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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8270
Loc: Beatrice, Nebraska
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Quote:
No one doubts that the AFOV's are as reported. They simply don't matter. All that matters is the RFOV and/or the field stop diameter. The AFOV will not tell you how much sky you will see. The RFOV and the field stop will tell you how much sky you will see. And the RFOV (or the field stop) will tell you without measurements of any nature, provided that the manufacturers get on the stick and accurately report these values.
I would most certainly like to know more about your method though!!!
The Apparent Fields *do* matter, especially to those who care how much they see. Indeed, with a few of my eyepieces, the apparent field used in the old AFOV/Mag calculation gave results that were *very* close to the true fields I actually measured. The problem is that the formula just isn't consistently accurate, which is why the field stop formula tends to be a bit better. The apparent field is real, as it can be seen and measured. The Field stop is real and can be measured (usually). The true field of view of a telescope/eyepiece combination can be measured. The RFOV is not a real characteristic of the eyepiece. It is an artificial construct to get an old approximate equation for true field to seem to work better. It already involves the field stop in its calculation, which is redundant. All that really matters is that if someone wants to know *exactly* how much true field an eyepiece will give in a telescope, it needs to be *measured* (star drift method, or an accurately-known field for comparison).
If the amateur wants to know roughly how much true field one can get with a given eyepiece with known physical parameters, there are *two* formulae which work. The old formula TFOV = AFOV/Magnification will give at least an approximate result (within 10 %) and the field stop formula will give somewhat closer results (within 2%). Introducing another concept that has less of a clear connection to reality might be confusing and is unnecessary, considering we already have two workable formulae for true field of view. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4736
Loc: N.E. Ohio
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What possible harm would there be to anyone in having the manufacturers report the RFOV? Why does Baader feel it is important to report the RFOV? It is a simple, practical, and quite useful tool.
And as the original poster ascertained, AFOV's do not tell anyone what they will actually see, but rather only how far the eye will have to glance from side to side to see the REAL field that the eyepiece and telescope are presenting to the viewer. In his actual example, as regards the true field that he expected to see based on the apparent field, the AFOV was off by 12.7% in one direction for one eyepiece, and 4.9% in the other direction for another eyepiece, for a combined error of 17.6% in his simple yet futile attempt to use AFOV's to compare one eyepiece against another. With RFOV this error would be reduced to near zero.
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3924
Loc: "Metrowest" Boston
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Quote:
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
You would rather have -11.7% angular magnification distortion than +6.1%? OK, I guess maybe I can see that, but where do you draw the line? How about an RFoV of 400 degrees in an AFoV of, say, 50. Of course there's a teensy bit of negative angular magnification distortion there, only about -700%, but it fits a lot of sky in 50 degrees! How much is too much?
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt, eq platform, Pentax 40XW and 5XO, Tele Vue 13E and 2x Barlow, ZAO-II 6mm
- 120mm f/8.3 home-built grab-n-go Newt with 7-21mm Nikon Zoom
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 12929
Loc: Amargosa Valley, NV, USA
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Quote:
Quote:
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
You would rather have -11.7% angular magnification distortion than +6.1%? OK, I guess maybe I can see that, but where do you draw the line? How about an RFoV of 400 degrees in an AFoV of, say, 50. Of course there's a teensy bit of negative angular magnification distortion there, only about -700%, but it fits a lot of sky in 50 degrees! How much is too much?
That's where "Majesty Factor" comes in! 
The answer to your question is subjective, of course. For general viewing, even the larger distortion is not going to be noticeable. But if you want to do imaging or sketching, then it can be a problem.
I would say it gets to be a problem when you can see the stars appear to follow curved paths across the eyepiece field, or when the image of an extended object is visibly distorted.
Or, when you're trying to take accurate measurements. Or star hopping.
I didn't realize the Hyperion had so much distortion until I used the formula that Lawrence provided. By that token, I would think it would not be a problem for visual observing.
