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Fluorite refracting telescopes

Attention has been generally directed toward fluorite as an optical material for its unique optical characteristics since 1886 when Abbe used fluorite CaF2 for its application in apochromatic lenses for microscopes. Following the introduction of the fluorite on the camera lens in the 1960’s, Takahashi Seisakusho Ltd. company started the development of fluorite objective lenses in 1970 for astronomical telescopes for the first time, and in 1977, the 90mm fluorite refracting telescope was announced and received domestic and foreign praise as the premier refracting telescope.  From 1981, beginning with FC-100, FC-65, FC-76, and then in 1982, FC50, FC-125 etc., small and medium aperture machine types announced the entire F8 series, and the FCT and FS series which improve capabilities to a greater extent were further developed while the superior optical capabilities of fluorite were pursued to the extreme, and finally, a product with good operation capabilities as a short focus telescope with no chromatic aberration was realized. Also, reducer development was carried out that could not be actualized in the former refraction telescope simultaneously with the successful mass production and sale of an epoch-making telescope which introduced the new concept of a high quality optical system where visual observation and a combined use of photographic set-up is possible.

Unofficial translation from Takahashi Seisakusho Ltd. product catalog document (1994)

 

 

 

 


 

FCT SERIES

 

1984/7/20-21: FCT-150 preproduction model at '84 telescope & binoculars spreading/displaying' exhibition.

1985/3: New product FCT-76 tube announcement (76-487).

1985/7: New product FCT-150 tube announcement (150-1050).

1986/1: New product FCT-100 tube announcement (100-640).

1986/7: New product FCT-65 tube announcement (65-320).

1986/7/25-26: FCT-125 preproduction model at '86 telescope & binoculars spreading/displaying' exhibition.

1986/10: Lenses becomes multiple coat on the FC and FCT series except Fluorite lens.

1986/12: Monthly astronomical January edition, 'FCT-65' test report.

1987/3: New product FCT-125 tube announcement (125-705).

1987/6: Astronomical guide 'New Face Test Report ', FCT-76.

1987/9: Astronomical guide October edition, ' New Face Test Report ', EM-10 & FCT-100.

1987/11/22-23: 'large only city' FCT-200 with EM-2000 equatorial mount at Yorii factory and the Nagoya showroom.

1987/12: The FCT-76 is modified to rack and pinion focuser system.

1988/1: Dark green paint becomes on light green paint on telescopes.

1988/2: Monthly astronomical March edition, EM-10 & FCT-100 test report.

1989/12: Astronomical guide January edition, ' New Face Test Report ', FC-125, FCT-125.

1990/January to April: Fluorite gemstone crystal sale available at FC or FCT tube purchases.

1991/10: New product FCT-200 tube announcement (200-2000).

1992/6/25: FC and FCT series, also Fluorite element becomes multiple coat.

2000/2: New product FCT-250 tube announcement (250-2500).

2002/11: New product FET-200 tube (Triplet Fluorite ED).

2006/2: New product FET-300 tube announcement (300-2400).

 

 

 

 


 

 

Recommended telescope as astrophotograph by Takahashi
Written by: Kenichiro Hata (optical designer)

 


Recently in astrophotography, we've seen great high growth of technical achievements. That made us to develop FC telescopes from 1980 and next to FCT as photo-visual telescopes. As many knows, it’s a design compromise if which should be based on a photograph or visual use.


In case of visual use, we should minimize the spherical and color aberrations for sharp center images and should set focal ratio F10 to F12 for magnify up to 20 times for large objective diameters as effective limit. In this case, we are allowed to narrow field correction because of the eyepiece field.


Another case of photograph, we needs not so severe correction of center images, because of the film grain or air streams which affects the image quality. Small focal ratio and short focus length for short exposures time forces the telescope as wide angle lens. As for the border zone of the photograph, we must optimize the another aberrations.


This conflict of alternative demand, after it was discussed by fumbling hard, finally we conclude a design as a system telescopes with sub-optics by reducer. FC-100 was the first telescope of this concept.



FC-100

We set a objective diameter 100mm and focal length to 800mm for visual use and the reducer would decrease focal length to 590mm. Then F ratio will be smaller, from F8 to F5.9 for utilize as astro-camera. About reduction ratio of reducer, we optimized as 0.7X. It’s said in visual use based on the human sensitivity that we should design the ray trace of wavelengths from 656.3nm to 486.1nm for adequate correction. However for astro-camera, we should design for wider bandwidth to 435.8nm g-line or even wider of 404.7nm h-line. For the balance of the objective and reducer, we should design the objective to minimize the color correction error, not for force the excessive power load to reducer. Therefor, we select the doublet objective using Fluorite crystal instead of the classical KzFS. Fluorite has wide range of ray transparency from 125 nm to 10 µm, nearly to the ultra-violet rays. This will be useful for the astrophotography. Reducer is mainly used for correction of field curvature and coma.
 

fig-1 shows the spot diagram of the FC-100 with reducer, calculated mixing each 250 of d, C, F, G-line. D-line is the standard line and used as the best focus point. Angle 0 degree, 2.5 degree and 5 degree i.e. height of 0, 12.9mm and 25.8mm.
 


