving it in 10" scopes. Photos of Saturn using a 16" scope have also not shown it - could also be that the imager didn't know about the gap and then went about eliminating it!!! :eyepop:

Heck! By strict resolution definition, a 7" Mak SHOULD NOT be able to resolve the Cassini division either! Yet the Cassini division is not even a test for an 80mm scope... Get the picture?

The testing doesn't stop there!

If you are able to do a side by side test of a couple of 6" and larger scopes, and both scopes can resolve the Encke, the second test is seeing related. If seeing is GOOD, but not excellent, then the image will be a little "fluttery", like a flag gently wafting in the breeze. If the image degrades with a scintillation or shimmer in one scope, but not the other, it is the second scope other scope that has the better optics as the photons are being much more tightly controlled to go where they are supposed to. In the first scope, there are some photons that are "stray" and it is this slightly less than perfect optics that has the image scintillate or shimmer instead of waft and stay sharply focused. But you don't need a scope scope to prove this. If seeing is good or excellent, but you also see a scintillation along with the gentle wafting, then the optics are a little lacking.

Happy hunting,

Alex.

PS: When I saw the Encke Division in the 7" Maks, Saturn was a little smaller in angular size than it is right now (20th of April 2020), so you can start doing your scope bashing now! :)

Alex.

[Saturnine]

Very timely article Alex, with planet season upon us, they're getting high enough in the early morning to start pointing telescopes at and seeing detail.
The two things to take into account when trying to see detail, as in the Encke division in Saturns' Rings, is the quality of the optics and the quality of seeing conditions.
The best optics in bad seeing will not allow fine detail to be seen as the atmosphere will smear the image at the eyepiece. Likewise , the best seeing will help bad optics perform at their best but the detail will still be compromised by the poor performance of the optics in question.
Still looking forward to having a ( long ) look at the planets with your Russian Mak, if and when we can travel and congregate again. I still haven't seen or imaged the Encke Div. with my 250mm newt, which means either the optics aren't good enough or the seeing is mostly way below optimum from my location.

Cheers

[mental4astro]

Thanks Jeff for noting that :thumbsup:

If your scope is smaller than 6", the Encke Division most likely will be too fine a detail to resolve, but nothing to stop you from trying. There are other planetary and lunar features that will serve the same purpose for scopes 6" and larger and of course also smaller. If you would like a few more just let me know :thumbsup:

Alex.

[Rainmaker]

I would think you will need perfect seeing, minimum 8" of high quality aperture and around 400x to see the Encke Division. At only 325km wide and being near the albedo feature often referred to as Encke Minima it is going to be beyond most scope/seeing combinations.

[N1]

This is possible because Seeing (i.e. detecting) something and resolving it are two different things.

Suppose your resolution limit is 3 units of angular size. Now suppose you are looking at an object that is 9 units wide but has a dark gap through the middle which 1 unit wide. Somebody now tells you that you can't possibly see the gap, being a third of what you can resolve. You then prove them wrong by seeing it anyway. That's possible because the integrated brightness of the middle 3 units is lower than that of the 3 units on either side. By resolving that difference in brightness you have detected the gap. But that doesn't mean you've resolved it.

On Saturn's rings, this effect becomes apparent when comparing the view of the Cassini division through different apertures. Smaller aperture: wider and lighter, larger aperture: thinner and darker (relatively speaking).

[Rainmaker]

I should have used the word SEE, instead of RESOLVE...... have amended previous post.....

There is a difference in that Cassini is bordered on either side by very bright rings whereas Encke is bordered by darker rings esp the 'Minima' zone.

I have only seen the Encke twice, both times with 18" aperture on nights of very good seeing and at well over 300x.

[mental4astro]

Nope, I've seen it in a Skywatcher 7" Mak and an Intes 715 Deluxe Mak. Not just my eyes, but had a witness too when I did the last of my Cat testing in 2018. The pic below is of that final Cat shoot out between two very fine C8s and the Intes. How the three scopes stood up to the Encke under the very same conditions is what showed which scope was the pick of the crop. In this case the Intes.

If I had to chose from either one of those two C8's, and the Intes was not in the equation, I would have been just as happy with either one of those two C8's. I also still know where those two C8's are in case someone is interested in getting their hands on a blooming fine C8! :D

For clarity sake, I tested 7 scopes between 7" and 8". The other 4 scopes didn't make the shortlist for only one of two reasons, either optically not good enough, or mould was an issue. Alas, that 7" Skywatcher Mak had a significant mould problem :(

[Rainmaker]

That is a very good observation Alex, I tried for it on a near perfect night with the TEC Mak/Cass 200mmF15.5 at 400x but could not claim it....

[Atmos]

It’s something I will have to try with my M250 but I don’t think I get good enough seeing from my backyard to be able to snag it.

[Camelopardalis]

Nice one Alex :thumbsup:

I remember the first time I saw it with my C8, in the suburbs of Sydney...didn’t know what I was looking at and had to look it up. Then didn’t believe it :eyepop: that first night it was really obvious.

I’ve subsequently seen it several other times, in good conditions, both with my C8 and other folks’ scopes.

[The Mekon]

More comments on observing the Encke division on this thread. http://www.iceinspace.com.au/forum/showthread.php?t=177034
I remain skeptical of observations in small apertures - remember it was discovered with a 36" refractor!

[astro744]

Luxury! When I was a lad we only had a 2.4” to see the Cassini division.

I have seen the Cassini division a few times with a Tele Vue 60 (2.4”). Just need excellent seeing, a wide open ring system and the planet closest to Earth as every km counts. Seeing probably the most critical.

Looks like I need to get a smaller telescope. A TV-40 has a nice ring to it. ;)

[mental4astro]

There is discussion about the Encke Minima. It is a valid point. The problem with the Minima is would we be able to pick up the subtle dynamic range variation on the brilliant rings. Considering how thin/narrow these features are, I would struggle with the minina.

A third possible thing could be the combination of the Division combined with the outer edge of the Minima which sits beside the Encke. Then here is a slightly, oh so slightly wider space, and only the Division itself added the necessary contrast. But this feature is still exceptionally thin. Seriously thin!

