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Patrik Iver
sage
Reged: 07/29/03
Posts: 201
Loc: Kaarina, Finland
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Hello everyone!
One thing has puzzled me for some time now:
I expect one could build a correct view binocular of porro-like configuartion using four mirrors per barrel just as one normally would use a pair of prims.
There has been discussions on the other forums here on CN about telescope mirror diagonals versus prism diagonals and there is also a technical article here about that issue. The consensus seems to be that prisms are preferable for longer focal ratio systems while fast systems would be better off with mirrors mainly due to less chromatic and spherical aberration.
Yet all binoculars that I know of use prisms despite focal ratios typically around f/4.
Logical reasons that spring to mind are:
* Less parts to collimate.
* More likely to stay collimated when used/abused.
* Longer life as the mirror coatings would degrade over time.
Or is it just that the chromatic and spherical aberration of the prisms is not a problem at low magnifications and that the aberrations in fast achromatic objectives totally dominate and that any aberrations from the prisms are thus insignificant?
Does anyone know of mirrors being used over prisms in any binoculars?
Other comments?
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Best regards,
Patrik Iver
60°N, 22°E
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patter1
professor emeritus
Reged: 01/19/05
Posts: 597
Loc: Canada
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Quote:
Logical reasons that spring to mind are:
* Less parts to collimate.
* More likely to stay collimated when used/abused.
* Longer life as the mirror coatings would degrade over time.
Patrik, I'm not a bino expert, but these three reasons sound very plausible. Also add light loss from mirrors...that would be significant.
Quote:
Or is it just that the chromatic and spherical aberration of the prisms is not a problem at low magnifications and that the aberrations in fast achromatic objectives totally dominate and that any aberrations from the prisms are thus insignificant?
I thought that spherical aberration originates in the objective, and not the prisms? But yes, chromatic aberration is less visible at lower mags, so it's less of a priority.
I have some questions myself about the chromatic aberration induced by binoc prisms (not the objectives)...
-at binocular magnifications (say 10x), is it even visible?
-is it longitudinal, or off-axis, or both?
-is it insignificant compared to the chromatic aberration introduced by the typical f/4 objectives?
There are a few mirror binocs...like the super wide angle Orion 7x32, and there's now a 10x50 that appears to have the same body.
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Patrick
8" f/6 NewStar dobsonian
Orion Starblast 4.5" f/4 mini dobsonian
42mm SuperView, 17mm Nagler T4, some other cheapies
Omcon 7x50, Oberwerk 11x56, Olympus DPS-R 7x35, Olympus Magellan 8x25
homemade 50mm right-angle bino-scope prototype
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Alan French
Night Owl
Reged: 01/28/05
Posts: 1484
Loc: Upstate NY
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I think you have hit the main reasons prisms are used.
For low power viewing, it is not necessary to produce a diffraction limited optical system. For day use, your eye stops down most binoculars, improving performance. At night, you eye's pupil is wide open, but the performance of the dark adapted eye is almost certainly the limiting factor.
Mirror might make sense in a large binocular designed for use with interchangeable eyepieces. Note that binoviewers designed for telescopes often include a "glass path compensator," that helps control the aberrations introduced by the prism.
Clear skies, Alan
Edited by Alan French (05/22/06 09:59 AM)
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patter1
professor emeritus
Reged: 01/19/05
Posts: 597
Loc: Canada
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Here's the ones which I think use mirrors...
http://www.oriontel.com/shopping/product/detailmain.jsp?itemID=9582&itemType=PRODUCT&iMainCat=5&iSubCat=16&iProductID=9582
http://www.scopesnskies.com/prod/bresser/binoculars/swa/10x50.html
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Henry Link
sage
Reged: 03/31/04
Posts: 316
Loc: Greensboro, NC
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I'd like to know more about the interaction between prism and objective aberrations in binoculars.
Recently I removrd the eyepiece and objective from an old Nikon 8x30E, and placed them at each end of a hollow tube to ellimimate the effect of the prism. The result was far worse aberrations than with the porroprism in the light path, infact a completely unacceptable image even at only 8x.
