Return to the Cloudy Nights Telescope Reviews home page


Observing >> Solar System Observing

Pages: 1 | 2 | 3 | 4 | 5 | (show all)
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Enckes new [Re: azure1961p]
      #5836745 - 05/02/13 05:00 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Enckes new [Re: Mark Harry]
      #5836987 - 05/02/13 06:50 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
David Knisely
Postmaster
*****

Reged: 04/19/04

Loc: southeastern Nebraska
Re: Enckes new [Re: Mark Harry]
      #5837605 - 05/03/13 02:46 AM

Mark Harry posted:

Quote:

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):

Quote:

"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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Enckes new [Re: David Knisely]
      #5837846 - 05/03/13 09:01 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-


Post Extras: Print Post   Remind Me!   Notify Moderator  
David Knisely
Postmaster
*****

Reged: 04/19/04

Loc: southeastern Nebraska
Re: Enckes new [Re: Mark Harry]
      #5838451 - 05/03/13 02:00 PM

Mark Harry wrote:

Quote:

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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Enckes new [Re: David Knisely]
      #5838782 - 05/03/13 05:25 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
leviathan
sage


Reged: 11/29/11

Loc: Azerbaijan
Re: Enckes new [Re: rgm40]
      #5838784 - 05/03/13 05:26 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.


Post Extras: Print Post   Remind Me!   Notify Moderator  
David Knisely
Postmaster
*****

Reged: 04/19/04

Loc: southeastern Nebraska
Re: Enckes new [Re: leviathan]
      #5839496 - 05/04/13 02:46 AM Attachment (16 downloads)

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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Schaden
sage


Reged: 06/30/08

Loc: Sonoran Desert
Re: Enckes new [Re: David Knisely]
      #5840759 - 05/04/13 09:18 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.

Post Extras: Print Post   Remind Me!   Notify Moderator  
greedyshark
professor emeritus
*****

Reged: 10/31/05

Loc: 3rd Rock
Re: Enckes new [Re: Schaden]
      #5841020 - 05/05/13 01:02 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


Post Extras: Print Post   Remind Me!   Notify Moderator  
David Knisely
Postmaster
*****

Reged: 04/19/04

Loc: southeastern Nebraska
Re: Enckes new [Re: David Knisely]
      #5846118 - 05/07/13 05:22 PM

In experiments done with a variable aperture mask on the Cassini division, it was determined that while a 50mm (2 inch) aperture would allow marginal "detection" of the Cassini division, it took a 2.4 inch (60mm) aperture to show it more firmly around the ends of the ring ansae (and the division itself was discovered in a 2.5 inch aperture). At apertures smaller than 50mm, the division basically bended into the darkness of the outer A-ring and was not visible other than as the generally darkening of the ring itself. This shows the same sort of effect that would make the actual Encke Gap vanish if too small an aperture was used. The Cassini Division's center separation from the outer edge of the A-ring is roughly 16,950 km, so at a mean opposition distance of 1277.42 million km, that distance would subtend an angle of about 2.74 arc seconds. This is around 1.2 times the Dawes resolution figure for a two inch aperture, which confirms the idea of considering the division and the edge of the A ring as two parallel lines. Extending this again to the Encke gap, it lends observational support to the estimate that it would probably take something on the order of a ten inch aperture to have much of a chance of separating the gap from the darkness of the outer edge of the A-ring. Clear skies to you.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Rick Woods
Postmaster
*****

Reged: 01/27/05

Loc: Inner Solar System
Re: Enckes new [Re: David Knisely]
      #5846318 - 05/07/13 06:39 PM

Quote:

Kind of makes you wish you were getting the view the starship below is getting.





That's the kind of view I had one magical, slightly-overcast night with my 14" SCT and a 7mm Nagler T1 (~508x). Just like I was in orbit!
It's never happened again, but that was enough to reassure me that it's possible, and can come again!

