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cirrus
super member
Reged: 05/10/03
Posts: 174
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I think a distinction should be made between "seeing" and "detecting". I consider myself very fortunate because I have had acess to relatively large telescopes and also very steady skies. I have clearly seen the Enke division many times with a 22 inch reflector. I have also seen it using a good 8 inch reflector; no, it was not anywhere as clear as with the bigger scope and it was only visible at the edge of the ansae. And no, it was not the minima, you can see that with any 5 inch instrument and it is a very different animal.
As with the deep sky buffs that seek dark skies, the planetary buffs seek steady conditions. Anybody that is interested in astronomy should treat themselves to both every once in a while. Perfectly clear skies are rare but also relatively more common in certain parts of the country. I'm lucky to live in one of those locations. When these conditions exist, the views with a good scope rival those obtained with the newer CCD equipment. I stand by my claim that I have never seen a CCD picture of Saturn that is better than some of the views I have been lucky enough to see with my own eyes. You can definitely see the Enke division with an 8 inch under exeptionally good conditions. Just do not expect it to be anything like the Cassini division.
cirrus
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Sol Robbins
Carpal Tunnel
Reged: 12/01/03
Posts: 1985
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Hi,
I've read a bit and also learned a lot regarding observing the Encke Division this thread. Thanks to all of you.
Though I don't believe I've seen it, I offer this.
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S.R.
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 13646
Loc: southeastern Nebraska
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Quote:
Hi,
I've read a bit and also learned a lot regarding observing the Encke Division this thread. Thanks to all of you.
Though I don't believe I've seen it, I offer this.
That is an *excellent* drawing Sol! It shows the "Encke Minimum" quite well in the middle of the A-ring (and maybe just a hint of the darkening where the Encke Division is). It must have taken a lot of patience and effort to get this kind of quality drawing. Keep up the good work and clear skies to you.
--------------------
David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right."
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Sol Robbins
Carpal Tunnel
Reged: 12/01/03
Posts: 1985
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Hi David,
Thank you for the complimentary words.
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S.R.
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cirrus
super member
Reged: 05/10/03
Posts: 174
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It looks like Sol's drawing shows a definite possibility of Encke gap detection. It also illustrates how the CCD- processed images do not do Saturn fair justice. Too much ring detail is washed-out in the processing, or maybe the detail was not even there because of seeing conditions.
At any rate, I have seen many of Sol's Saturn drawings before and they come a lot closer to what I have seen. My own drawings are nowhere near that quality.
Now, my question is, what is a Guan Sheng Newtonian?
cirrus
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Starman1
Vendor (EyepiecesEtc.com)
Reged: 06/24/03
Posts: 17639
Loc: Los Angeles
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GSO, Guan Sheng Optics is a Taiwn-based manufacturer of eyepieces and telescopes, mostly under private label to many of the world's companies.
Here is their web site:
GSO
Look carefully, and you will see a lot of familiar products.
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Don Pensack
www.EyepiecesEtc.com
12.5" Teeter/Zambuto, 5" Maksutov
Sustaining Lifetime IDA member
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Sol Robbins
Carpal Tunnel
Reged: 12/01/03
Posts: 1985
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Hi,
Guan Sheng manufactures a lot different badged scopes. My mirror came from Anteres in Canada. Guan Sheng also made or makes scopes for Orion, Celestron, Hardin and others.
Sky Instruments offers a service regarding optics. Basically
they get mirrors from both Synta and Guan Sheng. They randomly test batches of mirrors and the better ones get set aside with a better optical grade. I simply wanted a mirror that had no turned edge, zones, etc. My Newt star tests at about 1/6 wave at the eyepiece. Though I didn't optimize my scope for planetary observing, it does a relatively decent job.
Thanks, Sol Robbins
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cirrus
super member
Reged: 05/10/03
Posts: 174
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Very informative. Thanks for the info.
cirrus
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matt
Vendor (Scopemania)
Reged: 07/28/03
Posts: 10991
Loc: Chaville, France
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I'm proud to bump back up this thread, which started more than a year ago, mostly thanks to my obviously bad reports that I had seen the Encke Division with an 8".
After months of clouds, rain and snow, we are having some very good stable nights, and I'm getting pretty sure I have seen Encke, three out of four last nights (those four nighs are about as much clear nights as we had had since NEw Year's day!)
I'm pushing the 12" at 375x (with a 4mm radian) (I can get 600x but will never achieve focus there!), with a seeing at 8, or so I assess. Monday I used 500x but the dob's mount is too shaky for that.