Two possible explanations occured to me: 1) that the aberrations of prisms are complimentary to the aberrations of the objective and act to correct the objective aberrations rather than make them worse and 2) that the partial vignetting of the exit pupil by the prism shelf apertures reduces the contribution from the objective edge and that reduces the aberrations (there is no vignetting with the hollow tube) . To test the second idea I stopped down the objective to as small as 10mm. That did reduce the aberrations, but the image quality was still not as good as with the prism in place, so I have tentatively concluded that the objectives and prisms in at least some binoculars are designed to work together for lowest chromatic and spherical aberrations. Maybe Bill Cook could could help us out with this.
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Alan French
Night Owl
Reged: 01/28/05
Posts: 1484
Loc: Upstate NY
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Henry,
I talked to someone - Henry Paul, I think - at Stellafane long ago about my 25x120 "battleship" binoculars. I asked about using one objective for a Rich Field Telescope. He said I would be disappointed, because the objectives and prism systems were designed to work together.
Clear skies, Alan
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JCB
super member
Reged: 10/04/04
Posts: 151
Loc: France
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Binoculars with mirrors instead of prisms have been manufactured. See the links below :
http://main.yukonopticsglobal.com/overview.php?pid=6
http://www.russianoptics.com/binoc9.html
http://www.rugift.com/photocameras/manuals/newtonian-reflector-binoculars-nrb.htm
You can notice the very uncommon shape of the "prisms" housing, and the extreme compactness and lightness for a 30x binocular. Two mirrors on each side will invert the image, so an additional set of internal lenses is necessary.
According to a short review I have read in a French forum, these binoculars are very dim, but surprisingly sharp for a so high magnification.
About the aberrations introduced by prisms :
I have designed with OSLO a 40 mm f/4 cemented doublet followed by a piece of BK7 to simulate the prisms. The polychromatic strehl ratio of this set is 0.73.
Now, if I remove the prism and evaluate the doublet at the new best focus, the strehl ratio drops to 0.15 : there is a lot of spherical and chromatic aberration.
If I design a single cemented doublet, I can achieve a 0.59 strehl ratio.
The fact that the result is better with a piece of BK7 is probably due to my inability to correctly design an objective lens, but it is obvious that the effect of prisms must be taken into account for designing a binocular.
Jean-Charles
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1500
Loc: Connecticut
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Patrik,
You can achieve correct, upright images using just two mirrors per tube. There have been several threads about this topic in recent months. One product that uses this design is the EMS system by Matsumoto. Another system that uses mirrors is Astromeccanica. These do not have correct, upright images. Both of these systems are used to make bino-scopes out of telescopes and strain the definition of what most people consider binoculars.
One thing, that until recently, prisms do better than mirrors is % light transmission. Most binocular prisms utilize total internal reflectance (TIR). In the case of porro prisms, The loss of light is at the entrance and exit of the prisms. If they are bonded, thats just two surfaces that can have anti-reflection coating resulting in a very efficient system.
With mirrors there was always a significant loss of light at each reflective surface. Now with di-electric mirrors, that loss is much less than before.
Even so, a two mirror system may not be quite as efficient as a well designed prism system. But the performance gap is smaller than ever. A three or four mirror system is going to find it hard to keep up.
Using BK7 for prisms can reduce the abberations caused by the prism systems, but only in instruments that are slower than f/5. Despite this, most binoculars of any type use BaK4 nowadays.
Patrick (patter1),
Those UWA binos definately use mirrors and prisms on both sides.
JCB,
I wonder why they call the 30x russian binos "newtonian". I wonder if the "objective" is a corrector plate or simply a window. It may even be similar to a folded refractor. Even if it's a window, where is the secondary (tertiary)? Is it near the window or the primary. It'll be dim from mirror losses and central obstuctions compared to refractor style binos. The obvious advantage is the length and weight of the tubes. I suppose they could still work well if they are well designed and built. Sounds like a daytime bino to me.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
Edited by pcad (05/22/06 08:31 PM)
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jkevn
super member
Reged: 03/02/06
Posts: 175
Loc: Tucson, Arizona
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pcad knows the optics issues.
With 4 mirrors per side instead of Porro prisms, I wonder how many tiny "conditonal alignment" set screws there would be for each pair of binoculars, hidden in the darkness underneath rubber armor and blobs of RTV? And how long and complex would be the vendors web pages describing how to adjust them?
Also, using Porro Prisms is a more monolithic design; less likely to lose alignment.
jkevn
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BillC
on a new path
Reged: 06/04/04
Posts: 2108
Loc: Washington, USA
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We are now beginning to talk about what I wish all optical experimenters could know:
FOR THE BEST RESULTS, ALL OPTICAL SYSTEMS SHOULD BE DESIGNED AS SYSTEMS.
