Piazzi-Smyth Lens

a lens mounted directly in front of the focus of an astrograph to correct its curvilinear field. The surface of a Piazzi-Smyth lens facing the photographic plate is made flat; the surface facing the objective has a radius of curvature r = R(n – 1) /n, where R is the radius of curvature of the field and n is the refractive index of the glass from which the lens is made. The aberrations introduced by a Piazzi-Smyth lens are small. By also assigning curvature to the second surface of the lens, it is possible to correct the distortion of the astrograph objective. The Piazzi-Smyth lens was invented by the British scientist C. Piazzi-Smyth in 1874.