I can see the "equatorial" belt of Saturn, and two thinner belts a little like what Sol's drawing shows (including the lower belt which is lost in the "glare" of the white area, and of course the Polar region (which still does not appear green to me, sorry Ron.
The C ring is plainly visible as its shadow; the "irregularities" in brightness of the inside of the B ring; Cassini, and what looks tantalizingly like the Encke division, a little darker than what we can see on the outer edge of the A ring in sol's sketch (the "ansae" made by the visible parts of the division extend a little farther too).
What makes me think it's Encke is that the width of the A ring between the "feature" and its outer edge is about the same width as the Cassini Division - calculated by you guys to both be somewhere between .5" and .7".
I'll try making a sketch. The image is pretty much printed on my retina now.
So, are we getting closer?
Now I saw somewhere else a pic of Saturn showing a division on the outer quarter of the C-ring. Hmmm ... sounds like a job for a 16" APO!
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Matt
CI700 mount with various scopes on top.
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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In this month's (May 05) issue in "Astronomy"...article titled "Small Eyes on the Sky" by Damian Peach...I find the following:
"Figures often quoted as the resolving limits of telescopes only apply to point sources of equal brightness or to double stars resolution....Planets are extended objects, so these values don't apply to planetary detail....At the turn of the 20th century, famed astronomer William H. Pickering conducted experiments inot the resolution of planetary detail. Pickering found that a dark spot on a bright background could be resolved to a level a little more than twice as good as the Dawes value. Perhaps the most famous example of this is the discovery of Saturn's Cassini Division made with a 2.5 inch refractor. This observation exceeds Dawes Limit by nearly a factor of 3 for this aperture. (The Cassini Division spans no more than 0.7" at any time, and the Dawes Limit for a 2.5 inch telescope is 1.8".)"
I suppose this kind of info helps support stated observations of the Encke Division in relatively small scopes. David K reported above in this thread that the Encke Division is only 0.5" from the edge of the A ring. David pointed out that in fact, it's "worse" than this, because separation of parallel lines need to be 1.1 to 1.4 that of Dawes Limit to be resolved. This would mean scopes may need to resolve down to as low as 0.45" - 0.36" to resolve the feature, and thus limits this feature to scopes of 10 - 12" or more. But based on info in the above quote (based on experiment evidence), this resolution limit can be relaxed by perhaps up to 2.5x (probably if contrast is really high...e.g., black on white), but maybe more realistically only to 1.5x to 2x (because Encke divsion is black on GRAY, not black on white). Assuming a pessimistic case (the 1.5x case), a scope that can resolve down to maybe 0.54" (1.5 * 0.36") or 0.68" (1.5 * 0.45") has a shot at seeing the Encke Division. These values are in the ballpark of an 8" scope's capability. More optimistic values (the 2x case) of 0.71" (2 * 0.36") to 0.91" (2* 0.45") put the Encke Division in the realm of a 6" scope's abilities, and I have heard reports of this target being seein in 6" refractors.
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Hardin 12"
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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Incidentally, Sol says he doesn't think he's ever seen the Encke Division, but check out his drawing! On my monitor, his drawing shows the narrowest of lines right at the very very edge of the A-ring. Looks like E.D. to me! David Knisely pointed this out also: "maybe just a hint of darkening where the Encke Division is".
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Hardin 12"
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 13646
Loc: southeastern Nebraska
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Quote:
In this month's (May 05) issue in "Astronomy"...article titled "Small Eyes on the Sky" by Damian Peach...I find the following:
"Figures often quoted as the resolving limits of telescopes only apply to point sources of equal brightness or to double stars resolution....Planets are extended objects, so these values don't apply to planetary detail....At the turn of the 20th century, famed astronomer William H. Pickering conducted experiments inot the resolution of planetary detail. Pickering found that a dark spot on a bright background could be resolved to a level a little more than twice as good as the Dawes value. Perhaps the most famous example of this is the discovery of Saturn's Cassini Division made with a 2.5 inch refractor. This observation exceeds Dawes Limit by nearly a factor of 3 for this aperture. (The Cassini Division spans no more than 0.7" at any time, and the Dawes Limit for a 2.5 inch telescope is 1.8".)"