I don’t know how many times some young—or old—telescope maker has dragged in some humongous lens and wanted me to make a telescope out of it. It seems the consensus is that if it is a lens, it is a potential telescope. That may very well be true. But, quite often the telescope produced will be a poor performer.
About 15 years ago, Edmund Scientific was liquidating some 6-inch aerial lens that were designed to produce 22-inch images on film. For $250 people thought they were getting a steal. Had they made a camera, they COULD have been correct. But, as a short-focus telescope? Nope? Of course, rainbows ARE beautiful.
Finally, prisms do have a say in final performance. However, the main producer of aberrations in the objective. While thicknesses, glass types, and spacings in the prisms play a part, it is mostly related to chromatic aberration. If you want better optics, you’ll need to put your effort into the lenses, because the primary functions of the prisms are to offset and erect the image.
Cheers,
Bill
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William J. Cook, Chief Opticalman, USNR-Ret.
Founding Editor, Amateur Telescope Making Journal
20-year vet. of Captain's Nautical Supplies, Optics Dept. Mgr.
Optics Machanic, WG11-3306, Ft. Lewis, Tacoma,WA
Yata, Yata, Yata . . .
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1500
Loc: Connecticut
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Bill,
Henry, Alan, and Jean-Charles have all posted that the objectives and prisms of certain systems work best together.
You indicate that the objective is the single most important component of a prismatic binocular. Yet the prisms do have a role in determining optical quality.
It seems to me, that if you design the best possible objective for a refractor without prisms, this is not the same lens that would make the best instrument if a set of prisms were added to the design. There doesn't seem to be many choices to be made on the prism side. So the objective has to compensate for the effects of the prisms? Am I close?
It's probably a lot more complex than it looks. I'm sure glad I'm not the one designing the next batch of instruments. I like being an end user, thank you very much.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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JCB
super member
Reged: 10/04/04
Posts: 151
Loc: France
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Peter,
Look at this spotting scope :
http://main.yukonopticsglobal.com/overview.php?pid=26
The shape, similar to the binocular of the same brand, suggests to me that this is a folded refractor, without central obstruction. The bent reflection in the front element also shows this is not a window, but a lens with a spherical surface.
It is called Newtonian however. If I am right, it's only a pure marketing designation.
Jean-Charles
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DJB
Pooh-Bah
Reged: 02/23/05
Posts: 1267
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Hi Patric,
I have to side with Bill C. on his analysis. I, myself, support the idea that the ocular can be instrumental in further correcting the image from a very well designed objective doublet or triplet.
I do not follow the reasoning that the prisms can contribute much here. Sure, the glass selection introduces the redirection, possible vignetting, and FL compression of the light cone, which is its purpose. But FLAT prism surfaces will do little to correct aberrations already inherent in a poorly designed SYSTEM IMO.
As per the original question, I know that the Orion Expanse 7x32 and the Carson 8.5x42 UWA binoculars use mirrors. Also, a discontinued Bushnell did as well. The current models have a rather distinctive look to them--all very similar, and I'm sure several brands of these are available.
With such a very wide field, using prisms would be prohibitive in size, weight, and cost. The WWII SARD Mark 43 6x42 was a 10* MIL SPEC bino used in aircraft to spot approaching enemy aircraft. This model does use prisms, and it is heavy. Same goes for the B&L Mark 41 7x50.
The problem with mirrors has already been identified: less reflection and the inability to retain this percentage of reflection due to tarnishing of the first surfaces due to aging. So these UWA binoculars are probably not to be taken too seriously. Anyway, the price point seems okay.
Best regards,
Dave.
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Alan French
Night Owl
Reged: 01/28/05
Posts: 1484
Loc: Upstate NY
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Introducing a prism in a well corrected system adds overcorrected spherical aberration and longitudinal chromatic aberration (secondary color) on-axis. Off-axis a prism introduces coma, astigmatism, and lateral color. The off-axis aberrations are worse for for fast systems. Most binocular objectives are f/3.5 or so, so they qualify. It is no wonder the objective and prism system have to be designed together.
The aberration most folks seem to complain about is color fringing along high contrast surfaces away from the field center. This is lateral color, and it is largely produced by the eyepiece.
