A meniscus lens is a lens having two spherically curved faces, one convex and the other concave, so that it has the form of a shell. A positive meniscus lens is thicker in the middle than at the edges and serves as a converging lens; a negative meniscus lens thickens toward the edges and works as a diverging lens. Very large telescopes may use meniscus mirrors, which are lightweight and require an active control system to retain their shape against gravity.