{#sec:mbs}
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Beam-pattern calculations
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We now describe the simulation set-up used to compute the beam patterns for a dish with multiple feed-horns in the focal plane. Our simulations do not assume a far-field geometry; the telescope-source distance is left as an input parameter. We have done so to facilitate future studies of terrestrial and atmospheric transients [@dolin2014; @katz_perytons; @danish2014], which may be of great interest to atmospheric physicists, and at the very least, form a source of foreground ‘confusion’ to the astronomer.[^1]\
The multi-beam receivers on the Parkes [@smith1996] and Arecibo [@cordes2006] dishes have a central feed horn surrounded by an ‘inner-ring’ of 6 feed horns. Parkes has an ‘outer-ring’ of 6 additional feed horns. To compute the response of a feed-horn to a point-like radiator of spherical waves, we first compute the electric field on the dish surface. We employ Huygens’ principle and treat each segment of the dish as a secondary spherical radiator. We then sum up the electric fields of the ensuing spherical waves at each point on the focal plane. We finally average the aggregate electric field on the focal plane, over the aperture of the horn. The final averaging step gives us the response 