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Encke.....division ?

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#1 francesco italy

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Posted 29 August 2019 - 01:37 AM

Hi all from Italy!


A lot of observations of Saturn in this hot summer in Monfestino observatory with "Prometeo" refractor - Istar 250 f11.5 R35 - stopped down to 23.5, 20 and 15 cm, with and without Chromacor N :

In particular we are almost always able to clearly see the Encke Gap as a low contrast feature located about halfway out in the middle of the A-Ring, easy to spot with the big refractor.. but.. and this Is a big butwink.gif


We feel like we could see at times something more.. like a thin hair, narrow,  feature located near the outer edge of the A-Ring.


And here's THE question:


Is it possible In your opinion to detect the Encke *division* with ( of course very good ) refractors in the 8-10" range ?


I would like to share our observations with owners of big refractors, apo or achro.




prometeo 1.jpg

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#2 Joe Eiers

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Posted 29 August 2019 - 05:22 AM

 I don't have access to a high quality 8" refractor, but I have seen it with an 11" and 14" SCT - but it was in great seeing when it was nearly overhead some years back.   I'm sure I also saw it that summer in a nice C8 as well, but I really looked for and studied it in the larger scopes.  In this current apparition, it's so low for us in the USA that I haven't confirmed it in my GPS 11 with excellent optics.

  I am certain that with your refractor you should have no trouble when the seeing is good, and higher in the sky!

   Good luck!


#3 agmoonsolns


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Posted 29 August 2019 - 12:00 PM

I think I have seen it with a 6" refractor and a 7" MNT on a night of truly exceptional seeing. I know I have seen it with the 8" refractor and the larger Newtonians. Aperture is only part of the story. The seeing must be extremely stable and it's important to be able to really push the magnification. Also, position of the rings. 

Edited by agmoonsolns, 29 August 2019 - 12:01 PM.

#4 Richard Whalen

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Posted 29 August 2019 - 04:42 PM

I think what you are seeing most of the time is the Encke minima, and the thin hair line is the gap or division. I observed it with a few others back in 2003 in my 8" TEC at 827x under perfect seeing conditions. In a 10" to 12" with excellent optics and conditions it is a bit easier but still a real challenge from my experience. It takes a lot of magnification to see the actual gap, north of 700x as it is so small. At 450x to 655x the minima is not tough if conditions are excellent.

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#5 Stardust Dave

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Posted 30 August 2019 - 04:50 AM

"We feel like we could see at times something more.. like a thin hair, narrow,  feature located near the outer edge of the A-Ring."   Yup , bingo .Yes!


Late last night in 20" after the mirror settled we observed the enkles gap /division at 440X in excellent seeing.

Moments allowed 700X , fleeting glances of the planet like I'd not seen before .  It (keller's) appeared thin as a wire.

#6 luxo II

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Posted 30 August 2019 - 07:16 AM

Is it possible In your opinion to detect the Encke *division* with ( of course very good ) refractors in the 8-10" range ?


Hi Francesco, I will say yes, in theory it is possible with an extremely good scope of that aperture, but you may have a few years to wait as this requires Saturn to be very high with perfect seeing.


Last year at a club observing night, we were blessed with with perfect seeing with Saturn at 78 degree elevation. The Encke division was visible in my Santel MK91 at 750X but not in any of the other scopes which included two C11 and a Mewlon 250.  This was a very rare calm night when every object sat still, with no movement at all at any magnification.


At first I could not believe what I was seeing and had to check Hubble images and optical theory to see if this was possible.


This year I have not seen it, yet, however the seeing has been terrible.


Personal experience suggests anyone in the N Hemisphere who thinks they've seen it has only seen the minimum, not the division.  At elevations below 60 degrees IMHO it is not possible.

Edited by luxo II, 30 August 2019 - 07:23 AM.

#7 azure1961p


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Posted 31 August 2019 - 12:14 PM

You are all giving crap answers to this guy's query.


1st: The contrast in the middle of the A ring is known as the Enke Minima.  It's part real, part illusion.  The better the seeing the clearer it is.


2. At the EDGE of the A ring it's utterly ludicrous to suggest you are seeing the minima - and because you live up north, well Enke Gap just can't be.


Here's the deal:


A ten inch reflector at 400x (the lowest that works for this challenging line) is the accepted proven norm for threshold instrument requirements.  Now it's been said (over and over again) "a good 8" might do it" .    I'm the face of that , I've never read of success here with that size instrument. One of the BEST views of Saturn with good ring presentation I ever had with my 8" newt didn't offer so much as a crumb.  Still my opinions aren't made yet.  At the time of that best ever view of Saturn in my life I DID NOT use a fan to cool the mirror.  Since I've now been using that and seen the enhanced sharpness and contrast it's still plausible to me I just may see it yet at that 400x minimum.


I don't know.



A refractor here at 8" - provided it's got truly good optics just may pull it off as well.  I've got a 16% CO Newt but a 0% CO only has to be better.


I'll say this in closing - the 400x minimum seems to be unwavering in it's need.  Like resolving Io - either you reach a critical magnification limit or you don't and that is the precondition here.  The uphill challenge then is that you MUST have at least 400x but it also has to be BRIGHT enough as well.  Even at 350x the A ring in an 8" starts to look a tad shadowy and that is the aperture starting to redline it's effective use on this feature.


I for one don't doubt you saw Enkes Gap and the minima as well.