I like the challenge to the claims of having seen the Encke Division. Don't forget i did not see this alone, and the Encke's size does fall within a range of factor that is a plausible % of the Cassini Division to be detected. 1/14th part of the Cassini Division, a feature that is not dimension-less, but a measurable span of 0.65", which should be just about imperceptible for an 8" scope, but it is a thick line. This is certainly a possible fraction of the span of the Cassini that can be detected by the human eye - something that I back my vision for. Nor did I say that the Encke was an immediately noticeable feature. It isn't. It was even then a freaking difficult thin line to detect.

I have also spent hundreds of hours with the Moon, so my eyes have learned how to pull detail from a very high contrast situation. On many occasions I have shown very fine lunar and planetary features to others yet those features remained invisible to them, either their scopes were not able to resolve the features or their eyes could not pick them up.

That Nick and I saw it, or thing related to it, I still don't doubt. The feature was exceptionally thin. That what we saw was an exceptional feature, I completely agree. It was not on one occasion that we saw this, but two separate nights. So it was repeated. Therein lies the challenge that presents the Encke Division.

There are two lunar features that I can compare this to. There is a very thin rille that runs along the floor of the Alpine Valley and another down the sinuous Vallis Schroteri. The latter is particularly difficult. These two internal rilles are near equally difficult to detect as the Encke. What most helps with these rilles is there are two parts to them - the dark edge shadow beside the highlighted opposite face. The Encke is just the thin black line with no highlights to set it off. Spot these two lunar features, and you are well on your way to detecting what we saw. The black shadow line of the rille in Vallis Schroteri, THAT line is at the same scale as the Encke Division!

Below are two screenshots from Virtual Moon Atlas of the two vallies, Apline and Schroteri. The rilles in question run down the inside of the main channels. These thin internal rilles are very fine features.

Alex.

[doug mc]

Only ever saw it once using a10 inch Skywatcher dob about 12 years ago. My eyes aren't as good now, but aperture sure helps.

[mental4astro]

This is as much a photo challenge as a visual one.

Few photos are able to clearly show it. Many images show a diffuse and disjointed "ring". Others show nothing at all.

Good focus and resolution capability of the scope and camera are important here. If the image scale on the sensor is too small, detecting something here will be difficult if not impossible.

Up for the challenge with your camera or eyeballs? This is a very difficult feature for both. A test of optics, imaging, processing, and observing skills.

Conversely, what's the smallest aperture that the Cassini Division can be seen with?

Alex.

[glend]

As I understand resolution, point source descrimination involving Airy Disks as discussed by Alex in the OP here, (ala Double splitting for example) is almost completely dependent on aperture, but contrast differences, such as the case of the Encke Gap (or Division) are not. Hence, smaller scopes with very good optics, should be able to show the Encke Gap (or Division), if seeing allows.

[mental4astro]

From some correspondence that Wavytone shared with me:

"The most commonly quoted criteria for the resolution of telescopes are the “Rayleigh limit” (¼ wavefront error) and the Dawes criterion. Both of these concern the angular separation of a pair of equally bright point sources, at which they can be reliably distinguished as a pair, not a single point. However… Both limits are not particularly valid for a sharply defined linear edge or a line, such as the lunar limb or the divisions in Saturns rings, nor non-point (extended) objects (ie planets and their moons).

For an edge transition (white to black) such as the lunar limb diffraction theory provides a mathematical formulation of the way the light intensity drops off, and in much the same way as the point spread function, fringes can be observed, eg photometrically during an occultation of a star by the moon.

Similarly, for a bright slit (or a dark line) diffraction theory also provides the slit diffraction function - see for example http://labman.phys.utk.edu/phys222core/modules/m9/diffraction.htm and the location of the fringes is found from the equation:

w.sin(a) = m(L)

where w is the width of the slit, a is the angle from the centre, m is a positive integer (1 for the first fringe), and L is the wavelength of light.

However this has solutions where the width of the slit (w) is wider than the wavelength of the light (L). When the slit is narrower than the wavelength the equation has no solution - and no fringes are observed.

This means an extremely narrow linear feature - a bright line or a dark line will not produce fringes - but this does not necessarily mean it cannot be observed if the contrast is sufficiently high.

In small telescopes the Cassini and Encke divisions are good examples of this. The Cassini division at 4800km wide presents an angular width around 0.66 arc-seconds, yet it is seen with a 60mm refractor. The Encke division is far narrower at 325km, yet is seen on 14” SCTs and has been observed - and photographed - using smaller scopes with superlative optics (7” and 10” maksutov cassegrains)."

[Shiraz]

Bit old, but is this any use Alexander? I tried using a Hubble image and blurring it with an approximation of the PSF of my 12 inch scope and then applying some typical sharpening to the blurred image. Then compared that to what I was actually able to image in the real world - agreement was fairly good.
http://www.iceinspace.com.au/forum/showthread.php?p=965111#post965111

Added a revised version of the original image below, with a bit of explanatory text to hopefully make the process a bit clearer. Encke is definitely visible after realistic PSF blurring, although at low contrast.

Cheers Ray

[mental4astro]

Thanks Ray :)

Not old at all.

On the contrary, very topical really, because even then there is next to no mention of the Encke Division, and how IT should appear despite it being so fine. Only you, Ray, mentioned the Encke Div.. How many others knew what you were talking about, or the significance of its appearance is in the images?

The Encke Division is so poorly known that it counts for very little in conversation about images of Saturn. I've been using it as a gauge of the image quality of Saturn for a couple of years now - can it be seen or not.

Image processing is also a hindrance. Without knowing the Saturnian ring anatomy, features such as the Encke are overlooked or even eliminated altogether because the image processor doesn't know it is supposed to be there... they see it and then get rid of it thinking it must be an imaging artifact. That is if the Encke Division was even captured by the scope and/or camera.

Have a look through last season's crop of Saturn images.

Alex.

[mental4astro]

I got up early this morning to try a few eyepieces with the planets using my trusty ED80 refractor.

One thing I also wanted to do was start seeing how small an aperture I could use to see the Cassini Division.

