A planetary gauge to test out your scope with - Saturn's Encke Division: Encke Test
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.
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!!!
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.
Last edited by mental4astro; 29-04-2020 at 01:16 PM.
Reason: More info
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.
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
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.
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).
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).
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.
Last edited by Rainmaker; 27-04-2020 at 12:41 PM.
Reason: spelling errors
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!
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
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 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.
More comments on observing the Encke division on this thread. http://www.iceinspace.com.au/forum/s...d.php?t=177034
I remain skeptical of observations in small apertures - remember it was discovered with a 36" refractor!
“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!!!!”
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.
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.
Last edited by mental4astro; 30-04-2020 at 07:05 AM.
Reason: more info
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.
Last edited by mental4astro; 21-04-2020 at 12:01 PM.
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.
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/phys222co...iffraction.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)."
Last edited by mental4astro; 22-04-2020 at 07:58 AM.
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/s...111#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.
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.
what's the smallest aperture that the Cassini Division can be seen with?
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!
~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.
Last edited by mental4astro; 22-04-2020 at 10:05 PM.
