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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2748
Loc: Just passing through.....
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Quote:
After two years with the binocular, I completely agree with Holger's opinion of the Fujinon 10x50. The "moderate" level of light intrusion, although annoying when looking into shadows at twilight, seems inconsequential for astronomy.
From what has been said and shown, it appears that EdZ's, and Jean-Charles', Fujinons are exactly like mine. MOST of what Bill has said about his, too, is exactly like mine. The proximity of the light leaks to the eye seems like a good way of understanding the magnitude of the visual effect.
But, still there's a big problem here. I DO NOT SEE focused recognizable dim moon ghost images with the moon outside the field, and I DO NOT SEE any kind of scattered light phenomena at all with anything dimmer than the moon. Neither does anybody else, but Bill, an experienced observer. I don't think he has just flipped out and started seeing stuff. Replacing the bino seems worthwhile, if it's still in the no-questions-asked exchange period.
Ron
Well, I'm not the only person.....
"Now, I have seen at the eyepiece what you described !
In the APM 7x50 HD, the moon must be about 10° off axis, at 2 o’clock in the left barrel, and 10 o'clock in the right barrel. The ghost image is difficult to see because the leakage is very small, and the moon must be at a very precise location. But yes, I can see a ghost moon that moves obliquely near the edge of the field of view.
In the Fujinon 16x70 FMT SX, the moon must be about 5° off axis, in the same direction as noted in the APM. The ghost moon is more obvious, because it crosses nearly half of the field of view.
In both cases, the ghost image is due to the leakages I have described before. As I have tried to explain, the annoying leakages are not those you have drawn.
I have never noticed this effect before this thread. Of course, this produces a (very) slight reduction of contrast. I think that shields can be the right solution, but they must be rather long."
JCB
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10x50 Fujinon FMT-SX binos
15x70 AP binos + Paragon p-mount
Oberwerk 100BT 45 degree + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery split tube
35mm Pan, 26mm Nagler, 17mm Nagler, 13mm Ethos, 8mm Ethos
Member IDA
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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2748
Loc: Just passing through.....
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Also, I see the same reversed ghost image of both the moon and Jupiter in my BT100s as I mentioned earlier in this thread.
In fact, I reported the "fingernail" of prism and ghost moon in an email to edz several months ago way before the Fujinons asking him to independently verify that his pair showed the same.
The fact I observe this in both, along with the testimony of Jean-Charles, makes me wonder if trading the Fujis in for another pair would accomplish anything other than the hassle.
I take this as lessons learned about the limits of porro prism design.
In spite of this, I give the Fujinon 10x50s a "one and a half thumbs up."
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10x50 Fujinon FMT-SX binos
15x70 AP binos + Paragon p-mount
Oberwerk 100BT 45 degree + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery split tube
35mm Pan, 26mm Nagler, 17mm Nagler, 13mm Ethos, 8mm Ethos
Member IDA
Edited by Mr. Bill (09/16/08 03:23 PM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12515
Loc: Cumberland, R I , USA42N71.4W
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Quote:
In fact, I reported the "fingernail" of prism and ghost moon in an email to edz several months ago way before the Fujinons asking him to independently verify that his pair showed the same.
In a post somewhere up above in this thread, I named at least a dozen binoculars in which I could see this so-called fingernail. In fact, I've discussed it in several posts regarding 3 of the finest quality binoculars I've ever held in my hands. I'll repeat, I don't think that fingernail and your ghost images are related.
edz
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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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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2748
Loc: Just passing through.....
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Quote:
Quote:
In fact, I reported the "fingernail" of prism and ghost moon in an email to edz several months ago way before the Fujinons asking him to independently verify that his pair showed the same.
In a post somewhere up above in this thread, I named at least a dozen binoculars in which I could see this so-called fingernail. In fact, I've discussed it in several posts regarding 3 of the finest quality binoculars I've ever held in my hands. I'll repeat, I don't think that fingernail and your ghost images are related.
edz
The point is, I have seen the ghost images in another binocular so it tells me that its not necessarily the result of my sample of Fujinons being defective or substandard. OTOH, it doesn't "prove" that there is nothing wrong, just that it's seen in a different pair of binoculars.
This is besides the issue of a correlation between fingernails and "ghost moons." It may be that fingernails are not necessary for this effect to occur. In my case, both are present in both pairs of binoculars; that's all I can say with certainty.
Again, no ghost images in my pair of roof prisms.
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10x50 Fujinon FMT-SX binos
15x70 AP binos + Paragon p-mount
Oberwerk 100BT 45 degree + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery split tube
35mm Pan, 26mm Nagler, 17mm Nagler, 13mm Ethos, 8mm Ethos
Member IDA
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Fiske
Carpal Tunnel
Reged: 03/14/04
Posts: 2057
Loc: Missouri / United States
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I've been following this thread with a sort of bemused wonder.  I mean, we're surprised when some stray light appears in the FOV looking within a few fields of the FULL MOON!
