Beam calculation process of the two-step PO method. The most computationally expensive step 2, field propagation from M2 to M1, indicated by blue solid arrow, is divided into two sub-steps (blue dashed-line arrow) using the imaginary focal (IF) plane. First, the field in the IF plane is computed from the field on M2, where it is confined to a limited region around the focus; then it is propagated to M1, which scatters the field into the sky, producing the final beam map. The fields on M2, in the IF plane, and on M1 are denoted by E_M2, E_IF, and E_M1, respectively. The field calculated in the source region, located 300 m from the telescope, corresponds to the telescope near-field beam map and is denoted by E_Beam. The same flow is used for the scalar diffraction method, with the fields represented by scalar quantities (V_M2, V_if, V_M1, and V_Beam).