Achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. The most common type of achromat is the achromatic doublet, which is composed of two individual lenses made from different optical glasses with different refraction value. Typically, one negative lens with high dispersion material glued together with a positive lens with a lower dispersion material. Such as F2 negative lens glued with a BK7 positive lens to create an achromatic.
PHYO OPTICS still supply the apochromatic lens, which has better correction of chromatic and spherical aberration than the common achromat lens.