What is the difference between Achromat and Apochromat?
* Achromat An Achromat lens system compensates the wavelength differences of two colors to diminish chromatic aberration. For observation the two colors usually compensated are blue and red, and for photography, purple and red. An achromat diminishes chromatic aberration so that only a little remains. * Apochromat An Apochromat lens system compensates the wavelength differences of three colors or more. For example, an apochromat system is designed to make purple, green and red focus at the same point so that chromatic aberration is significantly reduced. It has become standard to use Fluorite Crystal, or a kind of XD (ED) lens for this type of lens system. yCautionz The above are general explanations and differ among optics manufacturers. Recent refraction telescopes are called apochromat when Fluorite or XD (ED) lenses are used and achromat when other lens materials are used.
