Measured GBT response pattern P, as in Fig. 9, but for sidelobes closer to the main beam. The Arago spot, peaking at  ≈ 0.25, is visible at the right, along with its surrounding rings. The double-peak feature at (H,V) ~ (±1°, −3°) appears just to the left of the (truncated) main beam, with height P ≈ 2.6. This contains the power scattered from the forward spillover lobe by the reflecting screen on the feed arm (Fig. 3). The peak of the main beam would be far offscale at P = 1.12 × 10⁵.