KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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Joad ,
For what it's worth , your subtlety , and mastery of the art of verbal communication , is neither lost nor wasted on this reader .
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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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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Edz et al,
I'd like to bounce a few thoughts about the bk4 vs. bk7 issue off you.
I am under the impression that porro prisms use total internal reflection (TIR) in order to bend the light path. I think this is true regardless of which glass is used. The quote from Roland Cristen said that bk4 is better for wide field applications. Does this mean that bk7 prisms would "leak" light and not have TIR for the entire cone of light. He pointed out that for a narrow light cone bk7 works fine as prisms. So it seems to me that bk7 and bk4 operate with TIR when the light enters the face of the prism close to 90 degrees. However, it seems bk4 has a more forgiving critical angle than bk7. Would this explain the loss of edge illumination in bk7 prisms used in binoculars? Also I belive all this is tied up with the index of refraction. So the suitability of bk4 over bk7 in binoculars is based on the two materials having different indexes of refraction.
Well, I've laid bare my preconcieved notions. Have at it! I think I'll just go find some cover.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
Edited by EdZ (09/18/05 08:20 AM)
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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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Hello Peter ,
< However, it seems bk4 has a more forgiving critical angle than bk7 >
- - I like the way you worded that !
That's not to say it's necessarily TRUE of course , but it has a ring of truth about it to me , if no -one else :-)
Although irrelevant to this particular X vs Y discussion , I might add that I once found a long list of glass types which COULD be used for prisms , etc , if cost was no object.
In a list containing perhaps 20 different quality types , from what I recall of it , both Bak 4 and Bk7 were WELL down the pecking order , sort of " mid - table " at best , if not struggling to avoid relegation to a lower division !
Im sorry , but I can't seem to find that list now .
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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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Looking through a pair of recent expensize Celestron binos with full sized BK7's, I think the outland 10X50's, the exit pupil appeared to be fully illuminated but the images were noticably dimmer compared to any other, Pentax Nikon etc. 10X50 with Bk4's.
We are talking prisms here, if the Bk7's tend to pass more light through, does this effect their performance as a prism? It may be fine for an optical lens which Roland is talking about but the different characteristics of the prism could lead to other performance issues couldn't they?
-Chris
All the notes that I posted above in regards to BK7 vs BaK4 where from a discussion by Roland regarding prisms, not lenses.
This QUOTE from the discussion
" BK7 is the clearest, most defect-free optical glass available for prisms. BaK4 is close, but not quite the same. That's ok for low power applications of a typical binocular, but not for high power low contrast situations seen in telescopes." Roland Christen
the rest of the discussion I described above.
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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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Peter,
what you say is correct. This does explain the loss of illumination at the edges of BaK7 prisms when we see them used in binoculars with a wide light cone.
I might just add one note. The appliction is really in reference to a wider angled light cone, not a wide field view, as that could be achieved behind the prisms with the eyepieces. An f/4 binocular has a light cone with twice the angle as an f/8 instrument and you are correct that is the internal angle of reflection where BaK4 prevails over BK7 for use in binoculars.
Yes, we do rely on fast instruments to give us wide angle applications. So in this sense you are are correct. But I could still vary the Tfov from an f/4 objective and I could still vary the Tfov from an f/8 objective by choosing the appropriate eyepieces.
For example, a Pentax 20x60 has a 2.2° field of view, meaning the eyepieces have a field stop giving an Afov of 44°. I could take that same Pentax housing and slip in a pair of eyepieces with an Afov of 72° and I would have a 20x60 binocular with a 3.6° field of view, pretty wide for a 20x binocular. But I haven't changed a thing with the incoming light cone, it's still f/4. So BaK7 prisms still would not work.
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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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Joad,
I didn't want to leave other readers with the potential to misinterpret that a green reflection from a coating is bad. It was a good opportunity to explain the reflective properties of various properly and improperly applied mixtures of coatings.
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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rboe
Numbfinger
Reged: 03/16/02
Posts: 39766
Loc: Phx, AZ
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It is nice to hear that again - and some new details I was not aware of from the last time we went over coatings over a year ago.
The whole bak7 vs. bak4 issues has been beat to death by the binoviewers people so it was very odd to see it debated here.
However; cost still seems to be main constricting factor. 
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Ron
NS11GPS
Pronto
16" dob
15X70 Obies
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Quote:
I test bino coatings on Luna, if there are reflections then they are not so great, if not they are fine. Now just because there are some reflections doesn't mean the bino-s are un-use-able either. Many of the cheapie 10x50 under $20 binos do a fine job of sweeping star fields and general DSO observations, yet give reflections on Luna.
I think checking for internal reflections by observing the moon would show you reflections in almost any binocular. Some of that would be due to reflections off of the internal glass surfaces, even fully multi-coated surfaces. But some of those reflections could originate from poorly blackened internal barrel surfaces, shiny edges at eyepiece field stops and either poor or no baffles. The light from the moon is so intense, it will cause all these things to show up. It could be a number of things and it would be very difficult to separate them.