*sigh*


Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Enckes new [Re: Rick Woods]
      #5846747 - 05/07/13 10:12 PM

That had to be very impressive Rick and the good news is odds are you'll have it again.

Pete


Post Extras: Print Post   Remind Me!   Notify Moderator  
David Knisely
Postmaster
*****

Reged: 04/19/04

Loc: southeastern Nebraska
Re: Enckes new [Re: Rick Woods]
      #5846833 - 05/07/13 10:51 PM

I had one of those nights with my binoviewer. In the 14 inch, it was like looking at 3-D model of the planet based on an HST image, but sadly, those nights tend to be few and far between. Clear skies to you.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Enckes new [Re: David Knisely]
      #5847246 - 05/08/13 07:31 AM

Quote:

In experiments done with a variable aperture mask on the Cassini division, it was determined that while a 50mm (2 inch) aperture would allow marginal "detection" of the Cassini division, it took a 2.4 inch (60mm) aperture to show it more firmly around the ends of the ring ansae (and the division itself was discovered in a 2.5 inch aperture). At apertures smaller than 50mm, the division basically bended into the darkness of the outer A-ring and was not visible other than as the generally darkening of the ring itself. This shows the same sort of effect that would make the actual Encke Gap vanish if too small an aperture was used. The Cassini Division's center separation from the outer edge of the A-ring is roughly 16,950 km, so at a mean opposition distance of 1277.42 million km, that distance would subtend an angle of about 2.74 arc seconds. This is around 1.2 times the Dawes resolution figure for a two inch aperture,
***
which confirms the idea of considering the division and the edge of the A ring as two parallel lines.

*** Extending this again to the Encke gap, it lends observational support to the estimate that it would probably take something on the order of a ten inch aperture to have much of a chance of separating the gap from the darkness of the outer edge of the A-ring. Clear skies to you.




-------
No, it does not. That is -NOT- the situation. I also know that you did not personally make this assessment; for I read it years ago as well.
***
I have a good friend the other side of town. You once made a remark about visual acuity not mattering; only the physics of light.
This guy was born legally blind. He can 'SEE' but at a much reduced level of acuity. He's not resentful of the fact, but accepts it as the hand of cards he was dealt. One of the finest mechanics I've ever known.
But I'm sure he could relate how this handicap has affected his whole life; and how he's had to make accomodations for it.
Think how it would be to never be able to posess a driver's license???
He's never said anything, but I know he would like to be able to drive; and not be reliant on someone else doing it for him.
He would be quite contented with a 1/2-1 wave error scope of any type, -IF- he could use typical eyepiece formats in use today (1.25 & 2")
--------
Back last page, I related and described a duplicateable "test". In simplest terms, I have described basic rez of the aperture in question--- It's first hand, and I have strived to make it the least subjective as possible; and understandable by anyone.
Your replies and arguements have no personal observations or information derived as close to original source as this. All they contain, is second- or third- hand knowledge; and some of this, I question the validity of such. It's anecdotal, to say the least.
****
There -HAS- to be a reason for the existance of custom optics makers the likes of Zambuto, Stevens, Dodds, Cowan, etc. A look thru the eyepiece generally is all the proof needed. The claim you have stated where there are no returns after 1/10th wave optics, is just where most of these aforementioned guys are getting down to serious figuring. People can see the difference, and are willing to pay big bucks for it.
There was recently a claim of viewing Saturn in another thread, where 200 and 400x were used. It was remarked how @ 200x, it was sharp and clear, whereas @ 400x the view was softer, and not so crisp. This brings up another point you have objected to, but others (and what I mentioned before here) have noted. Mags can work against high resolution views. Good accurate optics will provide better views at lower powers; not just high ones. Being fully saturated with light allows those who have the acuity to pick up ever-so-slight details that other less capable individuals will miss at higher powers. I believe that's the case here; for you remarked about ~400x being used and failing to see the hairline detail in question with your 9.25" (?)
I appologize for the straying somewhat from the OP, but this may be of help in successful planetary observation; and to dispell misinformation. I don't think my observation last page should be discounted just because my last name isn't Sidgewick. (sp?)


