thumb|300x300px|Deep blue ray refers the radius of curvature and the red line segment is the sagitta of the curve (black). 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 [math]\displaystyle{ r }[/math] from the axis.
A good explanation both of this approximate formula and the exact formula can be found here.
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, [math]\displaystyle{ K }[/math] is the conic constant as measured at the vertex (where [math]\displaystyle{ r=0 }[/math]). The coefficients [math]\displaystyle{ \alpha_i }[/math] describe the deviation of the surface from the axially symmetric quadric surface specified by [math]\displaystyle{ R }[/math] and [math]\displaystyle{ K }[/math].[1]
Original source: https://en.wikipedia.org/wiki/Sagitta (optics).
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