It could very well be that the majority of the reflections from the moon are there just because the light from the moon is so intense. Just about any internal glass surface, even with only 0.5% reflectivity, is going to reflect a small portion of the intense light from the moon.
Keep in mind, the light reflected internally from a full moon (approx mag -12), (even if only 0.25% of the light, the amount that might be reflected from a fully multi coated surface), is still about mag -4 or -3, brighter than the brightest star in the sky.
So part of what is going on when you see reflections of the moon internally may not be indicative of poor coatings. Even the best coatings cannot overcome that kind of intense light. It would be nearly impossible to suppress reflections from an object so bright.
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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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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Edz,
Thanks for confirming my understanding about bak4 / bak7 issues. Of course this just leads to other questions.
1) Do you have a link to Roland Cristen's discussion on this subject?
2 With TIR there is essentially no light lost. That's why fiber optics work so well. You mentioned that mirrors, even good ones, can't reflect 100% of the incident light. Why then are mirrors used in diagonals rather than prisms. I know that there are prism diagonals available, but most of the high end diagonals are dielectic coated mirrors. What am I missing?
3) I am familiar with how porro prisms are used in binoculars. Are prisms used in the same way in a binoviewer? I would imagine that each side of a binoviewer would need only a 90 degree bend in the light path after the beamspliter. Sort of like a prism diagonal again.
4) I searched in the binoviewer threads for bak4 and bak7 and did not find any mention of TIR or index of refraction in the discussions. The conclusion that bak4 is best for binoculars and bak7 is best for binoviewers is there but they arrived at this using different reasoning. Perhaps the disscussion Ron mentioned took place on a differant forum?
5) Earlier in this thread (#602194) you suggested that bino's with oversized bak7 prisms might work. If we agree that TIF and critical angles are why bak7 prisms don't work for binoculars (fast optics) why would making bak7 prisms bigger do any good?
6) How slow would the objectives have to be in order for bak7 prisms to work in binoculars?
7) It's my impression (I might be wrong) that the astro-mechanica (forgive the spelling) binoscopes use mirrors instead of prisms. This makes adjustments easier in order to prevent double images, but how do they handle the light loss issues mentioned earlier?
Thanks,
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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1) Do you have a link to Roland Cristen's discussion on this subject?
No, but you can search the Astromart forums, happy reading.
2 With TIR there is essentially no light lost. That's why fiber optics work so well. You mentioned that mirrors, even good ones, can't reflect 100% of the incident light. Why then are mirrors used in diagonals rather than prisms. I know that there are prism diagonals available, but most of the high end diagonals are dielectic coated mirrors. What am I missing?
Effective focal distance of light passing thru prisms varies considerably from that reflected off a mirror.
It would be pretty hard for any prism to beat out a dielctric mirror.
The same light cone problems may be reason to not use BK7.
Remember BaK4 introduces aberrations.
3) I am familiar with how porro prisms are used in binoculars. Are prisms used in the same way in a binoviewer? I would imagine that each side of a binoviewer would need only a 90 degree bend in the light path after the beamspliter. Sort of like a prism diagonal again.
Even the beam splitter is a prism isn't it? Could be a mirror.
4) I searched in the binoviewer threads for bak4 and bak7 and did not find any mention of TIR or index of refraction in the discussions. The conclusion that bak4 is best for binoculars and bak7 is best for binoviewers is there but they arrived at this using different reasoning. Perhaps the disscussion Ron mentioned took place on a differant forum?
Don't know about this discussion.
5) Earlier in this thread (#602194) you suggested that bino's with oversized bak7 prisms might work. If we agree that TIF and critical angles are why bak7 prisms don't work for binoculars (fast optics) why would making bak7 prisms bigger do any good?
What happens to the light after it enters the prism? It is bent so that some of the light beam hits the sides of the prism rather than the back internal reflection surface that directs the beam to the next surface. Sure prisms are TIR, but that doesn't mean light hitting the side will not be lost. In order for TIR to take place the light needs to hit the back 45° surface and get reflected to the next 45° surface and finally out the other end of the prism.
If the diameter of the light cone is 20mm where it first enters the prism, and the internal angle in a BK7 prism causes say 10% of the light to bounce off the sides, than maybe making the prisms large enough, let's say entering diameter 25mm, so none of the light hits the sides, but it all hits the back 45° surface, then you would get TIR.
6) How slow would the objectives have to be in order for bak7 prisms to work in binoculars?
No idea
7) It's my impression (I might be wrong) that the astro-mechanica (forgive the spelling) binoscopes use mirrors instead of prisms. This makes adjustments easier in order to prevent double images, but how do they handle the light loss issues mentioned earlier?
