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#26 buddyjesus

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Posted 17 April 2013 - 10:43 PM

beautiful picture!

#27 John Boudreau

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Posted 18 April 2013 - 07:20 AM

Amazing how light the hex is in that image. Ok back on topic...

That image shows the south polar region--- Only the north polar region has a hexagon feature. That image was probably taken about 10 years ago. The north polar hex does show in HST images taken in the early 90's though.

#28 David Gray

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Posted 18 April 2013 - 08:06 AM

I've seen the Encke Gap with my past 20" f/4.3 Zambuto Starmaster dob during the winter of 2006 when Saturn was in its northern-most apparition in the sky and the rings were open all the way. I was bino-viewing at about 500X with Saturn directly over head. The Gap was seen steadily as a fine black line with a contrasty appearance, not the gray, blurry and thicker line shown in the photos. It was a lot thinner appearing, but very black. See attached NASA image. The Gap appeared more like 'this' image -- very thin and black.


It used to be said that 20" was about the aperture to get Encke as a true black gap.

Looking back through my old observations the darkest I got it, in excellent conditions, was intensity 9 which roughly equates to 90% black but still a little diffuse - so perhaps my 415mm D-K is on the edge!

#29 azure1961p

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Posted 18 April 2013 - 08:42 PM

John,

Thanks for the clarification. I didn't realize it was pole specific. How odd.

Pete

#30 Mark Harry

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Posted 21 April 2013 - 05:48 PM

Once for Encke, and once for 8 Trap stars 3 years ago- Both times with 8" F/6, and the UO VT 9mm Ortho. Fantastic eyepiece. Conditions have to be essentially perfect; the least disturbance makes them disappear.
M.

#31 Asbytec

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Posted 21 April 2013 - 09:38 PM

8 Trap stars in an 8"...wow, well done. :)

#32 azure1961p

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Posted 21 April 2013 - 10:02 PM

8 Trap stars????????

Ok there goes a gauntlet!

Pete

Ps: I agree on the UO volcano top 9mm - an excellent ocular.

#33 David Knisely

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Posted 22 April 2013 - 01:11 AM

Once for Encke, and once for 8 Trap stars 3 years ago- Both times with 8" F/6, and the UO VT 9mm Ortho. Fantastic eyepiece. Conditions have to be essentially perfect; the least disturbance makes them disappear.
M.


The Encke "minimum" in a 8 inch I would believe (especially at lower powers). However, seeing the actual 325 km width division in that aperture is problematic at best due to diffraction effects between the division and the darkness at the edge of the A-ring. Even under the best conditions, the division is just too close to the darkness at the outer edge of the A-ring (0.496 arc seconds from the edge at mean opposition). For this reason, I may have some doubts about claims for visibility of the gap itself in only an eight inch aperture, although I won't dismiss them entirely. Clear skies to you.

#34 Mark Harry

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Posted 22 April 2013 - 05:48 AM

Read EDZ's discussion on the link.(?)
***
Oh well, folks have been telling me I can't see squat for decades anyhow. Up to 2 years ago, I had 20/10, and almost reached senior citizen status with it. I feel fortunate. May your skies be clear as well.
M.

#35 Mark Harry

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Posted 24 April 2013 - 01:36 PM

No offense meant; almost as if this thread has shut down. Lets just say, I'm trying to grow old as gracefully as possible!!!
M. :gramps:

#36 azure1961p

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Posted 24 April 2013 - 01:45 PM

Mark,

I don't think any offense was taken. Its merely the momentum or lack there of in the topic. Without having seen it myself I don't think its impossible merely exceptional to rare.

Pete

#37 Schaden

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Posted 30 April 2013 - 11:09 AM

I saw Enckes in my Nexstar 8 last weekend. The seeing in Phoenix was phenomenal. Cleardarksky predicted 4/5 in the early evening and 5/5 after 1am for transparency and seeing. I was using an 11mm es82 barlowed to 360 x.

I've never seen it look so good at that high of power. It revealed more detail than 180. Don't know the name, but I saw the crisp shadow of a large lunar crater. Inside I could see the smooth wall that looked like a giant sand climb challenge and an endless array of tiny craters everywhere, they emerged into view with the higher mag, the seeing was superb. I used one extremely tiny one to focus on, the inside walls only visible momentarily.

When I saw Saturn it immediately looked better than it's ever looked in the C8 or Z10 before or since. The planet's bands were vibrant. The rings were tack sharp. I could see shades of contrast within each ring. I could see the rest of the A ring around Enckes, the gap appeared as a thin black line out on the edge of the A ring, flickering in and out in clarity from minute to minute but no doubt visible with direct vision.