Edited by Mark Harry (05/08/13 07:35 AM)


Post Extras: Print Post   Remind Me!   Notify Moderator  
azure1961p
Postmaster
*****

Reged: 01/17/09

Loc: USA
Re: Enckes new [Re: Mark Harry]
      #5847254 - 05/08/13 07:41 AM

Mark,

I appreciate your points on these matters though in the case if the mechanic it would seem his vision is so extremely bad it doesn't apply to a simple matter of nearsighted or farsighted with the focusing of the telescope essentially doing it for the cornea wether its up to it or not. If there's retina issues, astigmatism, horrendous floater obstructions - then that's another story. At anyrated Im quite nearsighted but I can focus everything I own to unerring sharpness - tho the large exit pupil in the binos begins to reveal astig as well as my 26mm plossl.

Thanks.

Pete


Post Extras: Print Post   Remind Me!   Notify Moderator  
BillFerris
Post Laureate
*****

Reged: 07/17/04

Loc: Flagstaff, Arizona, USA
Re: Encke Division new [Re: BillFerris]
      #5848056 - 05/08/13 03:47 PM

This discussion illustrates well some of the challenges one encounters when attempting, making and reporting a threshold visual observation. Visual observing is a wholly subjective activity. Regardless of the degree to which one intends or attempts to be objective, the act of making a visual observation is ultimately inseparable from the subjective interpretations made by the observer. In many cases, this is not an issue. The handful of Saturn observations published to my Website, Saturn, are generally non-controversial. The physical details captured in my drawings are known to exist and are commonly reported by other amateur astronomers and astroimagers. As such, they don't merit scrutiny. Even if I was mistaken about observing a reported detail, an experienced observer would acknowledge that many reliable observations of that detail exist. My potential error wouldn't create an unreasonable expectation of what can or cannot be seen.

Visual observing gets more complicated as the objects and details reported lie closer to the threshold of what is considered possible given the equipment being used and the conditions under which the observation is being made. An observation of an object or detail at the threshold of visibility is, by its very nature, of questionable reliability. The observation is considered to be highly difficult and, perhaps, even of questionable possibility. If I were to report a naked eye observation of a 10th magnitude star, that report could--I would say, should--be received with substantial skepticism. Our contemporary understanding of the limits of human visual acuity do not allow the possibility of a person seeing a star that faint. The historical record does not include reliable reports by observers claiming to see a star that faint. No living amateur astronomer has ever personally seen a star fainter than, perhaps, mid-8th magnitude without optical aid. A very natural and understandable response to the claim might be, "If I can't do it, and nobody else has reported doing it and medical science doesn't support the theoretical possibility of someone doing it...why should I believe you did it?"

Therein lies the crux of the issue, "Why should I believe?" As astronomy enthusiasts, most of us value logic and objective evidence as the arbiters of what is and is not to be believed. There is no physical sensation of being on a planet rotating about its axis at 1,000 miles per hour. But the objective evidence indisputably supports the statement that Earth does, in fact, rotate at that velocity. The objective data gathered by professional and amateur astronomers, alike, are daily reminders that phenomena like novae, comets and planets orbiting other stars do exist. These are unquestionable facts of nature. And the data collected in a rigorous manner can be used to confirm or refute our theoretical understanding of the how or why such phenomena occur.

Visual observations at the threshold are much less tidy. In a civil forum such as this, we can all agree to accept that all participants in a discussion are forthright, honest folk. Whether or not we believe what someone has claimed to have done, we can at least accept that the person genuinely believes they have done it. They believe they've successfully made a threshold observation. Beyond that, it is difficult to say much more with any degree of certainty.

