Schemes to reduce coma without introducing spherical aberration include Schmidt, Maksutov, ACF and Ritchey–Chrétien optical systems. Correction lenses, "coma correctors" for Newtonian reflectors have been designed which reduce coma in telescopes below f/6. These work by means of a dual lens system of a plano-convex and a plano-concave lens fitted into an eyepiece adaptor which superficially resembles a Barlow lens.[1][2]