= Dirty beam
{title2=$B(\theta)$}
The dirty beam is the <point spread function> obtained by directly inverting the weighted sampled <complex visibilities>. For sampling positions $u_k$ and nonnegative weights $w_k$ with $W=\sum_k w_k>0$, choose
$$
B(\theta)=\frac{1}{W}\sum_k w_ke^{2\pi i u_k\theta},\qquad B(0)=1.
$$
Include each $u_k$ with $-u_k$ and the same weight so that $B$ is real. Its unit peak convention means that an isolated unit point source contributes a shifted unit-height response. It is not a unit-integral convention. Sparse sampling can leave substantial subsidiary peaks, even when the longest <interferometric baseline> is large.
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