Apochromat

A refractor telescope with a two to four lens optical system that uses one or more elements of costly ED (Extra-low Dispersion) or SD (Special Dispersion) glass and/or calcium fluorite crystal to virtually eliminate chromatic aberration. Technically, any lens system that is corrected for spherical aberration at two wavelengths, or colors of light, and for chromatic aberration at three.


. . . our 34th year