The scope's objective cover has a subaperture hole in it that is 53mm in diameter. I didn't have any aperture masks made, so I just used that hole in the dust cap a my first experiment aperture. I used a TMB Planetary type II 5mm eyepiece for this.

Well, knock me down with a feather! I could just make out the Cassini Division with a 53mm aperture!

Seeing was not good, so it took some time, but there it was! :eyepop:

~x.X.x~

Again I refer to Rayleigh's Limit as being totally misunderstood as being the very limit of a telescope's resolution. Rayleigh himself NEVER said this was the case. Some of Rayleigh's work was trying to determine how good does the wavefront need to be to be indiscernible from a perfect star. From this work comes the Rayleigh Limit. And as such, Rayleigh Limit is now always about being able to discern the "pinch" between two equally bright stars, with the "Limit" itself being the angular separation of the two Airy disks. The pinch itself is much finer in size than the angular separation of the Airy disks, and has a distinct observable shape.

And for that matter, the Airy rings that surround the Airy disk, these themselves are much thinner than the Airy disk they surround. Yet no one who quotes the Rayleigh or Dawes limit and is adamant about this being the finest size that can be seen through a telescope, seems to notice this??? Most curious.

This is why Giovanni Cassini was able to see the Cassini Division with his 2.5 inch refractor, and why a good 7" scope can show the Encke, or how come I've been able to see the Cassini division with a 53mm aperture.

[N1]

What happens if the amount of time the seeing at least matches the theoretical limits of my optic (also allowing me to compare 2 optics in marginal seeing as described in the OP) is approximately never? How do I know that I saw a given feature despite my scope's limitations and not despite those imposed by the seeing?

Edit: visually, I should add.

[N1]

I'd estimate somewhere between 35mm and 50mm. This is a nice visual challenge in its own right as it reduces uncertainties introduced by the seeing. Many more nights allow Cassini than Encke. Downside: Not many HiQ Maks available in that range :P

[mental4astro]

Mirko, to what theoretical limit are you referring to?

The "theoretical limit" oft quoted by scope manufacturers is misused to begin with, because it is used as a scope resolution limit, which is an incorrect application of the Rayleigh limit, as we can see significantly finer detail than what this "theoretical limit" is quoted at for any given scope.

I would say most manufacturers quote the Rayleigh & Dawes limits as a "resolution limit" mainly out of ignorance. It is not "wrong", but the "limit" is only for double stars, and not the finest detail a scope can show, but the spin put out is published as being the smallest detail...

I've slowly come to see that there is difference between the Rayleigh & Dawes limits to what I'm seeing through the eyepiece. My sketching the Moon over the years was the start of this process for me. Then seeing the Encke Division was the final piece that had me investigate what is going on and understand that the Rayleigh & Dawes limits are misrepresented & misunderstood.

The Rayleigh & Dawes limits are not the smallest detail/size/angle that a telescope can resolve. It ONLY applies to double stars for telescopes. See the first post in this thread for an explanation how this is if you have missed it.

The reason for using the Encke Division as an optical test is because with the Encke Division we are actually pushing a scope to its TRUE resolution limit. And this limit is first determined by the quality of the figure of the lenses and/or mirrors. The Strehl or fraction of wavelength is what rates optical quality. But short of having your scope lab tested, the Encke Division is an excellent substitute. If your scope can show the Encke Division, then those optics are very good! This is the first part.

The downside of using the Encke Division test is it needs very good seeing. This is a time and luck factor.

The second part comes with comparing scopes side by side, and working out how each individual scope maintains the image of the Encke Division under the sames seeing conditions. A scintillating or shimmering image is not what you want to see. But again this test is dependent on seeing conditions to allow the Encke Division to be seen, HOWEVER not perfect seeing is wanted here. Here it is how the image of the Encke Division holds up in slightly less than perfect conditions between the two or more scopes.

If the image scintillates more in one scope than in the other, it is the other scope with the better optics. You want to see less, or better stiil NO scintillation or shimmer in the image during the comparison.

But seeing the Encke Division if the first part!

[croweater]

Hi Alex. Would it also be the case that when comparing two scopes that on one night a larger scope will perform better under excellent conditions but another smaller along side it may outperform it on a night with seeing not as good. Therefore not optical quality alone that decides.
Cheers Richard

[mental4astro]

Richard, yes AND no.

To begin with it comes down to the quality of the figure.

My 17.5" dob unfortunately is astigmatic. It means I cannot increase the magnification to the 400X I need to see the Encke Division, no matter how good the seeing is. My big dob is not one for planetary detail.

I've been able to see the Encke Division in a damn fine 7" scope, but not in an astigmatic 8" scope.

Seeing conditions can certainly affect what an aperture can show, yes. This is a very well known aspect. Often at home when I want to pull out a scope, and I see the seeing conditions are poor I will use an 80mm refractor, and the bigger scopes stay in hibernation, or I use low magnification with a larger aperture, which is what I do when at a dark site. You just have to work with the conditions at hand.

[mental4astro]

PLEASE NOTE!

If you have a reflector (Newt, SCT, Mak, or whatever other flavour you care for), GOOD COLLIMATION IS CRITICAL! to have the best chance at seeing the Encke Division.

The hardest part about "collimation" is ONLY the word itself! It only means aligning the optics, that's all.

There is plenty of info on how to collimate your particular scope, whatever it may be, and not the place of this thread to describe the process of any given reflector design. But if you have any concerns, please PM me. I'm only too happy to help.

ALSO...
MANY SCT's, Maks and other types of catadioptric reflectors that focus by moving the primary mirror show mirror shift. If you scope has mirror shift, you NEED to know how to use this shift to both collimate and focus your scope so that it always performs at its best for you.

Mirror shift trick.
If your scope shows it, you cannot collimate your scope both inside and outside of focus - mirror shift will not allow you to do this as the mirror will shift out of collimation when you go through both while focusing.

