PO simulations for co- and cross-polarization beam maps of the FYST telescope at 300 GHz, obtained using TICRA GRASP. Both beam maps are normalized to peak of the co-polarization beam. The simulation models a focused beam from a source located 300 m from the telescope. The receiver is positioned 705 mm behind the nominal focal plane to refocus the optical system. The maps cover an angular range of 0.57 × 0.57 deg². The peak cross-polarization level is more than 65 dB below that of the co-polarization beam. Axes are shown in the (u, υ) beam coordinates, with u = sin θ_sky cos ϕ_skv and υ = sin θ_sky sin ϕ_sky, where (θ_sky, ϕ_sky) are the polar angles of the field direction in coordinate system C_sky (see Fig. A.1).