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APQ JENA Fluorite Quadruplet Polychromat 150/1200 Refractor (Focuser: Feather Touch FTF3545)
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Special projects

Special projects of APQ JENA

By special projects we understand such development and production orders, as for example of fluorite polychromats with special optical designs and astrographs.

Classic CaF₂ doublet and triplet apochromats are also developed and manufactured on special order.


  • Special polychromats
  • Doublet apochromats
  • Triplet apochromats

From the quadruplet basic lens design, polychromats with extreme focal ratios of f/1 to f/8 can be derived, by adding additional lenses, replacement of fluid, optically transparent media, by air gaps or the execution of glass-air surfaces as aspheres and by substitution of the optical glasses by crystalline and special media.

These special polychromats are used e.g. as astrographs and other systems with a usable spectral range from 320nm to 2500nm. Such instruments include special Echelle spectrographs, focal reducers and camera systems of modern professional astronomy.


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A fluorophosphate glass objective can be realized with two lenses, but a high image quality and high image contrast reached only the FK compakt three lens design  Laux, Uwe: Astrooptik. Optik-Systeme für die Astronomie. 3rd revised and amended edition, 2017, p. 96.

This statement also applies to the two lens CaF₂ apochromats, in which the lenses are separated as well as the FK glass doublet apochromats via an air gap.


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The development of the three lens, oil spaced apochromatic lens type with a center lens made of fluorite dates back to James G. Baker, who first introduced this lens design in 1963. Baker could therefore be described as the "forefather" of oil spaced CaF₂ triplet apochromats.

As early as 1943, the first prototype was made of calcium fluoride and the two outer glasses BK7 and K10 from SCHOTT®  Baker, James G.: Planetary Telescopes, Applied Optics, Vol. 2, No.2, Februar 1963, pp. 111-129 (13.12.2017).


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