That taught me seeing is the single most important variable in planetary observing. I got an amazing view from a light polluted city with a nearly full moon.

#38 David Knisely

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Posted 30 April 2013 - 03:07 PM

Despite excellent seeing, I have never seen the Encke division in my Nexstar 9.25GPS XLT at any power. Seeing has sometimes caused a "doubling" of the outer edge of the A-ring that mimics a division there (especially if I have any tube currents in the scope or I have not used my Cat Cooler enough). However, once seeing settles down, the effect vanishes. I often use the visibility of Enceladus to check on seeing, as if it isn't rock stable, that faint little moon is often not seen in the 9.25. Indeed, my sighting of Enceladus so easly in the 9.25 when I was reviewing the scope for my Cloudynights review of it was one thing that convinced me that the scope was of excellent optical quality (and I eventually ended up buying the instrument). Clear skies to you.

#39 Schaden

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Posted 30 April 2013 - 10:36 PM

I didn't see it until my scope had been outside for 2 hours. There is no doubt in my mind it was Enckes, it wasn't a mirage of Cassini. I think you just need perfect seeing. That was the best night I've had in the last 4-5 years and approximately 100 sessions. I regret not staying up later but I was dead tired at 2am. The website had predicted 5/5 seeing again the next night but when I checked again the next afternoon it was only predicting an average 3/5 night. So I feel lucky to have had the hour of perfect seeing. It really was an exceptional night, knowing what my scope is capable of doing given the conditions is a good thing, but it's depressing to think of going out on an average night now, given how much I know is missing.

#40 azure1961p

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Posted 01 May 2013 - 05:41 AM

I agree that when truly pristine seeing comes in its a revelation and you finally get to see all the precision in your optics at work - its awe inspiring! Too, as you mention, it is a but of a let down when you realize what you've been missing!!! Thanks for your account here. It sounds like your cass is a keeper.

Pete

#41 Mark Harry

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Posted 02 May 2013 - 04:00 PM

With discussion with one of the participants. it kind of "P"d me off; with the condescending attitude of the questioner brought to the table.
******
I have been labeled as having 20/20 vision by a highly regarded optometrist;.
The discussion with the questionable respondent, commented the impossibility of seeing the small division; in MOA of the object in question. I never got a response in what the rez his particular eyeball was capable of. It didn't confirm what I know of optical theory, etc.
This got my 'interest' so I decided to make a SWAG.
While "blessed" with 20/20, I noticed I could discern (this last weekend; repeatedly and confirmed by another observer) a 1/4-5/16th" -LINEAR- defect at approx 175 yards, which would equate to ~ .17 MOA @ 100 yards: ~ which equals the rez @ 100 yards.
This is what a supposed individual with 20/20 vision could discern at that yardage.
FWIW, the width of angle @ 175 yds equates to .143 MOA at 100 yards. This partalleled exactly what ED Z quoted in his discussion a few years ago in a reference thread.
The individual has failed to reply to repeated queries about detecting this resolution. He also publicly doubted what I could have seen with an 8" aperture.
His insistance, I could not detect anything smaller than 2-3 MOA........Really???
-----------
Now, I wonder who's word that would appear as being acceptable???????
Don't ask for the name of the condescender. I will not give it. But this brings up another point.
********
"excellence is the enemy of the 'good'."

I know what I saw.
My only fault; I did not record the observation of Encke. I don't have the specific dates. But I do recall it was visible in my scope; and it wasn't in a particular Russian Mak.
(SCT's- don't even apply.)
M.

#42 azure1961p

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Posted 02 May 2013 - 05:50 PM

Hi Mark,

Thanks for your input here . Its a discussion after all so it will have views or opinions on both sides of the fence and some like me sitting in the middle (ouch). I appreciate your accounts here and weigh the other remarks by valid observers as testament to the exceedingly difficult nature of this feature. I'm not slathering it on - I have belief or faith it can be seen with an 8" though I've failed - in great seeing- and with s great system. Your scope sounds like its top wrung and your observations were careful.
I found your contribution enlightening and well explained.

I am impressed with your naturally good vision, I wear contacts. However since the scope focuses past the effects of near or far sidedness, I'm not sure its a defining attribute, like , no astigmatism would be.

Again thank you for your contribution to the thread.

Pete

#43 David Knisely

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Posted 03 May 2013 - 01:46 AM

Mark Harry posted:

While "blessed" with 20/20, I noticed I could discern (this last weekend; repeatedly and confirmed by another observer) a 1/4-5/16th" -LINEAR- defect at approx 175 yards, which would equate to ~ .17 MOA @ 100 yards: ~ which equals the rez @ 100 yards.


