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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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Bill,
You're right, I'm sorry about any confusion with my posts about which glass I'm referring to. Anything with a 4 in it = Bak4, anything with a 7 = BK7.
I have heard of people using shaving mirrors for scopes. At the time I wondered what focal ratio it had. Now I know.
About the suction cup, I often examine patients with 20/20 in one eye and 20/200 in the other. Many times they are blissfully unaware of this until I test the eyes individually. Point is, as long as one eye is working well, many people won't notice a problem with their vision. They might notice if both ojectives had suction cups on them, but I wouldn't bet on it.
Edz-
I see now what you mean by part of the light cone hitting the edge/side of the prism and being lost that way. I hadn't thought of that. I used a GLP and a glass of water to convince me of which way the beam was deflected.
My hypothesis was that some of the light reaching the 45 degree face of the prism was not reflected but passed out of the prism. I thought that the edge of the light cone, having the greatest angle, might fail to be reflected due to different critical angles between BaK4 and BK7. I'm not trying to convince you, just trying to make the musings of my mind clear. My theory may (likely) be so much hooey.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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By using our own "best of" I anwsered at least one of my own questions. Rboe quotes a source saying that older f5 binoculars used BK7 prisms and f4 or faster use BaK4 prisms. This doesn't prove the f5 designs were perfect, but provides a "historic fence" between workable BK7 and Bak4 designs.
Another post does state a higher refractive index for Bak4. So BaK4 bends light more at transistions and BK7 less so. So which glass needs bigger prisms? Looks like I have some more thinking to do.
If anyone is still interested, I reviewed all of Rolands posts on astromart that have anything to do with binoculars / binoviewers. The thread called "bino wars" is the most applicable. The message is the same but he never gets into the nitty-gritty of why Bak4 is better BK7 with fast optics. Perhaps it's like my old physics text says, The rest of the proof is left to the reader as an exercise.
Never one to beat a dead horse,
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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I did the math and at f4 part of the light cone fails to meet the critical angle requirement using BK7. With BaK4 the entire light cone meets the critical angle requirement.
A little more homework showed that for systems faster than f3.35 BaK4 starts to fail in the same way BK7 did. The solution would be a material with a higher index of refraction than BaK4. A nice list of these materials is found on the edmundoptics site. It's late, good night Gracie.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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< A nice list of these materials is found on the edmundoptics site.>
Peter ,
This could be the list I referred to in an earlier post .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
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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brt209
sage
Reged: 09/18/05
Posts: 222
Loc: London UK
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It might be a good thing that the americains don't build binoculars,because if they built binos as they build telescopes and cars we'd be better off using just our eyes instead!
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BillC
on a new path
Reged: 06/04/04
Posts: 2111
Loc: Washington, USA
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Quote:
It might be a good thing that the americains don't build binoculars,because if they built binos as they build telescopes and cars we'd be better off using just our eyes instead!
That, sir, depends the integrity and the optical knowledge of those doing the designing and manufacturing. I can assure you that some Americans would produce a product to compete with the best in the world.
If I were 20 years younger--and had Kenny's money--I would do it myself. 
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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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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Quote:
I can assure you that some Americans would produce a product to compete with the best in the world.
I would fully agree with you Bill, and even note that some are doing it right now, especially in telescopes.
But I would like your explanation, if you have one, how it came to be that the US economy, with a significant activity in manufacturing telescopes, is not manufacturing binoculars?
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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< If I were 20 years younger--and had Kenny's money--I would do it myself. >
Come on Bill , you know I couldn't even afford to buy your collection of guitars !
Regards , Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
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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brt209
sage
Reged: 09/18/05
Posts: 222
Loc: London UK
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I was just kidding of course!
I am sure that they are plenty of skilled engineers in the US who could do a great job.
I believe it's got to do with reputation and both the germans and the japanese have had time to gain a solid one in optics for decades if not longer.
No offence guys! 
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BillC
on a new path
Reged: 06/04/04
Posts: 2111
Loc: Washington, USA
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Quote:
But I would like your explanation, if you have one, how it came to be that the US economy, with a significant activity in manufacturing telescopes, is not manufacturing binoculars?
Because, before my hard working, God fearing, ancestors kicked King George's boys out, some of them learned to like something for nothing. Then, when FDR got us out of the depression, he did so by instituting a bunch of something for nothing programs. Consequently, to many of my countrymen--at least the young ones--"quality" is a word in someone else's dictionary. I love my country with every fiber of my body. However, I love truth more. And the truth is that, in the name of "fairness" and "political correctness," our country is becoming a shadow of its former self. But then, I have heard from friends in England, France, Holland, Germany, and Australia who parrot the same thing. Oh, well.
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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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BillC
on a new path
Reged: 06/04/04
Posts: 2111
Loc: Washington, USA
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Quote:
Come on Bill , you know I couldn't even afford to buy your collection of guitars !
Regards , Kenny
Heck, Kenny, I only have three these days, and one of those just came out of pawn, yesterday.
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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BillC
on a new path
Reged: 06/04/04
Posts: 2111
Loc: Washington, USA
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Quote:
I was just kidding of course!
I am sure that they are plenty of skilled engineers in the US who could do a great job.
I believe it's got to do with reputation and both the germans and the japanese have had time to gain a solid one in optics for decades if not longer.
No offence guys!