If your scope shows mirror shift, you need to get into the habit of turning the focuser knob ONLY IN ONE direction every time you want to focus the scope. For instance, turn the knob a little counterclockwise first, and then ONLY clockwise slowly to gain focus. If you went past focus, then wind the knob back past focus and slowly wind it back clockwise for focus. Having used an SCT with mirror shift for so many years, I still do this with all my scopes, regardless if they show mirror shift or not :lol:

In this exact same way, you should ONLY collimate your scope in the same way, ONLY with the focuser knob turned in the same direction as for focus. It is only in this way that the primary mirror will always be in the same position for its optimal collimation point.

[Shiraz]

there is some interesting stuff in this Alexander - have you seen it?

https://www.cloudynights.com/topic/4913-seeing-encke/

also, this is worth a read http://www.damianpeach.com/simulation.htm

[mental4astro]

Thanks for the links, Ray.

As I said all along leaving seeing conditions aside, for a scope to have any chance of resolving the Encke Division, its optical quality must be very good. This is one thing many in that CN thread struggled to understand, and most assumed all astro scopes are produced the same. They are not, and optical quality can vary greatly even within a brand.

This is why the "Encke Test" is a good substitute for having your scope lab tested to provide a quantified result. It will also test your camera and processing skills.

It will also test your collimation skills and the way you use your scope (such as if it has mirror shift, focus), and your own visual acuity.

Alex.

[bratislav]

Can I ask what was the magnification used when supposedly seeing Encke?

For anything less than about 600x you can frankly forget it.

For the record, I have tried many times in all sorts of telescopes to see Encke, and smallest so far is 11". This is about only 45 years of observing ... and counting

BTW, I have had extensive conversations about Encke/Keeler with many experienced observers, among them late Thomas Back who was eagle-eyed observer and had access to some of the best telescopes ever made (and best seeing conditions). His claim of seeing Encke with his (exceptional) 7" AstroPhysics refractor (likely to be better planetary scope by quite a margin than 7 and 8 inch instruments in question) was quickly retracted after the delivery of his 20" (Zambuto mirrored) Dob, and seeing it for real. It took initially 1000x, then down to 550x, and the night of exceptional seeing in Florida (Pickering 9+, stationary rings).

I've also tried countless times with my own excellent home made 7" Mak (and only during nights that could sustain 500+ magnifications), with no luck. Same goes for my own (also home made) low obstruction 8" (I could see detail on Ganymede with that!), no Encke. Both scopes resolve subarcsecond doubles and easily show Alpine valley rille and wealth of low contrast detail on Jupiter. And I had them for decades (still own the Mak).

The only credible sub 10" observation that I can relate to is another very experienced observer using 9" Clark refractor. That would be about the smallest I can believe.

So color me extremely skeptical about seeing it in a 7" Mak or 8" SCT.

PS the night I saw Encke in 11" I took the video recording as well. I still keep the raws, and playing back the recording one can actually see Encke coming and going on screen (just like in eyepiece). But Saturn on that recording's playback is nearly 5" across on my screen, subtending more than 12 degrees, which makes it much easier to see - that is equivalent of over 1000x in the eyepiece!!!).
What is remarkable about that night's seeing (recording was done about an hour before sunrise on 19th of March 2014) is that Saturn was always completely stationary and only occasionally wavered a bit (not enough to affect Cassini which was always visible) at 750x. The globe of Saturn was also visible through the Cassini 90+ % of the time. Seeing like that only happens a few times in one's lifetime. Alas, just one avi is nearly 4 gigabytes so I can't share ... but I can show you completed color pic

Correction - the observation and recording actually occurred on the morning of March 20th. I started imaging Mars earlier, hence folder was labeled as 19th

[bratislav]

And this is a raw stack of roughly 12000 frames (in R), showing the gap just as hard as it was in the eyepiece (eyepiece view was much sharper of course, but the gap was not visible 100% of time). Tiny bit contrast adjusted.

[mental4astro]

360X and up. You don't need 600X, but maybe with some scopes that's what it takes.

Skepticism is good! It keeps everyone honest :thumbsup:

That is why the Encke Test is so good - the scope either shows it or it doesn't. Not half pregnant. Simple as that.

[N1]

Hi Alex, yes I'm aware of the challenges around applying a "limit" relating to point sources to extended, and specifically, linear objects/features, or even what that limit means for geneal observing. However, I expect there will be a quantifiable and repeatable relationship between say the Rayleigh limit of an optic, and what apparent width linear feature the same user can detect through that optic, under the same conditions. So once I knew that relationship, I could quite happily use Rayleigh to formulate my expectations for a particular observing task with a particular instrument. I'd maintain that there is a difference between resolving and detecting/seeing. Imagine the Encke Gap were actually a string of black spheres (think necklace), whose width is that of the apparent gap. A peculiar property indeed, major discovery etc - except this would be undiscoverable in a 7" scope, because you'd need to actually resolve it.

I do enjoy a challenge but the difficulty with using the Encke Division as a test for my optic alone is that the test has so many variabes it's hard to find the result in there. You've named them yourself. If I can detect the gap, what does that actually tell me? I've got good optics sure, but if I don't see it, then I've still got good optics for all I know. Because, ya know, the producer is coveted, few units have been made, and I blame the seeing :D

[bratislav]

At 360X in a 7-8" scope you have absolutely no chance of seeing it. It is hard enough in 16" at 500 times. Contrast would be impossibly low to detect it.
Our senses (including eyes) are too easy to be fooled, it is simply a fact of life. Believing has nothing to do with it. I have an audiophile friend who is absolutely positive that he can hear differences after replacing mains lead for his laptop (not kidding). He would bet his house on it.

Listen, I know about what people like Thomas Back could see in the telescope.
He was convinced he could see Encke in his 7" refractor.
He could not. And he was man enough to admit it later.

Too many people that I know as truly critical observers agree that it takes minimum of about 9" of perfect (unobstructed) aperture paired with perfect seeing and wide opening of the rings to see Encke/Keeler gap. I'm sharing the sentiment.

[Rainmaker]

The only two times I have managed to see the Encke Division were at 510x in my first 18" F5.58 using a Pentax XW5mm and at about 420x in my 18"F3.54 (F4.07 with SIPS) using a Delos 4.5mm.

Every other time that I have tried (there have been countless tries) the seeing has not allowed it, and as Bratislav stated above, the seeing has to be near on perfect so that the planet shows no shimmering whatsoever....