FCT-100

FCT series, it's an upgrading of FC concept, should take advanced level of point spread function, is what we intended. The objective of FCT consist in a Fluorite triplet, eliminating the 2nd order spherical aberration and the color aberration to making a compact short tube with improved the visual use in sharpness and high-contrast. The center image at the best focus field, mixed 4 different wavelengths, will be 30% smaller than FC. In fact, you can easily be aware of it. Optional reducer for astrophotography consists of gorgeous three elements, which yields 458.7mm of focal length and F4.6. Wide field of 8 degrees angle, is 1.6 time wider than FC-100.
 

fig-2 shows spherical aberration with eye for the FC-100.
fig-3 shows spot diagram of the FCT-100 with reducer. Angle 0 degree, 4degree and 8 degree i.e. height of 0, 16mm and 32mm with mixed ray of d, C, F, g .
fig-4 shows spherical aberration with eye for the FCT-100.
fig-5 shows vignetting image of the peripheral zone of the FC-100 and FCT-100, both with reducer. FCT-100 is gathering more light, of 13% up.


Document source and translation by Mr. Hiromu Goto.

 

 

 

 

FCT-65 TEST REPORT (Japanese) -Author: Chizin Shokan, Gekkan Tenmon, Mr. Kenichi Abe

FCT-125 TEST REPORT (Japanese) -Author: Seibundo Shinkousha

 

 

 


 

 

 

 

 

FCT-100

 

 

 

 

 

 

 

FCT-125

 

 

 

 

 

Model

 

Aperture

(mm)

 

Focal Length

f Ratio Lt Grasp Resolution

 

Limit

Mag

 

Tube OD

(mm)

 

 

Tube Length

(mm)

 

 

Weight

(kg)

 

Image  Circle

(mm)

 

Image Field

(degrees)

 

Dew Shield

 OD (mm)

FCT-65

65 320 / 240 4.9 / 3.7 86 1.80 10.8 80 400 3 25 / 42 4.5 / 10.0 80

FCT-76

76 487 / 345 6.4 / 4.5 118 1.54 11.2 95 500 3 40 / 55 4.7 / 8.2 114

FCT-100

100 640 / 460 6.4 / 4.6 204 1.17 11.8 114 790 5.7 54 / 65 4.8 / 8.2 145

FCT-125

125 705 / 560 5.6 / 4.5 319 0.94 12.3 145 880 10 50 / 67 2.8 / 6.8 166

FCT-150

150 1050 / 750 7.0 / 5.0 459 0.78 12.7 166 1150 21 62 / 64 3.4 / 5 210

FET-200

200 2000 10 816 0.58 13.3 256 2300 62.1 65 / 2 ?

FCT-250

250 2500 10 1276 0.47 13.8 256 2920 85 65 ? ?

FET-300

300 2400 8 1837 0.39 14.2 324 2900 150 65 ? ?

 

FCT System Charts

 

 

 

 

 

 

 

 

 

FCT Series Characteristics

The FCT series utilized the design technology of the fluorite lens which was cultivated in the FC series and a higher performance objective lens was pursued and made.  Aberrations were well revised with an objective lens constructed of 3 lenses including fluorite lenses so the FC series capabilities were exceeded both visually and photographically and the FCT is a high ranking telescope actualizing short focus and miniaturization.

*        Super Short Focus Refracting Telescope

Because focal length is short, the body tube is very compact.  In comparison with conventional very large body tubes, its rotation can even correspond to small machine types so operation capabilities have rapidly improved.

*       Clear Sharp Image

Because the contrast has no chromatic aberration, it is very good so even faint galaxies become seen easily.  Also, pattern shades and shapes when observing the moon as well as the planets are easily distinguished and marvelous images that exceed the ability of the aperture in all observations are shown.

*       Unrivaled Star Field Photography Capability

When star field photography is used together with a reducer, features are displayed.  As the focal length is further shortened, F is made bright while the image circle is expanded and can obtain a sharp stellar image up to the field of view periphery.  In order to use this capability, of course a 35mm single lens reflex camera and installed with a Mamiya film holder, medium format size film can be used.

*       Telescope System

Because of the exceptional basic capabilities, enlarged photography of the moon and planets with a camera adapter, Vari-Extender, etc. is possible with the free use of the many Takahashi multi-purpose accessories and each can be used with the maximum capability for all observations.

Unofficial translation from Takahashi Seisakusho Ltd. product catalog document (1991.12)

 

 

 

 

 

 

 

 

FCT System Charts

 

 

 

 

 

 

FCT-65 & FCT-76

 

FCT-100, FCT-125 & FCT-150

 

FET-200, FCT-250 & FET-300

 

 

 

 

 


 

Acknowledgements:

Masao Tanikawa

Akihiko Ito

Temmon Guide

Takahashi Europe

Takahashi Seisakusho Ltd

Texas Nautical Repair

 

This is an unofficial site about Takahashi FCT series telescopes and for further information please, refer to Takahashi web site.

 

 
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