With regards to the current thread, we should all be in agreement that visual detection of the Encke Division is a difficult observation in any amateur telescope. Unlike the Cassini Division, the Encke Division is not an obvious feature. This razor thin division just inside the outer edge of Saturn's A-Ring was observed by James Keeler in 1888 using the Lick 36-inch refractor. It has since been observed in smaller apertures but--there's always a 'but'--reliable visual sightings in small to moderate aperture are uncommon. If an observation of this delicate feature is difficult in a quality 24-inch aperture, an reported observation with an 8-inch aperture certainly qualifies as a threshold detection.

Now, I'm not going to take a position for or against the report. What I will do, is suggest that folks keep a couple of items in mind. One, the person reporting the observation honestly believes he observed the Encke Division. He's openly given details about the telescope used, the conditions and what he believes he saw. No one should question the integrity of the report and certainly not of the person. Two, it's a threshold observation. As such, many observers attempting to repeat the observation will--even if it is possible--fail. As such, there is room to doubt the accuracy of the person's honest interpretation of what they saw. This is the nature of threshold observations: there is room for doubt.

If this were an annual conference and I was presenting peer-reviewed research and a new theoretical model for some natural phenomenon, it would be fully appropriate for my professional colleagues to hammer me with questions probing for errors in my procedure, analysis or conclusions. But that is not this forum. This is a public forum of astronomy enthusiasts. And despite the misgivings those of you who are ardently anti-Facebook will have, I would argue this forum is not much different from any other social media site on the Web. As such, I would suggest that when it comes to reported threshold observations, it is reasonable and appropriate to point out the difficult or challenging nature of such an observation. But it's probably best not to push a questioning or doubting response too far. At some point, we should acknowledge that a person is honestly reporting what he believes he saw...an object or detail at the threshold of visibility which others may or may not have success in seeing for themselves.

Bill in Flag


Post Extras: Print Post   Remind Me!   Notify Moderator  
David Knisely
Postmaster
*****

Reged: 04/19/04

Loc: southeastern Nebraska
Re: Enckes new [Re: Mark Harry]
      #5848219 - 05/08/13 05:03 PM

Mark, I'm sorry, but this may be getting a little personal. As you have blocked me from responding to you privately, I would suggest that you unblock me if you wish to continue to respond to me rather than posting it here. I never said that you didn't see anything in your 8 inch. In my personal communication to you, I stated that, "I don't know what you saw, but I don't doubt your honesty. Your observation is interesting, but I can't know exactly what you saw as I wasn't there. I stated on this forum early in this thread; "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". I won't rule out an 8 inch for this, but I for various reasons, I think a slightly larger aperture may be required. I gave the perfectly valid reasons why I feel this way and backed that up with scientific reasoning and evidence, but you summarily dismissed them. Seeing as how you have blocked me, I will have to respond to you here, although as I have stated, this would be better off-forum.

Mark Harry wrote:

Quote:

No, it does not. That is -NOT- the situation. I also know that you did not personally make this assessment; for I read it years ago as well.




You are, unfortunately, quite wrong here when it comes to my assessment of the visibility of the Cassini Division. You may indeed have read something about this years ago, but you definitely do NOT have facts about this story. The experiment on the visibility of the Cassini Division was performed by *me* in 1999 and 2000 using my 10 inch f/5.6 Newtonian and my variable aperture stop. I put out the entire results of my experiment a number of times in the early 2000's in an article posted on the Usenet newsgroup sci.astro.amateur, as well as here on Cloudynights. A small portion of its results was excerpted from my original posting and later formally published in an article on observing Saturn's rings in Sky and Telescope magazine. Surprisingly, I have been published a few times in that magazine (and in Sky Publishing's older Night Sky magazine which is no longer in print). You can go back through the Sky and Tel archives to see the original article if you like (and the original Usenet posting may still be out there somewhere), but the fact remains that I did the work. I can provide the portion of the article concerning the experiment here:

"...At smaller apertures, the division would tend to just blend into the darker color of the A-ring, rather than forming a distinct dark gap. The division exists of course, but at what point does it become visible as a curving arc, and not just as a contrast effect?