#8 Asbytec


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Posted 01 September 2019 - 08:45 AM

Yes, it's generally accepted a 10" aperture is required to observe Enke division based on the visibility of Cassini division in a smaller aperture. A more empirical study suggests we can see such features down to 1/15 Rayleigh limit. The Enke division is smaller than 0.05" arc and may be as thin as 0.03" arc. The larger figure puts it just within reach of 1/15 Rayleigh for an 8" if observing conditions and scope preparation are nearly perfect for viewing it. Chances are, even in the best seeing found near the zenith, the sighting will be fleeting at best.

But, being generally accepted is more of a rule of thumb with some wiggle room than a hard floor on resolution. Surely we all know how fickle extended object observing can be and it may involve the observer's acuity as much as the telescope's imagine properties. If it's resolved on the focal plane (as it can be in good digital images), the next challenge is to detect it visually. So, as different as imaging may be to human acuity, a good image of Enke at least shows the feature is on the focal plane and we might stand a chance of catching it. So, it may be a good idea to see if any 8" apertures managed to image it. If so, then just maybe a human can do it, too.

Another Study put's linear high contrast features with a contrast of k = 0.95, like Cassini, at 43/Dmm or about 0.2" arc for an 8" aperture in 1" arc seeing. That's not nearly enough, in theory and in practice, to spot Enke division. The Enke division may be a bit higher contrast, say k = 0.99. So, that study seems to suggest an 8" aperture in about 0.5" arc seeing (Said to be the best on the planet) can resolve high contrast linear features down to about 12/Dmm ~ 0.06" arc. Tantalizingly close. So, maybe...just maybe an 8" can do it.

To my way of thinking, it's been shown a 10" can show Enke division, an 8" might be able to do it under the best conditions found higher in the sky, and a 6" mostly likely not at all. However, a 6" and larger can easily show the Enke minimum when conditions permit at a good magnification. There is actually a slight dimming in the middle of the A ring relative to the inner and outer edge, but the dimming is not that strong. So, there may be some illusion associated with it, yet we can see it regardless. The Enke minimum might appear as a thin linear feature, but to my eye it's more of a broad dimmer space.

Optical quality may matter, so long as the scope is not terrible. Dunno how good it has to be, but at least good enough for high magnification and sharp images. Better is better, they say. However, both can form an Airy disc, yet Airy discs have little to do with seeing Enke or any extended feature. If they did, the Airy disc would have to be exceedingly small for both Enke and Cassini. However, diffraction still causes a loss of contrast and image contrast is required to see it. Interestingly, though, an obstruction might be able to assist in observing Enke because of the increased MTF at very high spatial frequencies with the maximum spatial frequency potentially being greater than 1 by a factor of 1/(1 - co^2)^2. Enke is definitely a very high spatial frequency feature. And it's at the very edge of the ring system which may present some problems, too, requiring some high magnification to separate it from the edge.

Edited by Asbytec, 01 September 2019 - 10:42 AM.

#9 azure1961p


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Posted 01 September 2019 - 02:51 PM

LOL, so it goes like this:


10" @ 400x is the accepted minimum.


8" - even if you do see it - NOBODY will believe you anyway.


The real problem is the A ring is too dim. That's a real player in the 8" argument against seeing it.  1/15 the Rayleigh ...I'm guessing that equals 1/11 Dawes?.  I'm basing that on the Harvard study where they observed a human hair at some crazy  distance.  


Well if I ever do it - you'll be the first to know?



#10 Asbytec


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Posted 01 September 2019 - 04:03 PM

Pete, it's a hard one for sure, and hard to get folks to believe such a difficult observation in an 8" aperture without staking your credibility on it. Otherwise folks would have reported it more often, it'd be more generally accepted. I forgot about the Harvard study, that's a good data point. I also did not consider the A ring brightness, not sure how that plays a role. Lower contrast? If so, then yea, that makes it more difficult, especially for the eye. Plus, it borders a slightly brighter ringlet at the edge of the A ring, that may play a role, too. 


Here's an example of what we might perceive or what to look for.



Browsing some 8" images of Saturn, so far none have managed to image it. But, the conditions where it might be seen are so very rare. Less rare when you stack a bunch of lucky snapshots, but still not an easy target. Maybe Damien Peach can do it. 


Good discussion.



In the image above, notice the Enke minimum which is interesting in itself. It kind of speaks to the resolution we see in the rings. For example, we normally see a bright ring in the B ring bordering the Cassini division, but there is really some dimmer rings between them. The same seems true of the Enke minimum, we see a broader stretch of rings as a thinner dark area bordered by two brighter ringlets. I'd love to resolve the bright rings in the B ring for what they really are.


In order to image it, all we need is a slight drop off in each pixel along its length. The enhanced contrast beyond the visual level, which probably does not bode well. 



More from that discussion.




"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."




Unfortunately, no mathematical approximation is mentioned to guide us. If it is possible, visually or imagery, it needs to be reported from high in the sky in the southern hemisphere with the wide ring tilt. So, we still have to just go look and believe our lying eyes. Or someone else's. smile.gif


I thought I saw it in my 6", but I am glad I was open minded despite the excitement of thinking I may have seen it years ago when Saturn was better placed closer to the zenith in very good seeing. I'm glad I recanted my claim to it and wrote it off as a diffraction or seeing effect. I'm convinced a 6" cannot do it, but open minded enough that maybe once in a lifetime...or two. Effectively impossible. 

Edited by Asbytec, 01 September 2019 - 04:07 PM.

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