Sagitta (optics)

In optics and especially telescope making, sagitta is a measure of the glass removed to yield an optical curve. It is approximated by the formula

$S={\frac {r^{2}}{2\times {\text{ROC}}}}$

or

$S={\frac {r^{2}}{4\times {\text{focal length}}}}$

A good explanation both of this approximate formula and the exact formula can be found at http://www.opticampus.com/popcourse.php?url=high_powered/#appendix

See also

• Radius of curvature (optics)
• Versine
• Chord