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SEFD = {\frac{T_{sys}}{\frac{\eta_a A}{2k_B}}}  \end{equation}  where $T_{sys}$ is the system temperature including contributions from receiver noise, feed losses, spillover, atmospheric emission, galactic background and cosmic background, $k_B = 1.380 \times 10^{-23}$ Joule $K^{-1}$ is the Boltzmann constant, $A$ is the antenna collecting area (thus we could also write $\pi \cdot D^2$), and $\eta_a$ is the antenna efficiency with $\eta_a = \eta_{\text{surface efficiency}} \cdot \eta_{\text{aperture blockage}} \cdot \eta_{\text{feed spillover efficiency}} \cdot \eta_{\text{illumination taper efficiency}}$ (see \cite{antenna}).  If we assume N apertures with the same $SEFD$, obserbing the same bandwidth $\Delta\nu$, during the same integration time $\t_{int}$, $t_{int}$,  then the weak-source limit in the sensitivity of a synthesis image of a single polarization is \begin{equation}\label{eq:sens}  \Delta I_m = {\frac{1}{\eta_s }}{\frac{SEFD}{\sqrt{(N(N-1) \Delta \nu \t_{int}}}} t_{int}}}}  \end{equation}  in units of Janskys per synthesized beam area.   with $\eta_s$ most important factor being correlator efficiency $\eta_c = \frac{\text{correlator sensitivity}}{\text{sesitivity of a perfect analog correlator having the same } \tau_{acc}}$ (see \cite{sensitivity2}). 
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