Clear skies, Alan
Edited by Alan French (05/23/06 08:22 AM)
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Patrik Iver
sage
Reged: 07/29/03
Posts: 201
Loc: Kaarina, Finland
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Quote:
Peter wrote:
You can achieve correct, upright images using just two mirrors per tube. There have been several threads about this topic in recent months.
Can you do this with two mirrors only and still have a straight configuration i.e. you look in the direction that the objectives are pointing?
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Best regards,
Patrik Iver
60°N, 22°E
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BoyntonStu
sage
Reged: 01/22/06
Posts: 263
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Sorry, 2 mirrors must have the EP at 90* in order to give a correct image.
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1500
Loc: Connecticut
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Patrik,
Stu beat me to it. These EMS-like systems work on the same principles as roof prisms. An oversized Amici prism is a good analogy. Oversized so the light path does not hit the roof edge at all.
I rechecked the Astromeccanica products. It looks like they use three 90° mirrors. This also results in a right angle bino-scope, but not a correct, upright image.
JCB,
Looking more carefully at those Yukon products I have to agree with you about their being folded refractors. Some of the other images looked like they had an aluminized spot on the inside of the objective, much like a Maksutov design. Thanks for correcting my confusion.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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BillC
on a new path
Reged: 06/04/04
Posts: 2108
Loc: Washington, USA
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Hi pcad:
You paraphrase me as saying, “the objective is the single most important component of a prismatic binocular.”
But that is not what I said. What I said was: “the main producer of aberrations is the objective.”
You also point out, “Yet the prisms do have a role in determining optical quality.”
Once my thoughts are placed back into context, I find your conclusion to be just fine. Of course, everything that comes between the eye and its target will have some effect on the image. In the most basic terms, I was saying that it takes a fortune in designing and optical elements to correct a poorly designed objective.
Can an eyepiece create or worsen aberrations? Absolutely. HOWEVER, the elements are smaller and the designer is apt to place more of them together (sometimes using aspheres) to get things under control. So, I will repeat myself in saying that the main producer of aberrations is the objective.
I would like to go one step further in explaining why.
Suppose you have two large baskets of colored stones.
One basket is filled with red, green, and blue stones that are the size of baseballs. The other basket weighs exactly the same and the stones are exactly the same colors. The difference is that each of these stones is the size of a BB.
Now suppose your boss comes to you and asks you and another worker to sort each basket into three smaller baskets, each containing only one color stone.
Your friend gets the basket with the large stones and you get the one with the tiny stones. You both start the project at 8:00 a.m. on Monday morning.
At 8:02 on Monday morning, your friend is done and going about his business. But, what about you? Tuesday morning comes and you are barely into the project enough to be noticed. You finish 6 work days later. And although the project seemed critical to your boss, it was so time consuming and boring, that you took some liberties—after all, with stones so small, green and blue can occasionally look alike. This is especially true if you are tired and bored.
Well, the first basket represents the correction of aberrations at the objective lens, as nearly parallel rays pass through a 50mm aperture. The second basket represents the difficulty in correcting those aberrations once the light path has dropped to, say 20 millimeters, and is shrinking rapidly on its way to the focal plan at a steep angle.
The bottom line: it is easier and less expensive to correct aberrations at the objective. Yes, prisms have a say in the matter. BUT, the main contributor here is in the choice of glass types.
Hopefully, no one is thinking that Bk7 and BaK4 are the only glass choices for binocular prisms. For, while they may be the most logical choices, a lens designer will have hundreds of glass types to choose from, coming from many manufacturers.
Cheers, 
Bill
--------------------
William J. Cook, Chief Opticalman, USNR-Ret.
Founding Editor, Amateur Telescope Making Journal
20-year vet. of Captain's Nautical Supplies, Optics Dept. Mgr.
Optics Machanic, WG11-3306, Ft. Lewis, Tacoma,WA
Yata, Yata, Yata . . .
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KennyJ
Reged: 04/27/03
Posts: 10142
Loc: Lancashire UK
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Bill ,
Thank you for a SUPERB analogy .
It has helped me to understand the subject matter more clearly than ANYTHING I've ever read about the subject !
Regards , Kenny
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Two eyes and a preference to use both
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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johnno
scholastic sledgehammer
Reged: 05/03/04
Posts: 807
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Hi Bill,
I agree with Kenny,
THAT,was an EXTREMLY, good analogy,one of the best I have ever read.
I thank you.
all the Best.
Regards.
John
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