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CaF2 “Fluorite” (Various manufacturers): 94.99 (incredibly expensive and hard to work with, often has inhomogeneity, internal stresses, can fracture during polishing, not very stable in a chemical or thermal sense).
S-FPL-53 (Ohara Japan): 94.94 (more even internal structure, freedom from internal stresses can be attained by baking and annealing, more chemically stable, less expensive and easier to work with than Fluorite, but harder to work with than S-FPL51).
S-FPL-51 (Ohara Japan): 81.54 (easier to work with, and much less expensive than S-FPL53, very stable by comparison).
FCD-100 (Hoya): 94.66 (less expensive than S-PL53, thanks Hoya!)
By combining multiple lens elements and using ED glasses in the same refractor lens cell, we can get very good or essentially perfect “colour correction” in a refractor, such that there are no false colours visible in the in-focus image. This is particularly evident in our FPL-53 based triplet refractors which have perfect colour correction to the eye both inside and outside of focus, and amongst the best photographic colour correction on the market today, especially in the red and blue ends of the spectrum which CCD cameras are very sensitive to. This is borne out in the profusion of images taken with our telescopes on the internet. It’s important to remember, however, that glass type and the Abbe number of that glass type isn’t the only factor in controlling colour correction or optical quality overall. The geometry of the surface being polished, the relative spacing between the lens elements, coatings, surface smoothness, and deviation from the ideal design, plus the final figuring process are all very important. In fact, we are able to get near perfect colour correction in focus in our FPL-51 based triplets, and better blue colour correction than many FCD1 and H-FK61 based lenses we’ve tested. This is achieved by using a better match of mating elements and geometry, and taking time to adjust the lens arrangement and surfaces during production. Once again, this is borne out by the images taken with our larger FPL-51 based refractors which show better blue channel colour correction than some competing refractors with FCD1 and H-FK61 lenses. The point is that, while a lower dispersion element helps, it's not the only factor. But for now, let’s concentrate on the main ED glass types we use:
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