To answer this, I used my off-axis variable aperture mask I built for my ten inch f/5.6 Newtonian to judge double star resolution. This mask provides me with 94mm, 80mm, 70mm, 60mm, and 50mm clear apertures, and I put on one additional mask to get 40mm and 30mm apertures. This way, I could stop down the scope in well-defined steps to see at which point the division would become invisible. I primarily used 176x and 141x for my tests, although I did try 235x and 101x as well. I did the tests in October in both 1999 and 2000 on nights when seeing was better than one arc second. I started with the widest opening 94mm (3.7 inches), which, at 141x and 176x, showed Saturn nicely and the Cassini Division much of the way around the planet. Even the main belt across the planet was easily visible, as well as the faint Crepe ring. Stopping down caused a drop in the brightness and in the ease of detail visibility, but Cassini's Division could still be seen down to 60mm, where it was still fairly traceable along a wide arc of each ansa. At 50mm, the actual division was becoming more difficult and was not very well shown, detectable mainly at the bend of each ansa. The outer half of the ring system looked somewhat darker as the division started to blend in a bit with the A-ring. At 101x, the division was not visible at 50mm aperture. At 40mm, I could no longer see a clear dark division between the rings, although the A and B-rings could still be seen as separate features with differing brightness and borders. Interestingly enough, even the 40mm aperture was still showing the main belt on the planet's disk. At 30mm of aperture, the A and B rings began to merge somewhat, with no clear signs of any division, and the only visible ring detail being a somewhat darker outer edge. I tried the same variable aperture sequence at 235x, but again, at 40mm, Cassini's division was not visible. To be realistic, while 50mm may allow "detection" of the division at high ring tilt, in general, 60mm seems to be about the minimum to clearly and easily show Cassini's Division.
..."

The results of the Cassini division experiment do tend to lend at least some support to the idea that a division and the darkness at the edge of the A-ring can be represented as parallel lines and may require a little more than the Dawes resolution limit to be seen as separate features and not just a diffuse band. By analogy, that representation may also be applied to the Encke Gap. By extension, it also suggests that it could take something close to 10 inches to also separate the Encke gap from the darkness at the edge of the A-ring as well. As I mentioned, I have not seen the Encke Gap in my home-built 8 inch f/7 Newtonian or in my NexStar 9.25 inch SCT, but I have (barely) detected it in my 10 inch Newtonian. Indeed, a friend of mine remarked about it last night when he recalled seeing it in my 10 as well. He has a custom mirrored 8 inch Newtonian as well (Enterprise Optics) that tests out as excellent (about 1/18.9th wave p-v wavefront, Strehl 0.994), so while he has yet to see the Encke Gap in his scope, he is still wanting to try again. I may have a few doubts that he will succeed, but he has excellent vision and equipment, so he will be trying again.

Quote:

Your replies and arguements have no personal observations or information derived as close to original source as this. All they contain, is second- or third- hand knowledge; and some of this, I question the validity of such. It's anecdotal, to say the least.




I'm sorry, but as I have demonstrated, you are incorrect. The information is not "third" hand. It is from the formal studies summarized and published in book form (along with my own experimental study on the Cassini Division). I provided full references here, so there is nothing "anecdotal" about it. Your statements are beginning to get maybe a little personal here, so again, this should be taken off-forum.

Quote:

The claim you have stated where there are no returns after 1/10th wave optics,




This again is off-topic. I stated to you that there are not any huge returns, not that there are *no* returns. You gain some contrast with a very smooth mirror (which anything significantly better than 1/10th wave p-v often tends to be) but it is not exactly an enormous image quality boost. I know the gains that can be had by going to something finer than 1/10th wave (I have two mirrors of that superb quality), but especially for large mirrors, other factors (seeing, thermal issues, optical support issues, tube currents, etc.) tend to negate at least some of that gain. In any case, you are again bringing things (like visual acuity) that are best handled off-forum. We are supposed to be talking about the Encke features (minimum and gap) here and not other things which may not have much bearing on the subject of this thread.



