BAK4和BK7的区别？

好像望远镜上都说BAK4比BK7好。
 
   但是著名的双眼装置Denk II却采用BK7玻璃而非BAK4。

----------------
GLASS TYPE
BK7 vs Bak4
Prisms: Our binoviewer prisms are made from Grade A Bk7 Glass produced by Schott Glass Technologies. This material has better dispersive properties (Abbe Number) and higher transmission in visible spectrum applications than Bak4. Bk7 is the industry standard for beam splitters. We have tested Bak4 prism performance and Bk7 prism performance side by side and were extremely pleased with the images that Bk7 rendered in terms of sharpness and color-free performance. We made our choice of prism substrate material based on science, and actual observational results under the stars.

Internal transmittance at various wavelengths (in nm)
BAK4
700nm: 0.996, 660nm: 0.995, 620nm: 0.994, 580nm: 0.993, 546nm: 0.993, 500nm: 0.992, 460nm: 0.991

BK7
700nm: 0.998, 660nm: 0.997, 620nm: 0.997, 580nm: 0.996, 546nm: 0.996, 500nm: 0.996, 460nm: 0.994
The Abbe Number of a particular glass type refers to the reciprocal value or dispersive power of the material. A higher Abbe number indicates that less chromatic dispersion occurs for light passing through such a material. This would indicate a superior common focus and less chromatic aberration due to unfocused light of various wavelengths.
Abbe Number of BAK4 Glass: 56.1
Abbe Number of BK7 Glass: 64.2

还是google，越来越懒得自己写东东了。
http://bbs.nsysu.edu.tw/txtVersi ... /M.988040699.A.html

> 1. What exactly is BaK4?  Is it a type of prism design?  A type of
> glass?

It is a type of glass.  The main difference is a slightly higher index of
Bak4是指鏡材,非設計....                折射率較高.and貴一些....
refraction than the less expensive BK7 glass.  This means that the prism
will provide "total internal reflection" at steeper angles than with BK7
     內部全反設的入射角度比較大,(請想一下高中課程)...
glass.  The result is that BK7 prisms will lose some of the light at the
edge of the pupil, but the BaK4 prisms will not.
    所以bk7作稜鏡,邊角失光較大,而bak4不會.....
>
> 2.  How can you tell if your binocular uses them?  I've always thought
> if you examine the exit pupil, a non-BaK4 prism binocular will show a
> "squarish" disk...or more accurately a disk with illumination fall-off
> such that the brightish portion of the exit pupil appears square.  Am
> I correct?

Yes, this is correct.
     這就表示.bak4稜鏡所作的雙筒,出射光瞳(exit pupil)是正圓,而bk7
     '常看出是被外接的方型...大家可拿雙筒出來 遠遠的看目鏡那個exit pupil...
>
> 3.  Why ensure such a prism is used or what happens in a non-BaK4
> binocular?  Kind of a follow-up to question #2...is it just light
> loss?  If so, is it light loss across the entire field of view or just
> at the edges?

The light loss is all across the field of view, but the effect may be
slightly more pronounced (and asymmetric) at the edges of the field.
  邊角失光還繞使視野四周,但蠻明顯不對稱...
>
> 4.  Are roof prism binoculars subject to this problem in any way?
>
Yes, they are.  But in a roof prism of the type used in binoculars, one of
the surfaces won't support total internal reflection at all and is
silvered.  For a very good coating, this gives about 10% less reflection
than a total internal reflection, so the image won't be quite as bright
anyway.  If you want the absolute brightest image, you will get a porro
prism design.
     屋頂稜鏡式..(H型雙筒)有一面沒有全反射..LOSS約10%的光...所以反兒沒有
    &nbspORRO PRISM BINOCULAR亮
Grieg
       (譯的不好,請不吝修正)

BK7在国内的牌号是K9，Bak4在国内的牌号是BAK7。K9是标准光学玻璃，与BAK7在质量上并无差异，两者的区别主要是折射率不同，BAK7折射率更高，BAK7相比K9更贵一些，差价好像有10元左右/公斤，使得用在保罗正向系统中不会产生蓝色切边现象，而K9会有切边现象。在光学设计中设计者根据情况采用不同材料，能用低价的材料解决问题就不会用高价材料来做。

那么为啥那个denk II要用BK7(K9)而非BAK4呢？
   
    为什么普通的双眼要用BAK4而非BK7?

那个denk 给出的对比似乎BK7指标数值更加高些。

不管多么好的BK7，都会有切边现象。
至于那个双眼装置用BK7，也许是因为它透射率略高，色散也略小。这样牺牲边缘像质提高中心像质。

而且它的镀膜还是单层MgF2

好文章，建议置顶加精！

最初由 老咸鱼 发表
[B]而且它的镀膜还是单层MgF2

   denk standard是MgF2，那个denk II是MC的。

双眼鏡のプリズムは屈折率の高いクラウンガラスから作られています｡
　カタログの中に使用プリズムについて書かれているのを見たことがありませんか｡
　ひょっとしたら本体にも「ＢＫ７」とか「ＢａＫ４」とかかれているかもしれません｡
　「ＢＫ」は硼珪クラウン、「ＢａＫ」はバリウムクラウンの略で　それぞれ光学ガラスの材料をさしています｡

　ＢＫ７は昔主流だったプリズム材料で屈折率が低く、視野の一部では光が全反射しなくなってしまいます｡
　ＢＫ７を使用した双眼鏡が射出瞳の一部で暗くなるのはこのためで、現在では安物の双眼鏡でしか見られなくなってきました｡


射出瞳が正方形の影で囲まれている｡
光の損失と像の悪化は避けられない｡

　ＢａＫ４は光が全反射する角度が広く今の双眼鏡の主要なプリズム材料となっています｡
　ＢａＫ４を使った双眼鏡では瞳のケラレは見られません｡


ほぼ正円の射出瞳の例

　現在ではＢａＫ４より反射率の高いプリズム材料も使用されるようになっており、双眼鏡の小型化に一役買っています｡

　ちなみにガラスの単価としては、ＢａＫ４はＢＫ７に対して１．５～２倍の価格差があるといわれています｡

　

離題一會兒
我想問智通說他們的十寸主鏡(未磨)是k9玻璃
那麼k9玻璃對自製望遠鏡好不好??
這種玻璃好些還是pyrex好些?
謝謝

当然是PYREX 好, 热膨胀系数小.

没啥影响。

派勒克斯可大为缩短研磨到检测的冷却时间。

从热胀冷缩的性质来看，Pyrex 玻璃仅次于熔凝石英玻璃，做反射镜主镜足够了。

PYREX好啊，热膨胀会导致主镜面形的改变

使用BK7棱镜的双筒，从目镜看过去，你的视野不是圆形的，而是比圆要小的一个方形，部分边界处的光没有反射过来。

bandofbrother你从智通订做10英寸反射镜吗？多少钱  呢？

反射镜的话，如果因为价格太高，不用pyrex的话，也没有必要用K9（即BK7），用工业化生产的浮法玻璃（flat glass）就可以了，19mm是国内容易买到的最厚的规格了。

BK7在普通折射镜上好像比BAK4常用