论坛很多文章有详细描述，转其中一篇
http://bbs.imufu.cn/forum.php?mod=viewthread&tid=73450
天顶镜:采用直角棱镜和平面镜的比较
---Gary Hand 01/31/04
(这篇文章原文有些小丢失,我根据自己理解做了补充.另外有些说法感觉怪怪的,翻译不对之处还请大家多多批评)
直角棱镜和平面镜没有谁更好,如果你注意下会发现,每个优良品质望远镜的制造商对于短焦比(F/D小的)望远镜会采用平面镜为元件的天顶镜,对于长焦比(F/D大的)的会采用直角棱镜制的天顶镜.这不是什么意外,你得到的是与你的望远镜相匹配的天顶镜.
假定相同质量的产品,直角棱镜的光线散射更少,这是件好事,并且不像平面镜,它还是自准直的.但是当它用在短焦比的望远镜上时会产生色散.
正如同你在初中时期的实验课程,当你以一定角度除了法线方向外,将光线射入棱镜时它会引起色散.短焦比的望远镜会以一个更加倾斜的角度汇聚光线进入棱镜式天顶镜,事实上它将色散引入了本来不存在色散的镜子里.角度越倾斜(镜子焦比越小)引入的色散越多.在制作更大的尺寸时,棱镜也要比平面镜花费更多的钱.这也解释了为什么在制作2”甚至更大的天顶镜时,采用平面镜的占绝大多数.
从另一方面来说棱镜不会散射光线,因此能够提供优良的对比度.同时它不会年久失效.据说在折射光线时,每一个都依赖镀膜.一个多层镀膜的棱镜和镀铝的平面镜都能够达到大约98%的光线折射率.但是这真的不重要.
对于天顶棱镜的讨论大部分集中在反射率上,在制造一个好的天顶镜所包含的要素中,反射率是其中之一但不是最重要的.但是它是最符合市场销售的要素,“96%一定比95%好”.虽然有大量的实验数据表明人类是不能感觉到1%左右的轻微的光线入射改变量的,但是反射率却是最经常被引用来定义一个天顶镜是否优秀的要素.买X因为它的97%的反射率,不买Y是因为它的反射率只有95%.然而甚至Meade和Celestron公司关于他们的高反射镀膜的广告也提到了你不可能真正的看到那些细小差别,他们只是讨论10%的不同.但是另一方面,相机能够很容易的记录下这些细小差别,所以当你摄影时就不要用天顶镜了.
这样说来,为什么那些要求提供关于他们的价值5000美元望远镜光学精度的完整资料的人们,仅仅只关心他们天顶镜的亮度而不关心面型精度呢?即使不说精度,在这种情况下,平面度也比亮度重要.1/7波长和1/20波长的面型精度之间的对比所感受到的差别相较之94%和98%反射率对比所感受的差别,前者将带给你更大的感官上的冲击.少数被提供的关于天顶镜的精确报告数据都是它们在镀膜之前的信息,不是之后的,因为这将有很大的不同.这些数据使你感觉很棒,但是(很抱歉)它们却没有反应你收到的产品的真实情况.通常,镀膜越亮,镀膜的层数越多,但是(再次抱歉)也就越少的反映它最初精确平整的面型精度.标准镀铝有一层(88%--反射率?),半加强型的镀铝层有两层(91%),加强型的铝膜有三层镀膜(94%).电介体镀膜能达到22层.电介体镀膜的平面镜的优势是它会很耐用,除非你虐待它否则它会让你用一辈子.
简而言之,使用最适合你的望远镜的天顶镜.它可能是和你的望远镜是一个生产商的产品.
1,寻找面型精度最高的天顶镜,然后才是最亮的.
2,寻找用全金属的,机器加工的框架是最好的但不是必须的.
3,真正的90度很重要,那意味着一整块CNC加工的金属框架.
4,使用者可以进行一定的调整。
5,基体很重要. Cervit, Zerodur, Astrosital and Quartz比Pyrex玻璃好. Pyrex比BK7好一点点. BK7比做盘子的玻璃好的材料.
(最后那几种类型的玻璃哪位给介绍下?)
这些是英文原文:
Gary Hand is the owner of Hands on Optics and has over 30 years of astronomy experience.
Neither mirror nor prism are better. If you notice, every manufacturer of quality scopes with fast focal ratios uses mirror diagonals. Every manufacturer that sells quality scopes with long focal ratios sells prism diagonals. It抯 not an accident. You get the diagonal that matches the scope.
Assuming the same quality of manufacture, prisms give less light scatter, a good thing. And unlike a mirror, they are also self collimating. But they also create false color in scopes with fast focal ratios. Just like your old Jr. High experiments, when you send light into a prism at an angle other than perpendicular, it will cause false color. Fast telescopes send light into a diagonal prism at a steeper angle and can in fact introduce false color into a scopes where there was none. The sharper the angle (faster the telescope) the more false color is introduced. Prisms are more expensive to make in larger sizes than mirrors and may account why in 2?and larger diagonals, mirrors dominate.
Prisms on the other hand never scatter light and therefore can produce excellent contrast. Also, they never age. In terms of transmission, each depends on the coatings. A multicoated prism and enhanced Aluminized mirror can both achieve about the same 98% transmission. But that really is not important. (WHAT DID HE SAY?)
Most of the discussions about diagonals have centered on reflectivity. Of all the characteristics that make a good diagonal, reflectivity is one but not the most important. But it is the easiest characteristic to market, 96% is better than 95% right. There is a vast amount of laboratory data that proves that human beings cannot perceive a few % of light change, yet it is the characteristic most often quoted to define how good a diagonal is. Buy X because its 97% and not Y because its only 95%. Yet even Meade抯 and Celestron抯 new ads for their high transmission coatings state that you CAN NOT really 搒ee?the difference and they抮e talking about 10% difference. On the other hand, a camera can easily 搑ecord? the difference. But you don抰 use diagonals when photographing do you.
So, why is it that people that demand full documentation on the optical accuracy of their $5000 telescope, only worry about the brightness of their diagonal and not accuracy? Isn抰 accuracy, in this case flatness, at least as important than brightness. Doesn抰 the difference between a 1/7th wave vs. a 1/20th wave surface have a greater impact on your viewing than the difference between 94% vs. 98% transmission. The few that supply accuracy papers on their diagonals do so before they are coated, not after and there can be a huge difference. The papers make you feel good, but do not reflect (sorry) the actual product you receive. As a rule, the brighter the coatings, the more layers of coatings had to be laid down, and the less it reflects (sorry again) the accuracy of the original precision flat surface accuracy. Standard Aluminum coatings (88%) have one layer, semi enhanced (91%) 2 layers, enhanced Aluminum (94%)has 3 layers of coatings. Dielectric coatings can have up to 22 layers. The advantage of a dielectric coated mirror is its longevity. Unless abused, they will last a lifetime.
In short, use the diagonal that works best your scope. It may be the one the manufacturer supplied.
1.
Look for the most accurate diagonal, then the brightest.
2.
Look for all metal parts, a machined housing is best but not required.
3.
A true 90.00 degrees is important. That means 1 piece CNC machined housing.
4.
User collimatable is nice.
5.
The substrate is important. Cervit, Zerodur, Astrosital and Quartz are better than Pyrex. Pyrex is a bit better than BK7. BK7 is better way than plate glass.
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