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What is Fluorite?
In 1979, Takahashi became aware of
the superior optical performance of a
telescope that used a fluorite element
so Takahashi started development of a
90mm fluorite telescope in 1979. FOA-
60 is a doublet apochromatic refractor that
makes the best use of its fluorite element.
With less curvature and high resolution,
its focal length can be changed freely by
dedicated reducer and extender. As shown
below, its transmission area and ratio are
outstandingly large. In addition, multi-coated
element can produce rich illumination
from short wave to long wave. Different
from reflectors, there is no light loss by the
secondary, FOA-60 can produce brighter
images even with f/8.8 optical system,
which will enable viewing and imaging faint
objects.
The fluorite element with its excellent
performance is used in various high quality
objective. Chemically the fluorite element is
as hard as optical glass and can be multi-
coated for long time use. The focal length
can change with fluctuating temperature. So
it is advisable to take the telescope out at
least 30 minutes before using it to allow it to
equalize with the temperature of the outside
air to produce stable images. It is good to
refocus with a temperature change of 3
degrees C.
The fluorite lens of FOA-60 is multi-coated
to harden the lens surface to protect it
scratched and is placed at the rear. So there
is little chance to get scratches on the lens.
Wave Length
(nm)
Focal Length
(mm)
Comparative Color Aberration Curves
CaF
2
BK7
FK01
A’ (768.2)
C (656.3)
d (587.6)
e (546.1)
F (486.1)
i (365.0)
h (404.7)
g (435.8)
-1 -1 0 1 2 3 4 5 6 7 8 9 10
Wave Length
Light Transmission
Comparative Diagram (non-coated)
CaF
2
(t=10mm)
BK7 (t=10mm)
0.2
0.4
0.6
0.8
1
100
80
60
40
20
0
2 3 4 5 6 7 8 9 10 μ
%
