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.
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.
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.
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
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.