What Are K9 Vs K7 And Bak4 Prism Binoculars?

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Refractordude
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What Are K9 Vs K7 And Bak4 Prism Binoculars?

#1

Post by Refractordude »


Came across K9 for the first time. How do K9 compare against other types of glass/coatings used for binoculars? Thanks all.
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John Baars Netherlands
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Re: What Are K9 Vs K7 And Bak4 Prism Binoculars?

#2

Post by John Baars »


BK7 is the normal crown-glass for lenses and prisms. (K stands for the German: Krone) It has 7% lead-oxydants. High performing optical glass.
BK9 is als so used for lenses, chandeliers, jewelry etc. Less dense, high clarity, but as a result less capable of withstanding wear. So not as well suited for a front lens or use in a dishwasher. Common name: Crystal. 9% lead.
BAK4 ( Barium Crown) with slightly higher refractive index than BK7 has better capabilities for total internal reflection of rays at steeper incident angles in short focal systems like binoculars. So it is better in avoiding a bit of light loss.

One can see if binoculars has BK7 or BAK4 prisms by looking at the exit-pupil. BAK4 pupil is round, BK7 has a bright square.
Looking in the daytime with a 10X50 one sees no difference. The human entrance pupil is 2 mm, exit pupil is 5 mm. The slightly darker edges of the BK7 exit-pupil are vignetted by our own eye-pupil.
By night, with an eye-pupil of 6 mm for instance, one might expect to see some difference. Mostly this is not the case. A light-drop of 40% at the edge of our field of view is almost imperceptible to most people.

Advertisements in which BAK4 and BK7 prisms are compared you might notice a slightly darker BK7 image in the center. A difference that doesn't exist, only in the advertisement.

So what are BAK4 prisms good for? I imagine theorists, perfectionists and salesmen? It is better for nature though, for it contains only 4% lead. A salesman would say 50% less lead! :D
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
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Bigzmey United States of America
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Re: What Are K9 Vs K7 And Bak4 Prism Binoculars?

#3

Post by Bigzmey »


It seems that BAK4 is pretty much given nowadays. Even $20-30 binos have them. I am sure they find other ways to cut the cost. :lol:
Scopes: Stellarvue: SV102ED; Celestron: 9.25" EdgeHD, 8" SCT, 150ST, Onyx 80ED; iOptron: Hankmeister 6" Mak; SW: 7" Mak; Meade: 80ST.
Mounts: SW: SkyTee2, AzGTi; iOptron: AZMP; ES: Twilight I; Bresser: EXOS2; UA: MicroStar.
Binos: APM: 100-90 APO; Canon: IS 15x50; Orion: Binoviewer, LG II 15x70, WV 10x50, Nikon: AE 16x50, 10x50, 8x40.
EPs: Pentax: XWs & XFs; TeleVue: Delites, Panoptic & Plossls; ES: 68, 62; Vixen: SLVs; Baader: BCOs, Aspherics, Mark IV.
Diagonals: Baader: BBHS mirror, Zeiss Spec T2 prism, Clicklock dielectric; TeleVue: Evebrite dielectric; AltairAstro: 2" prism.
Filters: Lumicon: DeepSky, UHC, OIII, H-beta; Baader: Moon & SkyGlow, Contrast Booster, UHC-S, 6-color set; Astronomik: UHC.

Observing: DSOs: 3106 (Completed: Messier, Herschel 1, 2, 3. In progress: H2,500: 2180, S110: 77). Doubles: 2382, Comets: 34, Asteroids: 255
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Re: What Are K9 Vs K7 And Bak4 Prism Binoculars?

#4

Post by j.gardavsky »


Refractordude wrote: Sat Dec 19, 2020 3:51 am Came across K9 for the first time. How do K9 compare against other types of glass/coatings used for binoculars? Thanks all.
K9 is the CDGM China stuff when they save material costs replacing the BK7 (N-BK7) from SCHOTT Germany, or its generic from OHARA (Japan).

As the BK7 should be found in nearly every eyepiece, then when it is replaced by K9 you will see, what I have nicknamed, the green China tea reflections. And the same holds for the binoculars from the Far East.

Even the optics manufacturers in Japan take the K9 and other generics from China to save the material costs,
JG
6" F/5 Sky-Watcher achro, 2" BBHS Star Diagonal, 2" zenith prism, 1.25" Takahashi prism
Leica 82mm APO Televid
Eyepieces: Docter UWA; Leica B WW and WW Asph. Zoom; Leica HC Plan S and L, monocentric; Pentax SMC XW, O-, XO; Tak MC O, Carl Zeiss B WW, and Pl, E-Pl, S-Pl, W-Pl;
Swarovski SW; Baader Symmetric Diascope Edition; Nikon NAV SW, ; TMB supermonocentric; Rodenstock; Vixen HR; TV Delos
Filters: Astrodon, Astronomik, Baader, Balzers, Zeiss West and East, Lumicon
Binoculars (7x42 up to 15x85): Docter Nobilem, Leica Ultravid, Nikon Astroluxe, Swarovski EL Swarovision; BA8 (Kunming Optical)
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