I will keep trying.......

[mental4astro]

On another occasion last year, with the 9" Mak I am currently using, at Terry Hills during a North Syd. Astro Soc. meet, exact same time, sky and conditions, the 9" Mak was showing it clearly, but a C11 and 12" Meade not even hinting at it.

Again, to have any chance, it is a combination of seeing conditions and scope's optical quality.

Try the Encke Test.

If seeing conditions are excellent, and your aperture is 7", even 6", and larger, have a shot. You either see it or you don't. Your scope has the edge, or it doesn't.

Alex.

[mental4astro]

This is as much an imaging challenge as a visual one.

For imaging, image scale is important here. If your camera has a small chip, but with a dense pixel matrix (small pixel size, and a lot of them), it may be an advantage here as the image size of Saturn is able to be concentrated over a richer pixel field. This will give you a better shot at being able to resolve the Encke in your processing.

Use a large size chip, and the image scale of Saturn may be too small for the density of pixels, no matter how much the image of Saturn is magnified. The camera and processing may struggle to be able to show the Encke.

For imaging this is a technically demanding task. Not only does it involve excellent seeing along with good optics, but there is also quality of focus, image scale to chip size and pixel density, and processing skills - the Encke is not an imaging artifact, and you must be able to distinguish between an imaging artifact and the Saturnian ring anatomy.

If you look through last year's crop of Saturn images, few images show the Encke, some show a broken, disjointed line, and many show nothing at all.

~x.X.x~

There is another thread running parallel to this one related to the Encke Test:

Saturn's Encke Gap Challenge, 2020 (http://www.iceinspace.com.au/forum/showthread.php?t=182353)

You are most welcome to post your visual and imaging results there rather than here. This thread is more the technical discussion side of things, but you are also welcome to mention your current or previous successes here. And yes, skepticism too. This will keep people honest, and provide an important counter balance to the discussion.

Alex.

[Saturnine]

Just as a bit of a gauge of what may be needed to image the Encke Gap i've been checking images on the ALPO Saturn section. From what I've seen so far, the smallest scope that has picked out the gap is a C11.
Most of the guys that post their images on the ALPO site are using C14s' C11s" or largish newts , from 250mm upward and cameras with small pixels and fast frame rates and even then the seeing has to be good to definitely identify the Encke.
I would hazard a guess that excellent quality smaller scopes from 200mm and up, visually, in very good seeing and with good eyes a well, it may be possible to discern the gap.

[doug mc]

This resolution factor can be easily seen by finding a sun reflection on a power pole ceramic or bolt. You need a spider web close by and enough magnification to see the airy disc your scope delivers. You will see that the spider web is much thinner than the diffraction spot.

[ngcles]

Hi Alexander & All,

You can count me in on the "sceptic's" side of the argument as to the visibility of the Encke Division with telescopes of less than 25cm aperture. I am not calling anyone making such a claim a liar -- merely mistaken (and probably coupled with a bit of wishful thinking). The error being made is mistaking the subtle surface brightness variations within the "A" ring, commonly known as the Encke minima for the actual Encke gap.

First a little history: The gap is not so named because Encke discovered it. It was named for Encke by James Keeler in 1888 to honour the very careful extensive observations made by Encke at Berlin Observatory using a very fine 9" Fraunhofer refractor who reported several variations in surface brightness within the A ring. It is notable that with this a fine 9" refractor Encke did not report the existence of a "division".

Between the 1820s and 1888 when the Encke Division was actually discovered at Lick Observatory with the mighty 36" refractor, more than thirty "great refractors" from 12" aperture up (many in the 18-30" range) by manufacturers like Cooke, Grubb & Parsons and Clark, were installed around the world and ... nobody reported the Encke Division until finally a 36" was turned on Saturn and Keeler found it.

The division itself is only a little over 325km across. At maximum width, it is less than 0.1 arc-seconds across (closer to 0.05 actually). The pitfalls of using the Dawes limit for determining how small a high-contrast black on white albedo feature are well discussed here and I won't enlarge on them. Suffice to say, that such high-contrast features, somewhat less in angular diameter than the Dawes Limit might still be observable is perfectly credible.

The Dawes limit (more correctly approximation) is given by the simple formula R=116/D (where D is the aperture of the objective in millimetres. For several small/moderate apertures this formula yields:

150mm = 0.77 arc seconds
180mm = 0.64
200mm = 0.58
250mm = 0.46
300mm = 0.38

I'd be happy to concede that a very high contrast albedo feature around 1/2 to 1/4 the Dawes approximation might very occasionally be visible with a very high quality telescope in superb (read: functionally perfect) seeing conditions. For those using telescopes of 250mm aperture and less, that have reported seeing the Encke Division, I would respectfully suggest that what you are almost certainly seeing is the Encke Minima -- the broader but lower contrast albedo feature(s) first observed by Encke himself within the A ring -- but not the actual division.

As I've said elsewhere, Maksutov telescopes are not "magical" nor are they able to outperform other designs inch-for-inch assuming good optical quality. One quality they generally do possess that makes them attractive for planetary observing is their very long native focal length meaning that relatively high magnifications are obtained with eyepieces of moderate fl and comfortable eye-relief. They are simply more pleasant in use at higher magnifications than shorter fl telescopes of similar aperture. Their relatively large central obstructions (usually approaching 30%) (compared to an optimised Newtonian <20% or a refractor, 0%) is actually a contrast killer.

In all-but 50 years of visual observing (an in addition to long-service, I'm no mug either), I've also observed the Encke Minima probably more than a dozen, maybe twenty times with 254, 307 and 456mm apertures and moderately high magnifications. I've seen the actual Encke division twice during many hundreds of hours observing Saturn. The first time was with a very high quality 307mm f/5.3 (optimised) Newtonian at almost x400. I glimpsed it fleetingly no more than two or three times over the course of about an hour with exceptional seeing. Not merely good or very good but exceptional.

