Achromatics represent the classic, popular, modern lens telescope design. An achromatic consists of not one but of two lenses, usually made of crown glass and flint glass, and have the effect of a positive and a negative lens.
The lenses have differing refractivity and dispersion, so that by using them most chromatic aberrations can be eliminated. Nevertheless small levels of chromatic aberration may still be detected. In order to eliminate even this, one must revert to apochromatic devices.