Correction efficiency η_c(ν,Ω_source) for the sources listed in Table 1 and dark sky (gray). The plot shows the variation of η_c with the frequency for each source. The blue dotted lines indicate the efficiency predicted by the optics model. The black dashed lines indicate a smoothed efficiency function of an extended source. The ratio between the two is indicated by the red dash dot lines. The two black stars are the far field efficiency measured from prototype arrays in the lab (Chattopadhyay et al. 2003).