I suppose this kind of info helps support stated observations of the Encke Division in relatively small scopes. David K reported above in this thread that the Encke Division is only 0.5" from the edge of the A ring. David pointed out that in fact, it's "worse" than this, because separation of parallel lines need to be 1.1 to 1.4 that of Dawes Limit to be resolved. This would mean scopes may need to resolve down to as low as 0.45" - 0.36" to resolve the feature, and thus limits this feature to scopes of 10 - 12" or more. But based on info in the above quote (based on experiment evidence), this resolution limit can be relaxed by perhaps up to 2.5x (probably if contrast is really high...e.g., black on white), but maybe more realistically only to 1.5x to 2x (because Encke divsion is black on GRAY, not black on white). Assuming a pessimistic case (the 1.5x case), a scope that can resolve down to maybe 0.54" (1.5 * 0.36") or 0.68" (1.5 * 0.45") has a shot at seeing the Encke Division. These values are in the ballpark of an 8" scope's capability. More optimistic values (the 2x case) of 0.71" (2 * 0.36") to 0.91" (2* 0.45") put the Encke Division in the realm of a 6" scope's abilities, and I have heard reports of this target being seein in 6" refractors.
Well, you have got it a little backwards. The smaller the resolution figure, the higher the resolution of the telescope is said to be and the finer the detail is that can be resolved by the scope. The 1.1 to 1.5 times Dawes figure is the resolution for a given aperture and represents a *drop* in a telescope's resolution capability for that kind of detail (i.e. it isn't resolving as small an angular detail as for a pair of point sources like a double star). For a 10 inch, this effect on somewhat linear features *decreases* the scope's effective resolution capability from a nominal Dawes 0.46 arc seconds (point sources) to from 0.51 to 0.69 arc seconds (for a pair of parallel lines). Thus, for linear features, the resolution of the 10 inch has *decreased* to something found on a smaller scope like a 9 to 7 inch aperture. At mean opposition, the Encke division is about 0.5 arc seconds away from the outer edge of the A-ring. This is *smaller* than the effective resolution for a pair of equal lines that a 10 inch can probably separate, and thus the division might not get resolved to become visible. Even being somewhat optimistic about things, it would take at *least* a ten inch and perhaps something as large as a 13.7 inch aperture, since for linear detail, the separation (0.5" arc) is too small to be resolved. For this reason (and my own experience observing the Encke division), I have significant doubts that telescopes smaller than 10 inches can clearly resolve the division from the outer edge of the A-ring. Clear skies to you.
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David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right."
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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No David, I don't think I've got it backwards, though I apologize if I was unclear in how I described things. Like you said, the 1.1 to 1.4x Dawes figure represents a drop in the scope's ability to resolve. So, a 0.5" feature becomes more like a 0.45" to 0.36" feature (as I wrote above). And thus a scope of 10-12 inches is required instead of a scope of 9 inches (just like you said originally). I haven't said anything contradictory here.
My point is that while the parallel-line argument works against you (by 1.1 to 1.4x), the extended object argument (that I cited in Astronomy article) works in favor by up to 2.5x (though probably less so in this case because Encke Div is black on gray...hence my more pessimistic "adjustment" of 1.5x). The net gain is in favor of the observer.
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Hardin 12"
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 13646
Loc: southeastern Nebraska
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Quote:
No David, I don't think I've got it backwards, though I apologize if I was unclear in how I described things. Like you said, the 1.1 to 1.4x Dawes figure represents a drop in the scope's ability to resolve. So, a 0.5" feature becomes more like a 0.45" to 0.36" feature (as I wrote above). And thus a scope of 10-12 inches is required instead of a scope of 9 inches (just like you said originally). I haven't said anything contradictory here.
My point is that while the parallel-line argument works against you (by 1.1 to 1.4x), the extended object argument (that I cited in Astronomy article) works in favor by up to 2.5x (though probably less so in this case because Encke Div is black on gray...hence my more pessimistic "adjustment" of 1.5x). The net gain is in favor of the observer.
Well, there are still problems with using the parallel line argument. First, the Encke division and the darkness of the A-ring next to it are not sitting against a white background as was present when the line resolution experiments mentioned in Sidgwick were done. This will reduce the contrast somewhat and make seeing the division more difficult. Second, the Encke division and the darkness of the A-ring are not quite two very thin parallel lines, but are arcs. This may make things more difficult. If you want to take the "discovery" comparison, the Cassini Division was discovered using a 2.5 inch telescope, although I have done a series of resolution experiments indicating that, for full ring tilt, the threshold of bare detectability is more like 2 inches. The Cassini division is about 2.3 arc seconds from the outer edge of the A-ring at mean opposition, while the Encke Division is about 0.5 arc seconds out. This is a factor of 4.6 in angle change, so instead of 2 inches needed to just barely show the division, the Encke would now require roughly 9.2 inches. Again, this comparison wouldn't be quite fair, as the Cassini Division is much thicker relative to the size of the A-ring than the Encke Division is, so one would expect it to take a bit larger aperture than 9.2 inches to resolve the Encke from the outer edge of the A-ring.
