萤石的优势The Fluorite Advantage

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色差是折射式望远镜自18世纪初推广以来所具有的主要缺陷。最早采用的减小色差的方法是尽可能制造长焦比的望远镜。这就意味着视场必须缩小，而且仪器变得很笨重。在19世纪末，Dollond双镜片设计被采用。这种光学设计的消色差的能力有了显著的提高，但仍然只能对光谱中的两个部分予以纠正。典型的效果是，蓝光沿着光轴聚焦在不同的点上，在恒星和行星的外周产生蓝色的光晕。

Since their introduction in the early 17th century, refractors have suffered from one major defect: chromatic aberration. The earliest attempts at minimizing this problem involved making instruments with very long focal ratios. This meant that fields of view were necessarily small and the instrument extremely cumbersome. In the late 18th century, the Dollond doublet was introduced. This achromatic design offered significant improvement but still only corrected for two parts of the spectrum. Typically, blue light would focus at a different point along the optical axis leaving blue halos around stars and planets.

20世纪末，反射式望远镜正被专业的天文学家所青睐时，一种新型的折射式望远镜为天体摄影学制造出来。这就是APO（复消色差）折射式望远镜。根据使用的玻璃密度的不同，至少采用3片透镜来获得消色差的图象。然而，当这种新型复消色差镜还是很稀罕的时候，真正的全消色差折射镜正将主宰天体摄影领域。

Towards the end of the 19th century, just as reflecting telescopes were coming into favor with professional astronomers, a new type of refracting telescope was built for the new field of astrophotography. This was the apochromatic refractor. By using at least 3 elements of varying densities of glass, one could achieve an image almost free from lateral color. The new apochromats were rare instruments, however, as totally apochromatic reflectors began to dominate astrophotography.

折射式望远镜的制造技术在整个二十世纪中不断地进步。早期APO的最大问题是光线的损失，其次是高昂的成本和制造的复杂性。光线每次从空气中进入玻璃介质时会有4%的光线被反射掉。另一个与此相关的问题是在镜片的内部形成的反射或称为“鬼影”。德国的Carl Zeiss和Alvin Clark以及美国的Sons采用一种新技术，称为“油分隔（用油作为透镜之间的介质）”技术，以此解决上述问题。到了1930年间，油分隔的透镜已经相当普遍了。虽然油也有自身的反射常数和吸收特性，它能减小内部反射并增加每个透镜表面2%的通光量。还有一个优点是，油能弥补透镜表面的粗糙使成像的轮廓变得平滑。这也意味着一副三片式油镜的内表面的抛光程度不必象空气分隔的物镜那样高。该技术的缺点是：必须保证油不会从物镜组的小室中泄漏出去。由于温度的波动，镜片、油、物镜之间的小室都必须随着热胀冷缩一起膨胀或收缩才能防漏。通常，要实现这些就得制造较重的物镜，花费很高，而且必须定期维护。使用几年之后，漏油问题或者油质浑浊而必须进行一次大修。

Throughout the 20th century, the refracting telescope nevertheless continued to improve with refinements in technology. The biggest problem with early apochromats, other than their sizable cost and complexity, was light loss. Each air to glass surface gave up 4% of the light that struck it. Another problem related to this was internal reflection or ghosting. Carl Zeiss of Germany and Alvin Clark & Sons of the United States introduced a new technology called oil spacing to solve these problems. By the 1930's, oil-spaced lenses had become fairly common. While the oil had its own index of refraction and absorption characteristics, it eliminated internal reflections and increased transmission by over 2% at each surface. Of additional benefit was oil's ability to smooth out errors caused by roughness of the lens surface. This meant that the internal surfaces of an oil-spaced triplet objective did not have to be polished as precisely as air-spaced objective elements. The downside of this technology was insuring that the oil would not leak out of the lens cell. Because of temperature fluctuations, the lens elements, the oil, and the objective cell all had to expand and contract so as to prevent leakage. Generally, this necessitated a rather expensive and heavy lens cell as well as periodic maintenance. The oil would leak or become cloudy after several years and the cell would have to be overhauled.

