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

${\displaystyle S(r)\approx {\frac {r^{2}}{2\times R}}}$,

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 ${\displaystyle r}$ from the axis.

A good explanation both of this approximate formula and the exact formula can be found here.