The second time was with my 456mm f/4.9 Newtonian on a night of singular seeing at Mudgee, about 7 years ago. The seeing was so good that night, you could observe planets (Jupiter, Saturn & Mars approaching opposition were on display) at magnifications around x600 without a discernible quiver in the eyepiece image for over 30 seconds. On this occasion, I glimpsed it several times over the course of a couple of hours (we spent a lot of time back & forth between three planets).

On both occasions I saw the division itself, I also saw the minima.

If the Encke division were genuinely visible with fairly considerable regularity using quality 7-8" telescopes at powers around x300, one has to ask how, given the large numbers of very talented 19th century professional visual observers with over +30 high quality large to giant refractors, missed it between the 1820s and 1888 -- when it was finally seen -- in a 36" at 4,200 ft elevation, at one of the best "seeing" sites in the world?

Those observers weren't mugs and the telescopes they used were some of the finest refractors the world has known.

Remember also that the division itself isn't situated in the brightest portion of the A-ring -- it is right near the edge where the contrast is less than elsewhere -- it isn't black on white, more black on light grey (albion grey?) -- making detection even more difficult.

I would respectfully suggest the absolute minimum aperture assuming both excellent (functionally perfect) optics in absolutely perfect seeing for an experienced observer would be 28-30cm.

I would suggest respectfully, that those reporting it in 180 - 200mm aperture telescopes at around x300-x400 are seeing (like Encke himself) the Encke Minima -- coupled with a bit of wishful thinking. I am not suggesting that you are lying, merely mistaken.

Best,

L.

[croweater]

Thanks Les. That is a good read mate. I also didn't realise that the Encke Gap (according to wikipedia) is caused and kept clear by the small moon Pan (about 30 km across).
Cheers Richard. :)

[ngcles]

Hi Alex & All,



Firstly, I'd like to see the source (anecdotal or otherwise) for the statement:
" ... the actual resolution capability of a scope can be 10 to 20X finer than the Rayleigh or Dawes limit."

Secondly, the assertion "When it comes to extended objects, there is no diffraction pattern at play, no Airy disk - the possible diffraction pattern is totally disrupted, and the possible resolution limit is much, much finer." I am sorry to say, is incorrect. The level of detail observable on a planetary disc in an unobstructed telescope is directly proportional to the size of the airy disc it produces. Because the visual image of a planet is a mosaic of cheek-by-jowl airy discs.

Larger telescopes (given consistent quality optics) produce smaller airy discs = more detail (given equal contrast elements). Add a central obstruction (one type of contrast element) to any given aperture and more light is pushed from the central dot of the diffraction pattern and into the surrounding diffraction rings -- contrast drops as the size of the central obstruction increases. Add in other poor contrast elements like internal reflections/scatter and dust on the optics and seeing and the image is further degraded.

Keep the central obstruction small (ie less than 20%) and its diffraction effects (in transferring light from the disc to the rings) are quite small to negligible. Once you pass about 25% (rule of thumb) it begins to become noticeable on nights of very good to excellent seeing -- all other things being equal.

This is quite simple and well established physics.

This is fundamentally why, telescopes with large central obstructions show less visual contrast in planetary images compared to those with a small, very small or better no central obstruction -- inch-for-inch of aperture. This is why the instrument of choice for dedicated planetary visual observers, inch-for inch will always be a well made refractor that has no central obstruction and inherently very high contrast features.

This is also why, given excellent seeing and thermal stability, there is no rule, ever; that a good little telescope can beat a good big telescope (assuming equal quality of optics).

The good big telescope will always form smaller airy discs and show more detail visually on planets (given equal contrast elements) than any good small telescope.

Best,

L.

[glend]

Well, certainly some things to think about for those of us hoping to observe or image the Encke Gap in the lead up to opposition on 21 July.

[mental4astro]

A source?

Herein lies the whole problem with the understanding of resolution with telescopes. So many of us get caught up with statistics and the orthodoxy, but so few question things or allow themselves to make the connection between what the orthodoxy "says" vs what they are actually seeing - or photographing. We also believe that the gear we have spent hard earned dollars on is the bees knees, & "if I can't see it, then no one else can".

The source of my claims and that of many others in this thread is our own eyes. I very much hear the arguments put by the skeptics, but there it is, the Encke Division in the eyepiece. And at the same time it isn't in other scopes of larger aperture that are immediately beside the scope that is showing it. And not just one pair of eyes, but many.

I also started from the point of wrongly "believing" that the smallest detail I could see through a scope was predicated by the Rayleigh or Dawes limits. But I noticed that I was seeing much finer detail than these limits - the Cassini Division through an 60mm scope as an easy tangible example. And a 60mm scope is quite capable of showing finer detail than the Cassini Division. How many people have actually rationalized this for themselves, that the Cassini Division is finer than the Rayleigh Limit for a 60mm scope?

Then while studying and sketching the Moon I firstly began to be amazed by how fine a detail I could pull, but then followed it up with simple high school trig to work out that what I was seeing was pushing the resolution limit of my scope exactly to 1/10 and finer of the Rayleigh Limit.

Trust your eyes and do the maths.

I can easily resolve details below 500m on the Moon (as always, when conditions allow). But let's just take 500m. On the 10th of May when I did my last sketch of the Moon it was at 2am in the morning at 363776km.

At this distance, that 500m detail is 0.0476" in size. Oh, and guess what, the Encke is 0.045"... Hmmm.

For a 180mm Mak, Dawe's and Rayleigh limits are 0.64" & 0.77" respectively. This is the smallest aperture that I've seen the Encke Division with. The 9" Mak, Dawe's and Rayleigh limits are 0.51" & 0.61". With both scopes, both the Encke and a 500m lunar feature are less than 1/10 of the Dawe's and Rayleigh limits. 1/17 of the Rayleigh with the 180mm.

There's my proof.

The Rayleigh Limit is a value that has to show a very specific figure 8 shape between two Airy discs. That pinch between the two airy discs is much, much finer than the angular separation between the two airy discs. Rayleigh's Limit is not and never was the finest detail resolvable through a scope.