作为在第二次世界大战期间开发的技术，复消色差透镜的设计又有了大促进。氟化镁镀膜技术已经广泛用于透镜了。镀膜技术能减少内部反射并降低光线从空气经过玻璃表面的损失。与此同时，另一项发现面世。一种称做萤石，氟化钙（CaF2）的晶体被发现在可见光谱波段具有极其优异的光学特性。如果采用该材料制造的透镜，复消色差系统将使用更少的镜片数量来实现。早期使用英石的光学系统的问题是难以获得足够大的纯萤石晶体。经过几十年发展，只有一些小的萤石镜片被加工出来，用在显微镜的物镜上并收到很好的效果。

As a result of technologies developed during World War II, advances in apochromatic lens design got a big boost. Magnesium fluoride coatings were developed for most lenses. These eliminated internal reflections and also diminished light loss on air to glass surfaces. Another discovery was made at this same time. A crystal called calcium fluorite (CaF2) was found to have exceptionally good optical qualities across the visible spectrum. If lenses could be manufactured from this material, apochromatic optical systems could be developed using fewer elements. The problem with early fluorite optical systems was the difficulty in obtaining pure fluorite crystals of sufficient size. For decades, only small fluorite elements could be fabricated but these yielded impressive results in microscope objectives.

最终在1977年，日本的Takahashi Seisakusho有限公司制造了世界上第一架用萤石做物镜的天文望远镜。通过与Canon公司的光学专家密切合作，制造出直径150mm(6’’)的萤石镜片的技术成功了。萤石镜片在望远镜中使用的显著性能是可以只使用2片透镜构成的物镜就能得到焦比为f/8。镀膜技术在该时期也发展成全面多层镀膜，进一步防止光损失和鬼影出现。采用空气分隔的复消色差双片设计出现了，对色差的纠正能力等于甚至超过了三片式的设计，对比度更胜一筹。

Finally, in 1977, Takahashi Seisakusho Ltd. of Japan introduced the world's first astronomical telescope with a fluorite objective. By working closely with optical experts at Canon Inc., the technology for making fluorite lenses as large as 150mm (6") in diameter was developed. The remarkable performance of the fluorite element allowed the production of f/8 telescopes with only two elements in the objective. Coating technology had also improved during this period so that the glass elements could be fully multi-coated to prevent light loss and ghosting. The result was an air-spaced apochromatic doublet. Color correction was as good as or even exceeding most triplet systems and contrast was far superior.

Takahashi并未对已取得的成绩满足。在1980年代，他们继续改进FC的设计。他们也生产过三片式空气分隔的物镜。这些望远镜对天体摄影家来说是一种恩惠，色差的矫正达到完美，折射式镜可以快到f/3.7。高桥FCT-150,采用6英寸的萤石三片APO物镜，能在f/7时产生高度细节的图象，如果采用减焦镜，能在f/5时实现相同效果。这是目前6英寸口径中最好的望远镜。高桥也能应特殊需要生产FCT-200和FCT-250的望远镜。这是世界上最大的APO折射镜了。

Takahashi was not content to rest on their achievements. They continued to make improvements on the FC design during the 1980's. They also brought out a series of FCT air-spaced triplets. These telescopes were a boon to the astrophotographer as perfect color correction could be achieved in refractors as fast as f/3.7. The FCT-150, a 6" fluorite triplet apochromat, yields exquisite images at f/7 or even f/5 with its optional focal reducer. This is currently the finest 6" aperture instrument available to amateur and professional astronomers. Takahashi also manufactures an FCT-200 and FCT-250 on special request. These are the largest fluorite apochromatic refractors available in the world.