For a budget widefield eyepiece with great views in a convenient image scale, the Hyperion is hard to beat. If you want the least distortion possible in the wide field, and can afford it, get the Nagler or Ethos. If you want the most distortion-free field possible, period, then get -- an Orthoscopic? A Brandon? A Plossl?
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"Since the process of science generates more mysteries than it solves, I predict that we'll never learn everything: and we'll continue to generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8270
Loc: Beatrice, Nebraska
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Lawrence Taylor wrote:
Quote:
What possible harm would there be to anyone in having the manufacturers report the RFOV? Why does Baader feel it is important to report the RFOV? It is a simple, practical, and quite useful tool.
In the Baader literature, I do not see anything except the apparent field of view listed (68 degrees for the Hyperion). As for harm, as I said, it can sow the seeds of confusion (especially for the beginner).
Quote:
And as the original poster ascertained, AFOV's do not tell anyone what they will actually see, but rather only how far the eye will have to glance from side to side to see the REAL field that the eyepiece and telescope are presenting to the viewer. In his actual example, as regards the true field that he expected to see based on the apparent field, the AFOV was off by 12.7% in one direction for one eyepiece, and 4.9% in the other direction for another eyepiece, for a combined error of 17.6% in his simple yet futile attempt to use AFOV's to compare one eyepiece against another. With RFOV this error would be reduced to near zero.
Ah, not quite. He didn't get the true field expected because he used the approximation TFOV = AFOV/Mag and did not understand that it *is* an approximation. Had he used the field stop method, he would have been considerably closer to reality. As for apparent field, it shows the angular width that the eye sees when looking into the eyepiece (from field stop edge to field stop edge usually). Thus, it *does* show what the observer will see, not in terms of true field of view, but in the width of what is presented to the eye. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4736
Loc: N.E. Ohio
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You must go back through this thread, where when I speak of Baader using the RFOV I specifically reference this to only the Eudiascopics and the Genuine Orthos. Two eyepieces which hail from the same Japanese manufacturer (who I quite strongly suspect is the former manufacturer of the Masuyama eyepiece line).
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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frisianstar
member
Reged: 01/29/06
Posts: 58
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Thanks, I did my measurements with a 50 mW green laserpointer, just on the floor! the laser on a couple of books to get the right hight...the board is a piece a plywood...
the eyepiece was held in place with a counterweight...
when you held everything in place and square this method is very accurate.
With 50 mW laserpower you can easy see the various lensses inside a optical system, some are cemented, others absorbs
more light, some lensses are thin or very thick.
Holland (real name: Gerrit  )
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3924
Loc: "Metrowest" Boston
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Quote:
If you want the least distortion possible in the wide field, and can afford it, get the Nagler or Ethos. If you want the most distortion-free field possible, period, then get -- an Orthoscopic? A Brandon? A Plossl?
As regards angular magnification distortion, the Type 4 Naglers and the Ethos are tops. Perhaps some other ultrawides may offer less angular magnification distortion than corresponding Naglers of other types. I'm not sure, but the Pentax XWs might be pretty good in this regard. I think they publish EFSDs, so I could check. Have to get back to you on that one.
As for the most distortion-free field possible, it really depends, and of course there is rectilinear distortion to think about also. Tele Vue Plossls have some angular magnification distortion, a few percentage points, IIRC, and I was most disappointed by an 18mm UO ortho (classic) in this regard. It's field stop was a lot smaller than I expected based on its AFoV, to the point that I didn't want to use it (as I'd bought it to be an eyepiece for a small finderscope). If that eyepiece is "orthoscopic" in any sense, it must be because rectilinear distortion is low. Contrast that with the 18mm Tak ortho I had--its field stop was pretty much dead-on for the zero angular magnification distortion. Unlike the Tele Vue Plossls, which show some stretching near the edge, I couldn't actuall see distortion in any of the so-called orthoscopics, though.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt, eq platform, Pentax 40XW and 5XO, Tele Vue 13E and 2x Barlow, ZAO-II 6mm
- 120mm f/8.3 home-built grab-n-go Newt with 7-21mm Nikon Zoom
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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Rob S
professor emeritus
Reged: 03/16/07
Posts: 501
Loc: NZ
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I was thinking about this this morning. Assuming the figures we see on the Tele Vue website for field stops are real dimensions, and not effective/virtual, when they decided to make the 8mm Ethos, would they have to start with the requesite sized hole in lets say a piece of metal, that gives a 100 degree AFOV (lets assume no distortion) for an 8mm focal length - and then build around that?