No idea
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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pcad
Carpal Tunnel
Reged: 01/17/05
Posts: 1501
Loc: Connecticut
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Thanks edz, I'm not sure I follow you on your anwser to #5. If the central ray enters the face of the prism at 90 degrees, I can imagine the vergence of the light cone changing but not the direction of the central ray. I'll think about this some more.
Peter
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Peter
Telescopes 25 - 318 mm
Binoculars 15 - 88 mm
Microscope 50x - 1000x
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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I don't think it's the central ray that is getting lost. it's the rays entering at the edges.
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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BillC
on a new path
Reged: 06/04/04
Posts: 2111
Loc: Washington, USA
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Quote:
The whole bak7 vs. bak4 issues has been beat to death by the binoviewers people so it was very odd to see it debated here.
As long as there are Newbies, you will see it again and again. As we grow, we get excited about NEW discoveries that have been known by the Oldbies for decades. It the nature of the beast. Hey, let's talk about metal mirrors, optimizing our Newtonian, or builing a research-grade telescope based on an f/1.75 shaving mirror from Wal-Mart.
For the fellow who has known people who can tell the difference in brightness between Bk7 prisms and BaK4 prisms, I would like to say that I think it is more in the differences in blackening, baffling, and field stops, than in glass types. When it comes to optics, things are not always as easy to pin down as it first may appear. But then, that's just my opinion.
I can place a 1.5-in, black suction cup over one obj. of a 7x50 (Obscuring 75% of its surface) and most folks will not know the difference. This is not opinion. It is test results.
Also, some are using the names BaK4 and Bk4 interchangeably. They should not.
In d light, BaK4 has an index of 1.56883
and
In d light, Bk3 (Bk4 is not in my Schott catelog) has an index of 1.49831.
Just some thoughts.
Cheers,
Bill
Edited by billcook (09/18/05 02:51 PM)
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DJB
Pooh-Bah
Reged: 02/23/05
Posts: 1269
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Hi all,
My guess would be around f/6.9 for the BK7 prism system. As EDZ so pointedly refers, the whole "problem" revolves around the optical system and the ?engineers who created it.
I base this on a quick, no-brains, calculation that I used. It seems like only the outer 10% would be degraded with a circle of dimming, not the square of dimming. My thoughts only.
Regards,
Dave.
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sixela
Postmaster
Reged: 12/23/04
Posts: 9499
Loc: Boechout, Belgium
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Quote:
Actually, I put asterisks on each side of my subjunctively intended "might" to indicate that I did not intend it in indicative certainty.
There's no need for the apologetical quotes - if you hadn't wanted what followed to be subjunctive (or, in this case, optative?), you would have written "may", wouldn't you have?
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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Quote:
My guess would be around f/6.9 for the BK7 prism system. As EDZ so pointedly refers, the whole "problem" revolves around the optical system and the ?engineers who created it.
Of course, going from f/4 to f/7 would have the affect of needing to make the binoculars 75% longer. So a 10" long binocular now would need to be 17" long. So it would seem the engineers know very well how to address the compromises needed to make what we want.
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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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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Quote:
it would seem the engineers know very well how to address the compromises needed to make what we want.
That in itself is a statement capable of generating a quite interesting discussion!
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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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< That in itself is a statement capable of generating a quite interesting discussion! >
Well , ANYTHING that might generate " a quite interesting discussion " is fine by me .
In fact , one of the main reasons I frequent this forum is in hope of some " quite interesting discussions " being created and participated in. :-)
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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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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You're right Mark, and I should at least say, maybe they don't always know what we want, but they do know about chosing and compropmise.
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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Glassthrower
Vendor - Galactic Stone & Ironworks
Reged: 04/07/05
Posts: 14703
Loc: Hurricane Alley
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Via recent discussions we have demonstrated that the vast majority (or totality) of astronomical binoculars are made overseas - Japan, Germany, China, whathaveyou. Do this mean that all of the parts are likewise made outside the US? I have heard in this forum, some time back, that NO binoculars are made in the US at this time. Does this mean that no prisms or lenses (of the bino variety) are made here either? Could it be that the engineers in far-off lands are using different criteria to establish what the consumer wants? Language, cultural, and socio-economic factors can always make a mess of things in translation, and perhaps binocular engineering is no different.
It is evident to me that there are some wise folks in here who should be designing and producing binoculars (in a perfect world).....I would have a large measure of confidence in a bino that Bill Cook helped engineer and EdZ put through the rigors of testing and evaluation. Maybe a Cloudy Nights edition binocular is in order! It could have over-sized Bk-7 prisms, f/7 optics with the increased mammoth size, and a magical red button that automatically speed-dials Bill Cook's house and asks for collimation advice!
MikeG
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
Galactic Stone & Ironworks - Buy/Sell/Trade Meteorites, Moon Rocks, Mars Rocks, & 35 different falls and types!
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