The Japanese deserve our deepest optical respect. Especially clever is the way they can make so many Japanese products appear to come from Europe. But, then, that is a story for another day.
Still, were it not for the 9 engineers and techs they imported from Zeiss back in the 20's, and our sending Demming to them, the Japanese would still be selling cheap transister radios. They learned TOO well.
Hey . . . how 'bout them new Hondas? Even the gas burners are quieter than my watch. (disclaimer: While Bill Cook claims to favor truth in advertising, he is not above lying through his teeth for the sake of humor.)
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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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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For Kenny's benefit, links for index of refraction tables http://www.edmundoptics.com/techsupport/displayarticle.cfm?article=259 and http://hyperphysics.phy-astr.gsu.edu/hbase/tables/indf.html .
air = 1.00029 = n1
BK7 = 1.517 = n2
BaK4 = 1.569 = n2
Snell's law
n1/n2 = sin (theta 2)/ sin (theta 1)
critical angle
theta crit = arc sin (n1/n2)
BK7 fails 100% TIR below f5.02
BaK4 fails 100% TIR below f 3.4
The nitty-gritty is left to the reader as an exercise.
Below these critical f ratios light loss occurs at the 45 degree faces of the porro prisms. Making them larger will have no effect, assuming they're not undersized to begin with. Since the angle of the light cone does not vary inside the prisms, This "clipping" effect will occur at each 45 degree face of the two porro prisms. This will produce a square or diamond exit pupil which has been described by others on this forum. A particularly nice photo of this effect is at http://fantao.home.att.net/information.htm .
There are two cheap and dirty solutions that will make the diamond into a circle again.
Stopping down the objective will block the light rays responsible for this effect. Guess what, This increases the f ratio which fixes the basic problem in the first place.
Another is to use undersized prisms which achieves the same effect of stopping down the objectives.
I'm sure there are other ways, but I'm not devious enough to think of them. Besides, the view with a diamond exit pupil would probably still be acceptable visually. The main problem is that you're not able to deliver all the light the objectives collect to the exit pupil.
OK, I'm done. I hope this is interesting to someone other than myself.
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: 2111
Loc: Washington, USA
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Quote:
There are two cheap and dirty solutions that will make the diamond into a circle again.
Stopping down the objective will block the light rays responsible for this effect. Guess what, This increases the f ratio which fixes the basic problem in the first place.Peter
I see other possibilities. We could NOT stop the objective at all, since our eyes will automatically crop the gray area during the day, and since, at night, our peripheral vision is more sensitive than the axial, thus making the argument almost moot . . . again.
Just a thought,
Bill
P.S. BK7 Nf - Nc = .008054
BaK4 Nf - Nc = .010135
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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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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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< Besides, the view with a diamond exit pupil would probably still be acceptable visually. >
Wouldn't a DIAMOND exit - pupil prove prohibitive ? :-)
Kenny
--------------------
If everyone is thinking the same thing , no-one is thinking - General George S.Patton
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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Mad Matt
member
Reged: 05/20/03
Posts: 94
Loc: Frankfurt, Germany
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EXCELLENT RESEARCH pcad!!!! I think you have solved the mystery 
Just an interesting side note, My somewhat dated Schott catalog shows that the Material cost of Bak4 is only 1.4 times the cost of BK7 (or 40% more than BK7)
-Matt
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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But if I understand correctly BK7 is more difficult material to work, so after labor involved, what is the cost difference of the finished product?
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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BillC
on a new path
Reged: 06/04/04
Posts: 2111
Loc: Washington, USA
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Hi EdZ:
I could very well have missed something. However, I have worked many pieces of Bk7 and have noticed no unusual problems. It's not as brittle as the f2 or f4 that usually accompany it.
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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milt
sage
Reged: 09/13/04
Posts: 430
Loc: Arizona
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Quote:
The nitty-gritty is left to the reader as an exercise.
Hi Peter,
I'm afraid this reader failed the exercise.
Using the refractive indices and formulae you gave, I calculated that BK7 has a 3.75° margin relative to 45°, while Bak4 has a 5.39° margin.
However in an f/5.02 light cone derived from on-axis parallel rays, peripheral rays are 5.69° off axis and in an f/3.4 light cone they are 8.37° off axis. These angles are derived from:
arctan (0.5 / focal ratio)
This would seem to violate the critical angle in both cases. I am sure I am missing something here, but I don't know what...
Thanks,
Milt
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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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Milt,
Did you take into account that light obeys Snell's law at the transition from air to glass as it enters the prism?
My calculations included some rounded off values. critcal f# are BK7 ~ 5.0203 and Bak4 ~ 3.3551. I found an error in my calculations for BaK4. Instead of 3.4106 I now get 3.3551. These numbers are associated with the critical angles. I rounded them to 5.02 and 3.36.
I agree with your values of 3.75 and 5.39. But these are for the light cone when it's inside the prism material.
I calculated 5.02 and 3.40 (should be 3.36) by working backwards. Your numbers take 5.02 amd 3.4 and work foward.
After refraction at the air-glass interface 5.39 becomes 8.48 degrees for Bak4 and 3.75 becomes 5.69 degrees. Solving for FL = 0.5/tan theta 1. Now the critical f# of BaK4 is ~ 3.3551 and for BK7 ~ 5.0203.
Don't forget the critical angle is measured from the normal to the 45 degree prism reflecting surface. But I think you took this into account already.
Thanks for keeping me on my toes.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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