Figure 1 : Elliptical array of telescopes . A giant paraboloïdal dish P, having its focus in C, intersects the ground plane G along an ellipse.E. Telescopes located on the ellipse have their optical axes parallel to the paraboloïd's and feed coudé beams towards a common focus in C. Optical path lengths are equal. To track a star, the paraboloid may rotate around C, and coherence can be maintained by moving the telescopes to follow the deformation of the ellipse. Alternately, additional moving mirrors can be inserted in the coudé trains . The IGT project involves 27 telescopes of 8 to 10 meters, mobile on a flat site spanning several kilometers.