[glend]

I have been doing some reading on the subject of Diffraction Limited optical systems, and ran across references to Extending Numerical Aperture, which is achieved in Microscopes via side illumination, which can improve resolution by a factor of almost 2x. My source is here, good old Wiki:

https://en.m.wikipedia.org/wiki/Diffraction-limited_system

If extension of aperture is workable in Microscopes, why not planetary observation, where illumination of the rings comes from, not just direct solar lighting, but reflection from the planetary body, which for Earth based observers, is effectively side illumination?

So moving to a planet albedo, there are two kinds, the directly lit one, and the reflected one, or Geometric and Bond. In the case of Saturn, it's Geometric albedo is nominaly 0.50, and the Bond albedo is 0.34.

Can the existence of this reflected albedo (Bond) improve our ability to resolve the Encke Gap, beyond that available through the diffraction point limit of our telescope?

[N1]

It appears the argument isn't so much over whether you can see/detect finer detail than the telescope resolves as defined by Dawes and Rayleigh (yes you can) but rather how much finer, exactly. And that it seems is determined by contrast, and observer acuity.

The notches between two partially separated airy disks are an artifact, like the airy disks themselves, they do not exist on the target and so cannot be "resolved" by the telescope in the classical sense.

[ngcles]

Hi Alex and all,

I can only conclude from this that the dozens of expert professional observers (all astronomers back then were expert professional visual observers before the age of astrophysics and lived or died on their visual acuity), using some of the finest giant refractors mostly much, much larger in aperture than a mass-produced 7-9" Mak, were clearly a bunch of mugs in failing to detect the Encke gap between the 1820s and 1888.

More than that, it's also clear that presently or in the recent past, a large number of highly experienced amateurs who have commented on this and other forums upon similar claims (I'll include myself in that count) with high-quality, considerably larger aperture telescopes that detect the actual gap on a shrinkingly tiny number of occasions -- quite often never, are similarly either using rubbish telescopes, observe in crap conditions or are similarly, mugs.

On my own visual acuity, it's pretty well known I had cataract operations back about a year ago. Before that (back in my 30s and 40s) my vision regularly tested at 6/4 (I had them tested every other year) -- somewhat better than most of the population in detecting fine detail and without glasses. I can see detail at 6m a person with "normal" vision can see at 4m. ie somewhat, if not substantially better than average. I used to enjoy the game with optometrists when he/she said "read the line nearest bottom of the chart you can see", I would read the manufacturer's name at bottom. Two weeks after my operations I tested at the eye-surgeon's rooms at ... 6/4. Three months after that (November 2019) at the optometrist -- ditto.

I'm sorry Alex, I'm not suggesting you are lying, but I would respectfully suggest that what you are claiming to detect here here is the Encke Minima, not the gap. You are misinterpreting what you are seeing in the eyepiece and mistaken. This is no stigmata -- think of all the highly credentialled individuals over the past few hundred years who have made what turned out to be "spurious" observations. There's a truck-load of them.

Some "eye-opening" reading that may help you "resolve" what you are observing and describing.

Again, just for abundant clarity, I'm not suggesting you are lying or falsifying your reports, merely mistaken.

http://articles.adsabs.harvard.edu//full/2011MmSAI..82..225B/0000230.000.html

Best,

L.

[N1]

Hi Les, interesting reading, and thanks for clarifying that



I am sure that, similarly, Alex isn't suggesting



I don't necessarily agree with Alex et al. that trying to observe the Encke Gap in a 7-9" optic is a particularly good way to establish whether that optic is good or not, because it might just take a couple of decades for the seeing to eventuate that would permit an observation that is beyond doubt - in any optic. That's certainly the case where I live. I wouldn't want to wait that long, especially if the scope is still under warranty :)

On the other hand, I would not consider it completely impossible for a skilled visual observer like Alex to detect the drop in surface brightness in the region of said feature, even if it can't be resolved.

[mental4astro]

Those early astronomers were certainly no mugs.

HOWEVER, we need to consider the equipment they were using, and not romanticise it.

The gear used were doublet achromats, not reflectors, and uncoated. They were not made using exotic glass types, though of the best glass of their time. Couple this with uncoated eyepieces made of the same glass types as the objectives. Sure these scopes had massive aperture, but they were severely handicapped in terms of resolution, contrast and colour rendition/correction. Yes, those scopes were very well made, but they had severe limitations.

It is not surprising that it took until the 36" Lick refractor to show the Encke. It was finally the 36" Lick refractor that was of sufficient optical quality to show the Encke, and not before. Aperture actually has little to do with it.

And this is where too any of us get hung up on aperture, aperture, APERTURE to see the Encke. And this is why a well made 8" MODERN scope, with MODERN coatings and MODERN eyepieces can outperform say a 24" 1880's professional refractor for resolution, colour rendition/correction and contrast.

We can see this just examining sketches of from the 1880's and older that used scopes other than the 36" Lick refractor, with high quality contemporary sketches. The ability of the respective artists is a constant, but what each could and can see through their respective telescopes at high magnification has changed, be this Saturnian or lunar or other planetary works. Improved manufacturing and materials has improved contrast, resolving capabilities and colour rendition in modern scopes that today can still be seen in those old scopes as being lacking using their original eyepieces.

Mirko and others have hit on the fundamental reason why the Encke can be seen in say a 7" or 9" scope, and a big aperture dob can fail - contrast.

It comes down to the signal to noise ratio - signal theory. How easy it is for a finitely thin line to be seen against a bright monochromatic background. If the background is too intense, that black line will be overwhelmed for our eyes to be able to see it. How our eyes work is also an important factor here.

High magnification is also an important element in this. It improves the signal to noise ratio to allow for finer detail to be seen. Much like increasing magnification improves contrast with DSO's. However, for a big dob, 360X, 400X, even 500X may not be enough magnification to improve the signal to noise ratio for the Encke. You may need to push things to 600X or more.


If you have a big dob, try using a filter to help attenuate the brilliance.

If you would like me to stump up the relevant maths to support the above, just say the word, :thumbsup:

Alex.

PS: a well made 36" dob will kick the teeth in of the 36" Lick refractor.

[billdan]

I'm not sure the date when the Encke division was discovered, but Will Hay alluded to it in his book of 1933.

