Apochromatic means "free from spurious color" - a refractor system without the faint
violet halos of out-of-focus light you see around the planets, the limb of the Moon,
and all the bright stars in an ordinary achromatic (crown and flint glass) refractor.
All Takahashi fluorite doublet optical tubes are true apochromatic optical systems,
with virtually perfect color correction.
Takahashi has been the acknowledged leader in ultra-premium apochromatic fluorite
optics ever since they produced the first commercial fluorite refractor in 1979.
That system moved Takahashi to a position of industry leadership in terms of optical
quality, and Takahashi has never looked back since moving to the head of the class.
The Takahashi two-element air-spaced objective lenses use a full aperture laboratory-grown
calcium fluorite crystal lens element, combined with a high refractive index flint-type
glass element. This yields very high color fidelity and vanishingly low levels of
spurious color. Laboratory-grown fluorite crystals are used, rather than naturally-occurring
crystals, due to the lab-grown variety's optical uniformity, clarity, freedom from
striae and internal stresses, freedom from being adversely affected by dew and moisture
(as early natural-crystal fluorite lenses could be), and their ability to be hard
multicoated in a vacuum chamber for high light transmission.
The Takahashi color correction equals, or exceeds, that of most triplet lens systems
regardless of cost or brand name. Contrast is generally superior to triplet systems
or doublet systems that use a low-dispersion crown glass element instead of fluorite
crystal.
Since the Takahashi two-element air-spaced designs have fewer light-absorbing lenses
than triplet systems, they generally have less glass to absorb light, therefore
yielding brighter images than most triplets. All optics are fully ion-deposited
hard multicoated (including the fluorite element) for maximum light transmission
and contrast. Maintenance is less than oil-spaced designs, since there is no oil
to potentially leak or become cloudy with age. In addition, Takahashi lens cells
are fully collimatable for peak optical performance by using a simple optional Cheshire-type
collimating eyepiece and the locking collimating screws on the lens cell.
The diagram below shows the differences in color correction between a conventional
BK7 optical glass lens (shown in red), an ED glass lens (in blue), and a lens of
calcium fluorite crystal (in green). The smaller the departure from the ideal focus,
shown by the vertical line labeled "0", the lower the amount of chromatic aberration
and the better the correction for the specific color shown at the left of the diagram.
You'll note that the green line of the fluorite lens shows much less chromatic aberration
across the entire visible spectrum than either of the other two glass types.
