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?
what's the smallest aperture that the Cassini Division can be seen with?
Alex.
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
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!
Last edited by mental4astro; 24-04-2020 at 08:44 AM.
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
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.
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
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.
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.
Last edited by mental4astro; 30-04-2020 at 07:18 AM.
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
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.
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
360X and up. You don't need 600X, but maybe with some scopes that's what it takes.
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.
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....
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.
Last edited by mental4astro; 27-04-2020 at 04:11 PM.
Reason: Typo
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:
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.
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.
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.
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:
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.
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.