Regards,
Rob.
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8" f5 Dob with Feathertouch
Brandon: 16
Pentax: 7, 10 & 20XW
TMB: 30 Paragon
TV: 8 & 13 Ethos;
5 & 13 NaglerT6;
24 Panoptic
Leica Trinovid 8x50 BN
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4736
Loc: N.E. Ohio
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Because the Ethos has a built in Barlow, its reported field stop is only apparent.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3924
Loc: "Metrowest" Boston
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Well, the field stops are "effective" whenever there are any lenses in front of the focal plane, and that includes everything except for the Plossls and Panoptics. To answer your question, however, I suspect they did essentially what you said in a "virtual" design environment (to say a computer program), and set initial guess at the effective field stop diameter to be about 8*100/57.3, which would be about right for no angular magnification distortion, 8mm focal length, and 100 degrees of AFoV. The effective field stop diameter would be the physical field stop diameter adjusted for the effect of the eyepiece lenses between the telescope and the field stop. Consequently, this would be a calculated constraint for any particular lens configuration. There are lots of constraints to juggle, however, and so it is not surprising that the final value is allowed to deviate slightly, as it might be considerably more important to control something else a little more tightly and to ease up slightly on the idea of dead-on zero angular magnification distortion. The general tenor of this thread notwithstanding, there isn't much practical benefit to hitting that particular mark exactly.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt, eq platform, Pentax 40XW and 5XO, Tele Vue 13E and 2x Barlow, ZAO-II 6mm
- 120mm f/8.3 home-built grab-n-go Newt with 7-21mm Nikon Zoom
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8270
Loc: Beatrice, Nebraska
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Quote:
Because the Ethos has a built in Barlow, its reported field stop is only apparent.
I wish people would not call the Smyth field flattener in some wider-field eyepieces a "built-in Barlow". Barlows act to increase the magnification of an eyepiece and may or may not help with flattening of the field. The negative Smyth lens set in the front of these eyepieces is an integral part of the eyepiece as a whole and cannot be removed without compromising the eyepiece performance. They are designed to compensate for the field curvature produced by a set of eyepiece optics that are correcting off-axis astigmatism. Also, these field stops for eyepieces such as these are *internal*, and the figures given by Tele Vue will result in true field figures which are dead-on accurate. Clear skies to you.
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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4736
Loc: N.E. Ohio
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There is absolutely no evidence at all that anyone named Smyth was ever involved in designing a negative lens grouping for a telescope eyepiece. As you are well aware, I ran an exhaustive thread here covering this very subject perhaps a year or two ago. The only possible person to honestly credit with being first with this accomplishment is Peter Barlow. Ask Al Nagler if his negative grouping in the Nagler is a Smyth and tell us what his answer is. Actually someone who reported it here has already done just this, and his answer was that if anything the negative grouping is a "Nagler" (meaning a purely Nagler concept and invention). Patents are generally required to reference the prior works leading up to any new invention. Show me an eyepiece patent whereby the negative lens grouping honors by reference anyone named Smyth. For that matter, show us a patent for a negative grouping specifically in a telescope eyepiece by anyone named Smyth. Somehow, about 90 or so years after his life, someone twisted his camera acomplishments and equated them to eyepieces, and the misplaced credit has stuck like glue. If such a stretch is to be permitted, perhaps we should call it a Petzval (just kidding of course). Telescope eyepieces were used in conjunction with Barlows for roughly 90 years or more through which no one ever credited Smyth (the father who was a close friend and associate of Peter Barlow, or the son who made strides in camera lens technology) with this accomplishment. Although admittedly this does not always happen (witness the Masuyama), it seems that within his lifetime someone would have given Smyth (meaning either of the Smyth's) credit if any was due.