It sort of reminds me when I was trying to explain CA to a birder friend and used the example of looking at a starling on a telephone wire against a cloudy sky. He stopped me and explained he didn't waste time looking at starlings on telephone lines, regardless of the sky conditions. I had to laugh, naturally. But his point was that birders do everything they can to get the best view of a bird, not the worst view. They avoid backlighting, where possible.
In the same way, I avoid looking at stars near the full (or even quarter) moon. Why bother? And if that causes some minor image issue for an otherwise outstanding binocular, who cares? How often do you study planets with binoculars? I mean, really!
Now, bright stars causing reflections is a bit more concerning, I must admit. Is it really that obvious? I mean, I have certainly seen stray reflections looking near the moon, but never from bright stars.
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Fiske Miles
Nikon 8x42 LX / 12x50 SE Binos
Mini Borg 60ED, TV-101, AT80Ach, XT-8, C11/CI-700, 22-Inch Dob
Way too many Nagler eyepieces
http://www.fiskemiles.blogspot.com/
www.fiskemiles.com
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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2748
Loc: Just passing through.....
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Quote:
I've been following this thread with a sort of bemused wonder. I mean, we're surprised when some stray light appears in the FOV looking within a few fields of the FULL MOON!
It sort of reminds me when I was trying to explain CA to a birder friend and used the example of looking at a starling on a telephone wire against a cloudy sky. He stopped me and explained he didn't waste time looking at starlings on telephone lines, regardless of the sky conditions. I had to laugh, naturally. But his point was that birders do everything they can to get the best view of a bird, not the worst view. They avoid backlighting, where possible.
In the same way, I avoid looking at stars near the full (or even quarter) moon. Why bother? And if that causes some minor image issue for an otherwise outstanding binocular, who cares? How often do you study planets with binoculars? I mean, really!
Now, bright stars causing reflections is a bit more concerning, I must admit. Is it really that obvious? I mean, I have certainly seen stray reflections looking near the moon, but never from bright stars.
This all started out with me trying to do an honest report about ghost images from bright stars while sweeping MW fields.
I'm beginning to be sorry I ever said anything and this will be my last post on this thread that I started.
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10x50 Fujinon FMT-SX binos
15x70 AP binos + Paragon p-mount
Oberwerk 100BT 45 degree + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery split tube
35mm Pan, 26mm Nagler, 17mm Nagler, 13mm Ethos, 8mm Ethos
Member IDA
Edited by Mr. Bill (09/18/08 05:12 PM)
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ronharper
scholastic sledgehammer
Reged: 02/14/06
Posts: 989
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Bill,
Ok, you owe us one next time we want to nail your favorite bino to the wall!
Now, everybody, a reminder to look at stars near the moon on the night of Sept 18-19.
Moon to Knock Star Cluster Out of Sky, Stars to Land in US
Ron
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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 574
Loc: Ottawa, Ontario, Canada
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These visible fingernails are almost certainly more prominent with higher-index BaK-4 prisms. That's because the critical angle for complete internal reflection is steeper. In other words, a light "ray" traveling inside a prism will fully reflect off a surface when striking that surface at a steeper angle than is possible in lower-index glass such as good ol' BK-7.
Near the critical angle where light is fully reflected internally in BaK-4, at the same angle in BK-7 a good proportion of the light passes through the surface out into the surrounding air. This is the source of the bluish-grey "diamond" pattern which is so visible within the exit pupil for BK-7 prism binos--four mutually perpendicular reflecting surfaces suffering incomplete internal reflection of steeper light rays from the outer parts of the objective.
So high-index prisms suffer from a propensity to more efficiently retro-reflect light which enters them at angles closer to on-axis than can happen with low-index prisms.
To put it another way, an older BK-7 prism-equipped bino will probably suffer retro-reflection only if light is so far off-axis that it could only enter at the appropriate angle from *outside* the objective's aperture. In such case, no external blocking shield is necessary in order to ward off ghost-forming off-axis light. (Of course, light scattering off the inner barrels walls, etc., is another matter entirely.)
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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CESDewar
GorillAstronomer
Reged: 01/16/05
Posts: 1802
Loc: Morganton, GA, USA
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Quote:
I wonder whether the same issues are present in binoviewers? It would seem they would show light leakage also.
Very definiitely.... My BV3's show very pronounced ghosts on bright stars, but usually far enough away from the star itself, that it's rarely that bothersome, but it's very definitely there. My Denk II's (premium BV's) are better, but still show this ghosting. I think when you have prisms around, it's pretty difficult to eliminate these off-axis ghosts completely.
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ronharper
scholastic sledgehammer
Reged: 02/14/06
Posts: 989
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Bill,
Yippee, I have at last joined the ranks of the elite observers! Tonight with my 10x50 I succeeded in seeing focused ghost images of the moon, when the moon was positioned at 8 or 4 o'clock, 5 degrees out of the field. I could make them behave as you said.
