Measuring Wavefront Curvature from the Tip of a Fiber using a Shack-Hartmann Wavefront Sensor

This video shows how to measure the radius of curvature of a the diverging wavefront emitted from a fiber tip using a Shack-Hartmann wavefront sensor using the Active Optical Systems, LLC (AOS) software

[@zhany0u]

Interesting ,what is the math behind such measurement? Is there any literature or textbook recommended ?