The interferometer’s optical benches have a complex optical layout and contain many elements. These elements can create spurious beams, e.g. due to non-perfect anti-reflecting coatings, which propagate on the benches. These beams are called ghost beams. It is possible to trace them using a software called Optocad, which is a Fortran 95 module for tracing Gaussian TEM00 (defined in Appendix A) beams propagation within an optical set-up. In Fig. 7.1 the propagation of a ghost beam on Optocad is shown. The incoming beam propagates and it is almost completely reflected by a mirror. However, a fraction of the beam is transmitted from the outer surface as the Highly Reflective (HR) coating has residual transmission (of the order of a few ppm). Subsequently, this beam arrives at the back surface and is partially reflected by Anti-Reflective (AR) coatings (a fraction of hundreds of ppm). The ghost beam generated in this way leaves the first surface and propagates on the optical bench where it can be scattered by diffusing elements, such as mirror mounts. The scattered light thus generated can re-couple with the main beam of the interferometer, introducing the noise described in Eq. 6.5 through the factor \(f_{extra}\) in Eq. 6.6 .