That said, I feel that to continue this discussion within the context of a thread for which it has no specific contextual relevance would be inappropriate.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Ronin
super member
Reged: 10/18/04
Posts: 111
Loc: Phoenix AZ
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Perhaps a search for Piazzi-Smyth might be more fruitfull. Scottish Astronomer he developed a negative lens group field flattener.
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Martin "Ronin" Thompson
Observatory Manager
Gilbert Rotary Centennial Observatory
16" LX200R on Paramount ME
Personal Gear
Nexstar 8 GPS
Nexstar 11 CGE
Celestron Onyx 80
Canon Rebel XTi (EOS 400D)
Canon Xsi (EOS 450D)(Yippeee New Toy)
5 Baader Hyperions (Just Love Em)
All things are possible, they just may not be probable
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jcjr
professor emeritus
Reged: 01/06/08
Posts: 563
Loc: TN, USA
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Quote:
Quote:
All else being equal (regarding resolution from center to edge, contrast, transmission, color purity, etc...), if I had the option to see a real field of 77 degrees within an apparent field of 68 degrees, vs. seeing a real field of 77 degrees spread over an apparent field of 82 degrees, I would choose the former, since my eye would be able to take in the same real field of view far more comfortably, with less wandering of the eye required to absorb the exact same chunk of sky.
You would rather have -11.7% angular magnification distortion than +6.1%? OK, I guess maybe I can see that, but where do you draw the line? How about an RFoV of 400 degrees in an AFoV of, say, 50. Of course there's a teensy bit of negative angular magnification distortion there, only about -700%, but it fits a lot of sky in 50 degrees! How much is too much?
Mike, there is an application where maybe that would be very desirable. I wonder if you have ideas if it has been done or could be kludged together (it would probably require more than just the eyepiece)--
I think it would be great to have a largish crosshair finder scope with good light-gathering, and maybe a 20 or 30 degree 'quasi-fisheye' true FOV. The eyepiece view would be close to rectilinear in perhaps the middle 20 degrees of the apparent field of view, where the crosshairs meet. But the view would get progressively wildly distorted/compressed toward the edges.
Why? Well, with a TFOV that wide, you could point the scope 'in the ballpark' of the target, then easily find the target in the distorted periphery, and then easily guide it into the crosshairs.
True fisheye's are near 180 degrees, which would be unnecessarily wide and make it difficult to see the tiny points of light. But a 20 or 30 degree telescopic fisheye might be VERY useful as a Finder?
I got a nice SV F80 Finder because it hurt the bones groveling on the ground to sight thru a Red Dot. But even with about 5 degrees of FOV in the F80, I STILL have to grovel on the ground to sight along the tube, because 5 degrees is often too narrow a capture range to 'just aim in the general direction' and find the object in the eyepiece.
A TFOV of 20 or 30 degrees should eliminate groveling on the ground to sight along the tube, even if most of the view is extremely distorted.
Anybody ever made a Finder like that? Any ideas how to make one?
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CPC 1100, C102SLT, SV F80, Meade 70 & 60 AZT
Q70 38mm, Pan24, Meade 5K 18mm UW, Axiom LX 15mm, 10mm, 7mm, Nagler 13T6, Expanse 20mm, 9mm, 6mm, BO/TMB 5mm, 2.5mm
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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4736
Loc: N.E. Ohio
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Quote:
Perhaps a search for Piazzi-Smyth might be more fruitfull. Scottish Astronomer he developed a negative lens group field flattener.
He is the "son". Already been there.
