An apochromatic is a lens or optical system that is virtually free of chromatic aberration. For practical purposes this means that light of at least three different wavelengths is brought to focus at the same point. The best apochromatic lenses use fluorite crystal and may correct three wavelengths with only two optical elements. However, because fluorite is expensive to manufacture, and, because of its brittleness, difficult to grind, polish, and mount, high quality apochromatic refracting telescopes are costly. Reflecting telescopes, on the other hand, are apochromatic in performance without the extra expense.