在1990年间，Takahashi对他的FC系列的f/8两片式系统进行了重新设计并创立了FS系列。高桥的新进展是采用硬镀膜技术，使萤石镜片位于低色散镜片的前面。以前由于萤石很难镀膜，FC系列的望远镜采用的是把非镀膜的萤石镜片放在多层镀膜的光学玻璃镜片之后，目的是保护萤石防止划伤和污染。

In the 1990's, Takahashi redesigned its FC series f/8 doublets and created the FS series. New advancements in hard over-coating allowed Takahashi to design an objective with the fluorite element in front of the low dispersion, flint-type optical element. Since fluorite is difficult to coat, the FC series employed a non-coated fluorite element behind the multi-coated optical glass element for protection of the fluorite against abrasion and staining.

新的FS系列使用了完全硬镀膜和低色散透镜，两片空气间隔的透镜f/8能提供最大的成像对比度、亮度和锐度。新型FCT三片式也使用多层镀膜萤石物镜以获得最大的通光量。

The new FS series employs fully hard over-coated fluorite and low dispersion elements in an air-spaced f/8 doublet design that provides the highest contrast, brightest, and sharpest images in an apochromatic refractor today. The new FCT triplets also utilize multi-coated fluorite objectives for maximum light transmission.

FSQ-106是高桥的最新一代折射式望远镜，这是高桥进入二十一世纪时的科技。FSQ采用4片透镜，包括2片萤石物镜， 能实现4寸f/5的优异像质。物镜组与FS设计相同，在镜筒的后段加上了Petzval纠正像场设计，使成像平坦。使用该系统，在f/5时能获得88mm周长的图象。这是为天体摄影爱好者设计的第一款望远镜。它的成像平坦，边缘锐利，色差矫正完美，效果令人震撼。它是计划从事天体摄影者的拍摄利器。

Takahashi's latest-generation refractor is the new FSQ-106. With this unique instrument, Takahashi enters the Third Millenium on the cutting edge of lens technology. The FSQ employs 4 lenses, including 2 fluorite elements, to create a 4" f/5 masterpiece. The objective consists of an FS-style air-spaced doublet while a Petzval field-flattener is built into the rear cell of the telescope. The correcting lenses are almost as large as those of the objective providing an incredible 88mm image circle on film at f/5. This telescope is the first instrument of its type designed for the amateur astrophotographer. It yields stunning flat-field sharpness to the edge with perfect color correction. It is truly a plan-apochromatic astrograph.

如果您想从现代望远镜中寻找一款光学和机械性能完美的望远镜的话，不必再犹豫了。Takahashi就是您的选择。其他的望远镜制造商宣称ED（极低色散）玻璃等同于萤石或者采用了新设计。不幸的是，他们未必诚实。尽管ED和含氟的冕牌玻璃能达到接近萤石的Abbe系数，它们在可见光谱波段要吸收更多的光。这就意味着萤石成像会更亮，对比度更高。Leica,Zeiss和Kowa在它们的观景镜和望远镜中已经都转向了萤石，以满足顾客对其产品提出的最高性能要求。而他们以前是采用ED玻璃的。显然，这些生产商们知道ED与萤石之间的差别。您也将知道。Takahashi是天文望远镜中使用萤石的先锋和佼佼者。不要妥协，选择萤石，选择高桥！

If you seek optical and mechanical perfection in a truly modern telescope, look no further. Takahashi is the answer. Other telescope manufacturers may claim that ED (Extra-low Dispersion) glass is the equivalent of fluorite or that their older designs will work as well. Unfortunately, they are not being honest. While ED and fluoro-crown lenses can achieve Abbe-coefficients approaching fluorite, they tend to absorb more light in the visible spectrum. This means that fluorite yields a brighter, higher contrast image. Leica, Zeiss, and Kowa have all gone to fluorite in their spotting scopes and telescopes to achieve the maximum performance levels their customers demand. Most of them previously used ED glass. Obviously, they know the difference between fluorite and ED. You will too. Takahashi pioneered the use of fluorite in astronomical telescopes and they are still the leader. Accept no substitute. Get the fluorite advantage. Get Takahashi!