It also takes a lot of power to bring the scale of this 0.5 arc second distance to a point where it can easily be viewed by the eye. Assuming you need to get it to a scale like that of the wide components of Epsilon Lyrae (3.5 arc minutes which is near the limits of the unaided eye), you would need around 420x. Clearly, you need enough aperture to support this power, and again, this argues for something definitely larger than 9 inches. The time I have seen the Encke Division in my 10 inch Newtonian, the division was fairly marginal, and it took between 400x and 500x to even get much of a glimse of it. Thus, I remain somewhat skeptical of reports of the division's visiblity in 8 inch and smaller apertures. Clear skies to you.
--------------------
David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right."
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Todd
Carpal Tunnel
Reged: 02/23/04
Posts: 1674
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Over the course of the last 3.5 weeks I've been fortunate to have 3 nights of exceptional seeing.
For two of those nights I was using my 12.5" Portaball. I've had it for about 19 months and until these two nights I have never been able to say that I had seen the Encke Division. I use a Nagler 3-6 Zoom for high-power viewing and I needed to use the 4mm setting at the minimum to see Encke (giving me about 380x). One night it was quite sharp and the following night I was getting fleeting glances of the division (nonetheless it was there).
On Saturday night the seeing was incredible once again and I was working at a local resort's observatory. The observatory has a 16" SCT and Saturn was _tack_ sharp. There wasn't a hint of any movement. Unfortunately, the highest power ep they have couldn't be found so I could only take the scope up to 320x.
Even with the excellent seeing, I could not see the Encke Division at that magnification.
Unfortunately, it was after 11:30 p.m. when I returned home and Saturn was too low for me to look at with my Portaball. I was able to see Jupiter like never before though.
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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Ok...David, thanks for your argument. I just wanted to make sure you were clear on why I originally saying. And while I don't challenge the overall conclusion that Encke is an extremely difficult target, I think there's still a relevant take-home message in my post (which is merely relaying info from the Astronomy article): that the Dawes Limit must be relaxed for non-point features. Whatever numbers you come up with based on Dawes Limit...e.g., 0.5 arc sec, adjusted "pessimistically" by 1.1x to 1.4x for parellel lines...you must then adjust this in a more "optimistic" direction for the fact that we're dealing with extended objects. The article made the point that in seeing the Cassini division with a 2.5" scope, Pickering was exceeding Dawes Limit by nearly 3x. There's a general result here...that extended objects are "easier" than Dawes Limit suggests. I won't try to come up with any accurate estimates, but to SOME extent, based on factors A, B, and C, Encke Div should be easier to detect than a purely Dawes Limit argument would suggest. That's all I'm saying.
And also, for everyone saying that "you can't see X because I can't see X"...remember that different people have different abilities that are a product not only of experience, but of anatomy. Some people just have better eyes than others, especially in terms of visual acuity. I accept that Encke MAY not be possible to see in less than a 10" scope, but I accept this based on good theoretical arguments, not just because person A has never seen it in his bigger scope. I already know that I can split doubles easier than people who've been observing much longer than me (e.g., split them at lower mags), and I can see the Cassini Div at lower magnification than most people who've been observing a lot longer than me. So, it's also probable that there are some eagle eyes out there who can see Encke Div at a lower magnification or in a slightly smaller scope than the norm.
--------------------
Hardin 12"
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BillFerris
Post Laureate
Reged: 07/17/04
Posts: 3587
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Quote:
In this month's (May 05) issue in "Astronomy"...article titled "Small Eyes on the Sky" by Damian Peach...I find the following:
"Figures often quoted as the resolving limits of telescopes only apply to point sources of equal brightness or to double stars resolution....Planets are extended objects, so these values don't apply to planetary detail....At the turn of the 20th century, famed astronomer William H. Pickering conducted experiments inot the resolution of planetary detail. Pickering found that a dark spot on a bright background could be resolved to a level a little more than twice as good as the Dawes value. Perhaps the most famous example of this is the discovery of Saturn's Cassini Division made with a 2.5 inch refractor. This observation exceeds Dawes Limit by nearly a factor of 3 for this aperture. (The Cassini Division spans no more than 0.7" at any time, and the Dawes Limit for a 2.5 inch telescope is 1.8".)"