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3924
Loc: "Metrowest" Boston
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Quote:
Mike, there is an application where maybe that would be very desirable. I wonder if you have ideas if it has been done or could be kludged together (it would probably require more than just the eyepiece)--
I only worked out the mathematical possibility of it. I have no idea how to put lenses together to make something like that.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt, eq platform, Pentax 40XW and 5XO, Tele Vue 13E and 2x Barlow, ZAO-II 6mm
- 120mm f/8.3 home-built grab-n-go Newt with 7-21mm Nikon Zoom
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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Ronin
super member
Reged: 10/18/04
Posts: 111
Loc: Phoenix AZ
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Indeed he is the son but he is credited with the development of a dispersive lens group to correct abberations in his work in Photographic lenses... and of course the name stuck to any such grouping in a lens/ eyepiece design
--------------------
Martin "Ronin" Thompson
Observatory Manager
Gilbert Rotary Centennial Observatory
16" LX200R on Paramount ME
Personal Gear
Nexstar 8 GPS
Nexstar 11 CGE
Celestron Onyx 80
Canon Rebel XTi (EOS 400D)
Canon Xsi (EOS 450D)(Yippeee New Toy)
5 Baader Hyperions (Just Love Em)
All things are possible, they just may not be probable
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8270
Loc: Beatrice, Nebraska
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Quote:
Perhaps a search for Piazzi-Smyth might be more fruitfull. Scottish Astronomer he developed a negative lens group field flattener.
I do believe that this is where the term "Smyth Lens" very probably originated. Indeed, the first time I ran into the term was in reading Harry Rutten and Martin van Venrooij's book TELESCOPE OPTICS (c. 1988 Willmann-Bell). On page 175 of the 1993 edition, the authors state:
"As mentioned above, it is possible to reduce astigmatism in a positive eyepiece system at the expense of introducing field curvature. By placing a negative lens between the objective and the positive eyepiece, however, we can compensate this field curvature. The reason is that the negative lens introduces curvature in the intermediate focal surface F2. If this is chosen to match that of the positive lens, the resulting combination of lenses can be free of both astigmatism and field curvature in the imaginary focal surface F2.
The design principle of the Nagler is not totally new. The combination was proposed long ago by Smyth; hence, the negative lens is called a Smyth lens (see ref. 16.1)."
Now some eyepiece manufacturers just slap in a negative lens in front of a simpler eyepiece design to try and make it work a little better at what amounts to a longer focal ratio. This would be a "built-in" Barlow. The Smyth lens, however, would be specifically designed so as to compensate as much as possible for extensive field curvature produced by the positive lens set that has already dealt with the astigmatism problem. There are a few eyepieces where the Smyth lens also serves as a sort of Barlow (the 5-8mm Speers Waler eyepiece for example). However, the overall design and intent of the negative field flattener is for field flattening and not just to bump up the f/ratio to a point where a simpler eyepiece can handle the narrower light cone. Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8270
Loc: Beatrice, Nebraska
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I got these results with my own eyepieces, but it is tough to format it for Cloudynights to get every column to line up.
Code:
To compare both the old AFOV/mag formula and the Field Stop Formula
with reality, I did some measurements for my own eyepieces and telescope.
The telescope is a 10" f/5.55 Newtonian (1410 mm actual focal length), and
the eyepieces tested were the following: (EFSD is eyepiece field stop
diameter in millimeters).
____Eyepiece_________Apparent Field____True Field (on sky)__EFSD__
40mm Mk-70 Konig 68.8 deg. 1.883 deg. 46.00mm
30mm Widescan III 84.0 deg. 1.798 deg. 44.00mm
30mm Ultrascopic (Orion) 52.2 deg. 1.063 deg. 26.08mm
27mm Kellner (Jaegers) 52.5 deg. 1.042 deg. 25.38mm
24mm Panoptic (Tele Vue) 68.0 deg. 1.113 deg. 27.00mm
24mm Koenig (University Opt.) 58.9 deg. 0.981 deg. 24.04mm
20mm Celestron Plossl 51.0 deg. 0.705 deg. 17.23mm
15mm Ultrascopic (Orion) 58.3 deg. 0.590 deg. 14.40mm
14mm Meade Ultrawide 80.5 deg 0.820 deg. 20.3mm*
10mm Ultrascopic (Orion) 49.8 deg. 0.339 deg. 8.32mm
6.4mm Super Plossl (Meade) 49.4 deg. 0.217 deg. 5.31mm
6mm Orthoscopic (Brandon) 44.2 deg. 0.188 deg. 4.62mm
*14mm Ultrawide's field stop is inside of the eyepiece inside the negative
doublet of the field lens, and thus its size is *approximate only*.