OK, a single dark line against a light background that was 1/4" across at a distance of 175 yards would subtend an angular diameter of about 8.2 arc seconds. Detecting that line at that distance with the unaided eye would be a fairly decent feat but definitely not even close to being the best that is possible for a good set of eyes. Indeed, Sidgwick (Amateur Astronomer's Handbook, 2nd edition, p. 430-431) mentions observers with good eyesight seeing a longer linear feature like that down to an angular width of around an arc second or so. In fact, a few observers with great eyesight have gone as small as a width of only 0.44 arc seconds (Barnard, using a wire against the daytime sky).

In telescopes, thin linear features have been detected having widths much smaller than standard resolution figures such as Dawes Limit or the Rayleigh criterion. This can be shown by the fact that Cassini's Division was discovered in a telescope (2.5 inches of aperture) that does not have enough resolution to resolve the angular width of the division. Indeed, in that same book (Sidgwick, p. 50), it is noted that detection tests managed to show a linear feature that had an angular width of as fine as 1/14th to 1/15th that of the usual Dawes figure for that aperture.

Encke's division has a physical width of 325 km, which at mean opposition distance, would mean an angular width of about 0.052 arc seconds. This would mean that considering the division as a linear feature and using 1/14th of the Dawes Limit for an 8 inch (0.041 arc seconds), *if* the division was in the middle of a white background with absolutely nothing anywhere near it, it should at least have the potential of being detected in an eight inch aperture.

However (you knew that was coming, right? :)), the problem is that the division is *not* isolated. It sits on a light grey background only around 0.5 arc seconds from the massive darkness at the edge of the A-ring. This dark edge would act as a second (and much higher contrast) linear feature sitting right next to the division. In that case, one would have to consider the resolution limit of *two* linear features and not just the detection of an isolated feature. For that, we can again refer to the experiments quoted by Sidgwick. Treating the division and the outer edge of the A-ring as "parallel lines" is somewhat more realistic here than just using one of the standard resolution criteria or a mere detection criteria. To quote Sidgwick (from AMATEUR ASTRONOMER'S HANDBOOK, 3rd edition c. 1980, Dover, Section 2 (Telescopic Function: Resolution), p. 50) where R' is Dawes Limit (4.56/D):

"C. PARALLEL LINES ON A LIGHT GROUND

(i) W. H. Pickering: minimum separation for resolution with a 10-in reflector was 0".63 (1.4R'),

(ii) A similar performance was given by the Arequipa 15-in, which resolved a pair of parallel lines when their separation was increased past 0".42 (1.4R', in good seeing. Slight atmospheric deterioration immediately raised the threshold to about 2R'. At less than 0".42 the lines appeared as a grey band of width about 1-1/2 times their separation.

(iii) Resolution of the lines at 12" arc with 0.4 in OG (1.1R'). See also sections 2.3, 24.6, 26.7, 26.9"


Having too small an aperture resulted in the two lines merging into a diffuse band rather than being resolved as two clearly separate features. Using the most "liberal" (iii) limit of 1.1R' and the 0.5 arc second mean separation between the Encke division and the edge of the A-ring, the aperture required for such a resolution of parallel lines would be about 10 inches. Using apertures much smaller than this would result in the Encke division being basically "swallowed-up" by the diffraction effects caused by the presence of the darkness of space at outer edge of the A-ring. This is the main reason I may be somewhat dubious about the prospect of seeing the Encke division in apertures much smaller than around 10 inches, although as I clearly stated before (and in a proper and civil manner), I won't exactly rule it out. Clear skies to you.

#44 Mark Harry

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Posted 03 May 2013 - 08:01 AM