I'm not sure if the difference between detection and resolution is dicussed elsewhere in the article, but detecting the Cassini divsion in a 2.5-inch aperture is one thing. Resolving it is another. There are countless examples of observations where high contrast objects and features are detected but not resolved. Take the thousands of naked eye stars, for instance. All can be seen. None are resolved; not with the naked eye nor with any visual telescope.
Cassini's division was discovered in a 2.5-inch aperture and, therefore, is indisputably visible in that aperture. But, at only about 0".7 in angular width, it isn't resolvable in that aperture.
Regards,
Bill in Flagstaff
--------------------
Grand Canyon Adventure
Lowering the Threshold
18" Obsession
4.5" Meade 4500
10x50 Swift Audubon
Cosmic Voyage
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photonovore
Moonatic
Reged: 12/24/04
Posts: 2792
Loc: tacoma wa
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Quote:
Some people just have better eyes than others, especially in terms of visual acuity.
Bingo. Forest for the trees...the eyeball is a part of every system...and *it's* ability to resolve varies from 3' apparent to less than 1' minute. This gets back to the old "can one detect the moons of jupiter with the naked eye" biz, which is quite possible for those persons who have the eyes that 99% of us *don't* have. (my own eyes are rather ordinary I'm afraid.) Using a telescope, what this means is that a person with say, 20/10 vision, will need one half the magnification to sufficiently resolve an object than that required for a person with 20/20 vision--which latter, tho often called "perfect" vision, is in fact only "normal" vision. With the foregoing in mind you can easily understand why some observers see much more at a given max instrument magnification than others are able to see using the same instrument--and why some need a larger instrument to detect details that are visable to others in smaller instruments--the larger instrument provides the capability for increased magnification and thus the increased angular separation that a less acute eye requires. Most of this "I can see this--No, you can't see that" business can be laid at the feet of the human eye's variances in visual acuity.
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Mardi
4" achromat, ETX-70, 8"cat.
Whitepeak Lunar Observatory Website
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matt
Vendor (Scopemania)
Reged: 07/28/03
Posts: 10991
Loc: Chaville, France
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If our corneas and eye lenses were made of glass, we might see the moons of Jupiter naked eye. Unfortunately most of us have eyes made of organic cells in fiber patterns, causing bright stars to have "rays" - and Jupiter too, which hide the satellites.
I'm afraid the eye tests we are dispensed are made separating close double stars, usually they make them up for dumb stuff like reading books.
I have been lucky enoug to meet two (2) people who claimed they could see the moons of Jupiter naked eye, but unfortunately for them when I tested them, the moons were in a 3-1 or 4-0 arrangement and they saw of course two on each side of the planet.
I also used averted imagination on this case myself- I thought I had seen them but the pattern did not match when I checked the satellites' positions.
--------------------
Matt
CI700 mount with various scopes on top.
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jmoore
Carpal Tunnel
Reged: 10/01/03
Posts: 1959
Loc: Beaufort, NC
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Quote:
I'm not sure if the difference between detection and resolution is dicussed elsewhere in the article, but detecting the Cassini divsion in a 2.5-inch aperture is one thing. Resolving it is another. There are countless examples of observations where high contrast objects and features are detected but not resolved. Take the thousands of naked eye stars, for instance. All can be seen. None are resolved; not with the naked eye nor with any visual telescope.
Indeed, though I THINK the article is talking about actually resolving this feature. Maybe not. Again though, point is that Dawes Limit is only valid for point features.
Regardless, seeing or resolving...the topic of this thread concerns whether either is possible for Encke Div below a certain aperture. I'd be content to say that I saw, but didn't resolve, the E.D. in my 8" scope. But many in this thread say event that's not possible. Perhaps. Not worth getting fired up about.
Quote:
Using a telescope, what this means is that a person with say, 20/10 vision, will need one half the magnification to sufficiently resolve an object than that required for a person with 20/20 vision
I'm glad you agree with me that eyes are important Mardi, but I'm not sure about this statement. A person with 20/10 vision can focus better on more distant objects, but this doesn't mean he/she can magnify them. Where image scale is important, I don't think having 20/10 will help. In fact, I have terrible myopia...I'm like 20/200 or something ridiculous like that...and therefore eyepieces focus at a different location for me than others. But once focused, the playing field's level, and in this case I seem to have very good visual acuity.
Quote:
I have been lucky enoug to meet two (2) people who claimed they could see the moons of Jupiter naked eye, but unfortunately for them when I tested them, the moons were in a 3-1 or 4-0 arrangement and they saw of course two on each side of the planet.
--------------------
Hardin 12"
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