Sagitta (optics)

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

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,

is the conic constant as measured at the vertex (where