A schematic figure shows the spherical wave observed with 3 telescopes located at positions A, O, and B. The 3 telescopes are settled at the spherical wavefront. The source S is the center of the circle. With the measuring of distance \(b\)=OO', one is able to get the distance \(D = {d^2 \over 2b}\), where \(d\) is the distance between B and O'.