= Field-dependent averaging losses in radio interferometry
{title2=$\delta\nu\,B\theta/c\ll1,\quad\Delta t\,\Omega_EB\theta/\lambda\ll1$}
After delay tracking and fringe stopping at a phase center, an off-axis direction retains delay of order $B\theta/c$ and rotation-induced fringe-rate offset of order $\Omega_EB\theta/\lambda$. Averaging across a wide channel or long integration then decorrelates that direction. The displayed inequalities give order-of-magnitude small-smearing conditions; exact coefficients depend on baseline geometry, averaging window and allowed loss. A larger field or longer baseline therefore demands finer frequency and time sampling.
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