Will Hay was a British comedian that did a lot of movies in the 1930's, but he was also a pilot (he trained Amy Johnson to fly) and a passionate astronomer. He had a 12 inch reflector and including the mount weighed 1.2 Tons.

He made headlines around the world for being the first person to see a large white spot (storm) on Saturn.

In his book (photo attached) he says that there is a division in the outer ring that can be seen occasionally.

[ngcles]

Hi All,

At the risk of making this an endless battle of opinions (I wasn't going to add further posts to this thread), I'd offer just one other solid fact that bears closely upon whether it is truly possible to observe/detect/see the Encke Division itself as opposed to the minima.

That portion of the "A" ring that lies outside the Encke Division at best measures 0.5 arc-seconds in angular diameter -- somewhat less (about 25% less) than the angular width of the Cassini Division that separates the "A" and "B" rings.

If the angular resolution capability of the telescope employed is higher than 0.5 arc-seconds (for the record 7" is 0.64") the simple effects of diffraction will blend the division into the darkness at the outer edge of the "A" ring, resulting only in a diffuse dimming of the ring edge, rather than the visibility of a narrow division. It matters not how good the observer's eyes are, if the 'scope has insufficient aperture, the wave nature of light will prevent the detail from being viewed.

I'm sorry, but I remain firmly unconvinced that the Encke Division is actually observable/detectable/see-able in apertures less than 25cm (more likely 28-30cm): Even with a perfect (unobstructed) optic set (these don't exist), in perfect seeing and assuming the observer has high visual acuity. Don't blame me, blame physics.

As Foghorn Leghorn said to the the young chicken hawk: "Son, I say son (listen to me when I'm talkin' to yer), you can argue with me, but you can't argue with figures".

Best,

L.

[mental4astro]

Les, why do you insist on the angular resolution capacity of a scope as being the Rayleigh or Dawes limits?

It never has been. Ever. Rayleigh own work says so. HIS figures say so. No where does his work day that the Limit is the final limit of resolution of a telescope. No where.

If this is what you are saying? :shrug: Because if it is, then we shouldn't see stars as they have zero angular width.

It is an honest question as I otherwise do not follow your last post.

Let's talk rather than continue this here :thumbsup:

[glend]

In my further readings on this subject, particularly the idea that resolution and detection are not the same thing, I found this article from Baader-Planetarium website by Wolfgang Paech:
https://www.baader-planetarium.com/en/blog/the-encke-gap-in-saturns-ring-system-imaged-with-only-17-aperture/

Notice this in the article:

"At Saturn's distance in July 2018, these 300 kilometres corresponded to only 0.05 arc seconds. However, the resolution of a 17" telescope at the deep red wavelength of 685 nanometres of the IR pass filter is theoretically only 0.33 arc seconds.*So how is it possible that the Encke Gap is visible on our image?

Well, we have to distinguish between "resolved" and "detected", i.e. imaged. The resolution is defined as the separation of two point light sources. In principle, however, objects below the resolution limit can also be detected if the contrast to the environment is high enough and linear structures can also be detected more easily."

Again we come back to contrast differences, and linear objects, which have been discussed before, like in Alex's post #17, and my post on contrast enhancement.

[wayne anderson]

For me rather than debate maths, equations and optical limits I would much rather spend my time just getting out there on a very clear stable dark night and actually try to " see" what the limits of my scope and eyes are.

I may be very wrong but for years I thought Dawes and Raleigh limits are related to the ability of optics to resolve 2 similar "light point sources" and this serves as a good guide as to a telescopes ability to separate close double stars.

A telescopes ability to resolve a dark line on light background requires other considerations such as Edge Spread Function (ESF) as noted on web site below.

https://www.telescope-optics.net/telescope_resolution.htm

Extract from web site below:

.... As mentioned, this limit applies to near-equally bright, contrasty point-object images at the optimum intensity level. Resolution limit for star pairs of unequal brightness, or those significantly above or below the optimum intensity level is lower. 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.

[Rainmaker]

I think this thread has arrived at a stalemate. We need to all go out there and spend time with the scopes....

I am not into the mathematics of θ = 1.22 * λ / d but I have been viewing Saturn for many years using some very high quality optics, including 210, 250 and 300mm Mewlons, TEC MC200 and 18" Newtonians from sites all around western and northern NSW including Coolah, Dubbo, Cookamidgera, Tamworth, Coonabarabran......

I have only seen it twice, both times with 18" scopes....

[mental4astro]

I've done some more reading to clarify what the difference is between the Encke Minima vs the Encke Division. The picture below shows the two.

With this thread, make no mistake I am talking entirely about the Encke Division (or Gap). Not the minima.

What no one has been able to say who doubts mine and others observations is if we didn't see the Division, what was it that we saw? Likewise the night where multiple people viewed Saturn through different scopes where a 9" clearly showed it, but 11" & 12" scopes didn't, again if it wasn't the Division then what was it? Nor why I am able to see the Cassini Division with 53mm aperture, little lone all of you who see it with 60mm...

If you are still adamant that it takes BIG aperture to see it because it was first seen in a 36" scope, read post No. 48.


Ultimately only time under the stars will tell.

I have never shied away from putting up my scope. You know where it is :)

This test is for both visual AND photo.

PS: a little food for thought in this S & T article (https://skyandtelescope.org/observing/viewing-saturn-the-planet-rings-and-moons/)

[AstralTraveller]

I am going to put my hand up as someone who has seen some dark feature outside the Cassini division on just a few occasions. Most recently was a year or two ago (time gets away), when we had a run of exceptional seeing, with a C-11 at 280x (czj 10mm ortho) and 311x (UO 9mm ortho). I don't keep notes (naughty boy) but I also recall seeing it years ago in a 10" f8.2 newt (20% obstruction) at 275 (Celestron 7.5mm plossl).


I am not saying what I saw. My question is: how could I tell? When the Encke Division is visible is the Encke Minima also visible? Or is the minima perhaps too subtle a feature? I don't think anyone has quite addressed this point. And how sharp does the division appear? I expect it appears much more diffuse than in the images.

[UniPol]

I'm just happy to see the cassini division in my telescopes, the mind boggles when you look at pics like the one attached, try seeing all this with your earth bound scope.