.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,使用者校准是不错的.(不懂??)
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:
Diagonals; Prisms vs. Mirrors
by Gary Hand 01/31/04 | Email Author
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.
