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The Sky-Watcher ED-APO refractors provide the finest images obtainable-bar none-in their design class. If you are on a quest for the ultimate in pristine, near textbook-perfect astronomical imaging, look no further. Sky-Watcher ED-APO refractors deliver dramatic coal-back skies and faithful, contrast-rich views of planetary and deep-space phenomenon that are second to none. Sky-Watcher ED-APO refractors are the logical choice as an investment in advanced instrumentation for exceptional viewing or photography.
The Sky-Watcher ED-APO design assures virtual elimination of secondary, residual "false color" normally present in 2-element achromatic lens designs using Crown and Flint glass. Sky-Watcher APO refractors utilize a second lens element made from the mineral fluorite instead of Flint glass. Fluorite, CaF2, is a halide with unique properties and a hardness of only four. It is an exotic and difficult material to fabricate into a lens. Hence the cost of manufacturing a fluorite lens is quite high compared to a Flint lens element, for instance.
Fluorite's molecular properties have several distinct advantages. First, it has superior dispersion characteristics compared to pure silica-based glass. It functions as a high-index lens with very low dispersion. The term ED to signifies "Extra-low Dispersion". Fluorite also corrects well for spherical aberration. Sky-Watcher utilizes only the very finest available fluorite lens, FPL-53.