The Fluorite Advantage

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.

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.

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.

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.

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 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.

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.

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.

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.

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!