Each eyepiece had its apparent field of view measured on the optical bench,
rather than depending on the listed manufacturer values, which tend to be a
little optimistic. The focal lengths used in the calculations are those
given by the manufacturer on the eyepiece barrel, as, due to some of their
construction and short focal length, they could not be accurately measured.
The true field of view (TF) for each eyepiece was measured at the telescope
using the star-drift method. Below are the calculated values for true field
and their percentage deviations from the measured true field for each
formula. All angles are in degrees. FS-FORMULA is the eyepiece field stop
formula TFOV = (180/Pi)*EFSD/TFL, and AF/Mag is the Apparent field divided
by the magnification.
EYEPIECE Mag True Field | AF/Mag % Dev. | FS-FORMULA % Dev.
-----------------------------------|--------------------|--------------------
40mm Konig 35.3x 1.883 deg | 1.95 deg +3.5% | 1.869 deg -0.7%
30mm WSIII 47.0x 1.793 deg | 1.79 deg +0.0% | 1.789 deg -0.2%
30mm Ultras. 47.0x 1.063 deg | 1.11 deg +4.7% | 1.060 deg -0.3%
27mm Kellner. 52.2x 1.042 deg | 1.01 deg -2.8% | 1.031 deg -1.1%
24mm Panoptic 58.8x 1.113 deg | 1.16 deg +4.5% | 1.097 deg -1.4%
24mm Koenig 58.8x 0.981 deg | 1.00 deg +2.0% | 0.977 deg -0.4%
20mm Plossl 70.5x 0.705 deg | 0.72 deg +1.4% | 0.700 deg -0.7%
15mm Ultras. 94.0x 0.590 deg | 0.62 deg +5.1% | 0.585 deg -0.8%
14mm Ultrawide. 101x 0.820 deg | 0.80 deg -2.4% | 0.825 deg +0.6%
10mm Ultrasco. 141x 0.339 deg | 0.35 deg +2.9% | 0.338 deg -0.3%
6.4mm S. Plos. 220x 0.217 deg | 0.22 deg +0.0% | 0.216 deg +0.5%
6mm Ortho. 235x 0.188 deg | 0.19 deg +0.0% | 0.188 deg 0.0%
The AF/Mag standard form gives values which deviate from the actual true field
by an average of about +1.6 percent (maximum deviation: 5.1%). Although the
apparent field could be measured with some accuracy, one has to depend on the
manufacturer's eyepiece focal length numbers, and this undoubtedly is at
least partly the reason for the errors with the AF/Mag method. The Eyepiece
Field-Stop formula is generally more accurate for calculating the true field
of view with a given eyepiece. The mean deviation with this formula is
-0.4 percent, with the maximum deviation of -1.4%. NOTE: filters for 1.25"
eyepieces, (LPR's, Nebula Filters, ect.) will often have a clear aperture of
about 24.5mm. This diameter may become the maximum field stop size available
when these filters are being used, thus limiting the field of view for 1.25"
eyepieces which have field stops larger than 24.5mm.
The chief difficulties with using the Field Stop formula are when you
cannot get direct access to the field stop to measure it (the case when some
eyepieces have a field stop just behind the field lens), or when you do not
have the ability to measure it with much accuracy for the smaller diameter
field stops (as in some very short focal length eyepieces). In addition,
many eyepiece manufacturers do not routinely provide the eyepiece field stop
diameters for their products (only the focal lengths and the approximate
apparent fields). Still, either the classic AFOV/Mag formula or the Eyepiece
Field Stop formula should be able to give the amateur astronomer an idea of
what size of true field of view to expect when viewing with a given eyepiece.
Clear skies to you.
--------------------
David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org
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