Post Extras: Print Post   Remind Me!   Notify Moderator  
Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: Enckes new [Re: David Knisely]
      #5849438 - 05/09/13 08:24 AM

Bill has outlined -PRECISELY- my whole point. (appreciated, btw)
I see a thinly veiled attempt to discredit my observational abilities at every opportunity. Your last post is quite obvious in the intent. Not only do you attempt to claim fame at someone else's expense, but I can see you have basically drastically reduced the credibility of your arguements. I knew you wrote that article, but I wanted to see if you'd 'rub it in'; which you've done. Thanks for giving me the true character insight; much appreciated, Dave.
****
But I think everyone has missed my point about visual acuity.
****
"Whitey" would be satisfied with a 1/2-1 wave instrument. He's albino. There is quite a larger issue than being just near/far sighted. I'm sure he would think this the best thing since sliced bread.
Dave's limit is what he has stated as 1/10th wave. this is where his acuity must "run out" and has himself claimed there is no/-LITTLE- improvement with any accuracy better than that.
I know several individuals who insist (myself included) that there's a noteable difference with optical trains with smaller errors than 1/20th. Degree of fine detail is improved slightly, but the -QUANTITY- of this delicate stuff is proportionally larger than the coarse and more easily seen detail. The image 'difference' is relatively large, and quite obvious to anyone seeing the actual comparison.
________
About your off axis mask-
Did you tilt the focuser on that 10F/5.6 when using such a mask???? Do you know -WHY- I am asking this????
About visual acuity-
At what lowest power setting can you reliably see a good tiny diffraction disc? (IE, the brightest, sharpest setting to use for maximum rez possible)???? Other than making detail larger, and a bit easier to see, all magnification does is reduce intensity and contrast which can, and does obscure or render fine detail harder to see, period.
These things I mention, are -NOT- off topic. They pertain to seeing, and not seeing the item of discussion. It has been -YOU- that has brought a negative context to this thread for personal gain from the beginning page after my report of seeing Encke and Trap stars. Again, thanks, Dave.
M.


Post Extras: Print Post   Remind Me!   Notify Moderator  
brianb11213
Postmaster
*****

Reged: 02/25/09

Loc: 55.215N 6.554W
Re: Enckes new [Re: Mark Harry]
      #5849516 - 05/09/13 09:02 AM

Quote:

Dave's limit is what he has stated as 1/10th wave. this is where his acuity must "run out" and has himself claimed there is no/-LITTLE- improvement with any accuracy better than that.
I know several individuals who insist (myself included) that there's a noteable difference with optical trains with smaller errors than 1/20th. Degree of fine detail is improved slightly, but the -QUANTITY- of this delicate stuff is proportionally larger than the coarse and more easily seen detail. The image 'difference' is relatively large, and quite obvious to anyone seeing the actual comparison.



Are we comparing apples & oranges here? A rough 1/20 wave RMS optical surface may well be far inferior to a 1/10 wave PV optical serface .... but going past 1/10 wave PV, however rough, the improvements are so small that they're at best very hard to detect ... and even then adding 10% to the aperture will more than compensate for optical "defects" in the 1/6 wave PV range.

Differences between observers' acuity, temperature stabilisation and atmospheric steadiness are much more of an issue when resolution of critical features is concerned.

So far as "Enckes" is concerned: I have no difficulty in accepting that observers with good acuity, a better than average 6" scope and good seeing can see some variation in brightness across the width of the A ring. But what is now known as the Keeler Gap (80-90% of the way from Cassini's Division to the outer edge of the A ring): I don't believe that can be seen properly with less than 12" & full resolution as a sharp edged, black gap takes a lot more than that.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | 4 | 5 | (show all)


Extra information
2 registered and 8 anonymous users are browsing this forum.

Moderator:  Rich (RLTYS), star drop, Mitchell Duke 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 4256

Jump to

CN Forums Home




Cloudy Nights Sponsor: Astronomics