Calculated all-sky response of the GBT at 1.4 GHz, the main beam and far sidelobes, as a function of angle from the main beam, θ, for three values of φ in this coordinate system: φ = −90° (towards the feed arm in the antenna symmetry plane), red; 0° (perpendicular to the antenna symmetry plane), green; 90° (away from the feed arm), blue. To show details of the sidelobes the scale has been magnified so that the forward gain of the main beam, 61.6 dBi at θ = 0° (Fig. 5, Table 1), is beyond the range of this figure. Vertical lines indicate the far edge (), center (), and near edge () of the main reflector in the φ = 90° half-plane.