The reason I had not seen them before is that they are not visible as long as the eye is looking at the field center. In my hand-held style, it is natural to keep the eye centered and point the bino around. But when the bino is mounted, as your photo indicates you've been doing, the eye must do the wandering. This is consistent with the idea that the ghosts are contained in the prism leak spots surrounding the exit pupil. I can believe that under dark conditions, similar ghost could be seen of bright stars.
The ghosts did not appear where I was looking, but off to the side, in my averted vision, so the view of them was too poor to say if the almost full moon was inverted or not.
A difficult observation, requiring creativity and luck, if I do say so! Maybe Ed will promote me to Pooh-Bah in advance of the requisite thousand posts?
Ron
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JCB
super member
Reged: 10/04/04
Posts: 151
Loc: France
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I don't know the Fujinon 10x50, but in the 16x70 version, the phenomenon is indeed so difficult to see that it passes unnoticed in normal use. It won't prevent me from purchasing one day this excellent Fujinon 10x50.
Unfortunately for Bill, he seems more sensitive than others to these ghost images.
Jean-Charles
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Mr. Bill
Carpal Tunnel
Reged: 02/09/05
Posts: 2748
Loc: Just passing through.....
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Well, I said I wouldn't post to this thread again, but I think that I have learned something about porro prism limitations, indeed any optical device with prisms.
Notice my signature....the 10x50s are off probation. I'm keeping them and will learn to live with this ghost imaging as the superior field flatness, contrast and pinpoint images of the Fujinon 10x50s are much more important to me for sweeping MW fields.
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10x50 Fujinon FMT-SX binos
15x70 AP binos + Paragon p-mount
Oberwerk 100BT 45 degree + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery split tube
35mm Pan, 26mm Nagler, 17mm Nagler, 13mm Ethos, 8mm Ethos
Member IDA
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JCB
super member
Reged: 10/04/04
Posts: 151
Loc: France
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What follows is intended for the few people who are interested in technical stuff.
I have made some ray tracing to simulate a 10x50 binocular. The objective lens and the eyepiece are supposed to be perfect. The focal length of the objective is 180 mm, and the field of view is 6.6°.
This first picture shows the normal ray path for an object that is 3.3° off axis, at the edge of the FOV.
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JCB
super member
Reged: 10/04/04
Posts: 151
Loc: France
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The second picture shows the light path for an object that is 12° off axis. There is a third reflection inside the first prism, then the light passes normally through the second prism, goes through the field stop where it is almost focused, producing the ghost image, and then passes in the vicinity of the exit pupil. The ghost image is weak, because there are two partial internal reflections in the first prism.
This effect and its solution are mentioned in this paper about binocular design :
http://www.optics.arizona.edu/optomech/references/papers/Vukobratovich%201989.pdf
(see figure 5). The cut in the prism is probably too small for blocking incoming light with a very wide angle.
For those who are intrigued, I don't work in the optics industry. I'm just curious, and I have access to several optical programs that I try to run.
Jean-Charles
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GlennLeDrew
professor emeritus
Reged: 06/18/08
Posts: 574
Loc: Ottawa, Ontario, Canada
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Thanks very much for this, Jean-Charles! Because of the small additional distance traveled by the rays within the prisms, the focus of the far-off-axis image point is a small distance beyond the focal plane for in-the-field images. This would imply that by racking the eyepiece farther out, the ghost should come to better focus.
The first diagram illustrates a common "defect" of Porro prism binoculars... The short focal length of the objective can result in part of the light from one side of the objective being clipped by the leading prism. This is visible as a small linear edge, or chord, in the exit pupil.
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Home-made 11X50 right angle bino, 8.1 deg. FOV
Modified 26X100 bino, 3.5 deg. FOV
Mediocre minds discuss people. Good minds discuss events. Great minds discuss ideas.
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pcad
Pooh-Bah
Reged: 01/17/05
Posts: 1488
Loc: Connecticut
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Thanks Jean-Charles, that's a really nice article and your diagrams are very helpful.
I think the groove across the large face of the prism would go a long way in reducing the ghosting that we've been discussing.
The give and take is that it costs money to add a step in making prisms and very few people will notice a difference if it's not there.
I doubt if a groove can be made large enough to totally eliminate these unwanted reflections without cutting into the on-axis light. A small groove would have been nice in a premium binocular like Bill's.
Bill, I'm glad you've decided to keep the Fujinons. We've all enjoyed this thread and have learned from it. I'm glad you brought it up and stayed with it until we were all up to speed. Thanks.
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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mooreorless
Carpal Tunnel
Reged: 07/05/05
Posts: 1539
Loc: Cornpropst Mills,Huntingdon,Pa
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I want to add another thank you to Jean-Charles. This has been an interesting thread. I have used a Fujinon 10x50 FMT-SX and I really didn't notice this ghost image. I must say I wish I had a 10x50 Fuji.:-)
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Regards,Steve M
"the eights are better" Jim Carmichel of Outdoor Life magazine
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