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The Sky-Watcher ED-APO objective lenses are fabricated to the industry's highest optical standards utilizing a front element of Grade-A Crown (BK7) glass free of straight and other image-degrading imperfections. The second element is formed from an exotic halide, calcium fluorite, insuring extremely low dispersion and excellent correction. The resulting design exhibits very low residual color (secondary chromatic aberration) and produces the highest contrast images obtainable from an ED-APO of their class.
Split double stars to the theoretical limit of the telescope's aperture. Each lens set is individually pitch-polished and hand figured by a master optician ensuring premium diffraction-limited optical performance that is second to none worldwide. All lenses are uniformly air-spaced. Each optical lens surface is coated with exotic anti-reflection metallic coatings (fully coated) to ensure the optimum light through-put. Four layers are applied to each surface (Magnesium Fluorite, MgF2 and Hafnium Dioxide, HfO2). The proprietary Sky-Watcher "Metallic High-Transmission Coatings" (MHC) are the finest light anti-rejection coatings in their class.
The objective lens cell is light-baffled to reject stray, unfocused light. The draw tube is likewise baffled to insure optimum contrast. The tube's interior is flat blackened to help prevent unfocused internal light reflections from reaching the focal plane. The optical tube is aluminum, powder-coated reflective gloss black with gold fleck accent. The front and back cells are cast-aluminum painted black. Optical focus is achieved by means of a mechanically ultra-smooth, backlash-free, precision 2" Crayford rack & pinion focuser-assembly which includes course and fine focusing controls.