In optics and especially telescope making, ****sagitta or sag is a measure of the glass removed to yield an optical curve. It is approximated by the formula where R is the radius of curvature of the optical surface. The sag S(r) is the displacement along the optic axis of the surface from the vertex, at distance r from the axis. A good explanation both of this approximate formula and the exact formula can be found here.

Aspheric surfaces

Optical surfaces with non-spherical profiles, such as the surfaces of aspheric lenses, are typically designed such that their sag is described by the equation Here, K is the conic constant as measured at the vertex (where r=0). The coefficients \alpha_i describe the deviation of the surface from the axially symmetric quadric surface specified by R and K.