This makes an interesting bone to worry.
****
Still curious, I took a scope that was apertured to 50mm, and of F/12-600mm focal length. In it, was a 42mm eyepiece for a mag of 14.3x. A refractor might not be the best type to use, but it was handy; regardless that it (achromat) will only focus perhaps green the best and the other 2 main colors would smear with any substantial magnification.
In my PMs, I mentioned I could see 1/4" spaces between white painted planks at 100 yards. One of the spaces is actually 1/8th inch. so I think that's pretty good.
1/8 MOA =7.5 arc seconds. Eye pupil was around 3mm; 1/8".
There are also 1" dots at that range, and they are plainly visible. A smattering of .22 bulletholes are for all intents, invisible by eyeball.
The refractor brought all the holes out plainly.
There are surface checks on the surface, and ends of the planks. .010" ones are easy to see, and .005" are regularly seen with little trouble. Even the glint off a spiderweb was visible at the same distance, but I have no idea of it's actual width. Suffice to say, I think it's realistic to estimate it as half a thousanth (.0005", but likely significantly smaller than that!) I will use .0005". This latter item figures to .03 arc seconds. Hmmm- with a 50mm aperture.
Well let's see what that would be with a 200mm aperture.(~8"):
.03 x 50/200 =.0075 arc second.
***
Now, if Encke is .052 arc second.......
If atmosphere cooperates, there should be no problem seeing Encke with 8": -IF- the .052 arc-second figure is correct.
******
It was 38 degrees this morning; right at sunrise. The refractor was working very well. The sun started to show on the ground a distance in front of this plank wall; at a very acute angle. Seeing deteriorated almost immediately. The .005-.010" surface checks took on a fuzzy characteristic.
And I think the most important thing; I didn't have to read a thing, or dig up any kind of source material. Just go out, do a little looking, measuring, and get the real deal.
Another aspect, is the discrepancy found in looking for essentially point sources, vs linear ones. Quite a difference. If I was to hazard a guess, a 1/2" dot at this 100 yard distance would be quite taxing by eyeball alone.
****
One can only surmise that a smooth accurate reflector with zero color can have the magnification raised without the degradation that goes along with a simple achromat. I used only the combination I outlined above, along with a 2" erecting diagonal to give the refractor a chance to show as little color error as possible. The Newt I had suffered from none of these issues. With a bit more "colorless" power applied, I imagine linear features could be detected 2-4x smaller.
Well, that's about all for my little experiment.(or rant, which depends on just what side of the fence you're on.)
Regards,
M.
PS: might be interesting to hear what other folks might find out-

#45 David Knisely

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Posted 03 May 2013 - 01:00 PM

Mark Harry wrote:

If atmosphere cooperates, there should be no problem seeing Encke with 8": -IF- the .052 arc-second figure is correct.


That 0.052 arc second figure for the angular width of the Encke Gap is indeed quite correct for a mean opposition distance of Saturn (1277.42 million kilometers). You take the physical width of the Encke gap (325 km) and divide it by the opposition distance to Saturn. Then, you take the inverse tangent and convert the result into arc seconds. Again, however, you seem to be continuing to miss the point that the division is *not* sitting all by itself. The division has a diffraction structure at apertures too small to resolve it, with a central maximum darkness at its precise geometric center and a gradually reducing darkness the farther away from the center that you look. Diffraction basically "softens" the division from a hard thin line to a soft broadened band. That kind of diffraction structure is also present on the edges of the A-ring. These two diffraction structures will interact if they are close enough together. As I clearly explained earlier (both in PM and here), the telescope must have sufficient resolving capability to clearly separate the division from the edge of the A-ring. Eight inches is just a little on the small side for this to occur. This has nothing to do with visual acuity, but concerns the physics of light. Clear skies to you.

#46 Mark Harry

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Posted 03 May 2013 - 04:25 PM

And -YOU- are -NOT describing the true situation presented, either. (now, or before) It's not just 2 simple parallel lines.
Quote all you want, and argue until blue in the face. I know what I saw, and have an idea how to determine seeing conditions, quality of optics, what can be seen, etc.
Good day to you,
Mark

#47 leviathan

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Posted 03 May 2013 - 04:26 PM

Just got it today for the first time in my 8" SCT at 30-37 degree elevation and rather good seeing, at 330x and 440x.

P.S.: I'm talking about Encke's minimum.

#48 David Knisely

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Posted 04 May 2013 - 01:46 AM

At least you got to see it (the planet and the minimum). I just wish the weather would cooperate for once. It has been cloudy, cool, and rainy for weeks (even had snow day before yesterday). The last time I was preparing to get the scope out for Saturn a few nights back, by the time I got things ready and went back outside to get the first piece of equipment out, the sky had completely clouded over. The forecast looks like it will be Monday at the earliest before I can get another look at that planet. Sheesh, what a spring! Kind of makes you wish you were getting the view the starship below is getting. Clear skies to you.

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#49 Schaden

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Posted 04 May 2013 - 08:18 PM

What I saw in the A ring looked like that picture posted by Peter Natscher. Enckes appeared as a black line, very similar to the way Cassini usually looks but thinner and fainter. The A ring looked the same on each side of it. Although the A ring did change as it approached Cassinis. And the B ring had a lighter and darker section within it too. Prior to that night, I had only been able to notice a simpler contrast between the different single shades within each the A and B. The cloud bands stood out too. They seemed more vibrant than usual, each had a distinct color instead of the normal very subtle differences between them.

#50 greedyshark

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Posted 05 May 2013 - 12:02 AM

I observed Enckes on two occasions in the Summer of 2001 using my superb orange tube C8. Outstanding seeing (rare). Using a UO 5mm Ortho. I've never seen it since. A view forever burnt in my memory.

Charles






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