= Dirty image
{title2=$I_D=I*B$}
The dirty image is the direct weighted inverse <Fourier transform> of the measured <complex visibilities>:
$$
I_D(\theta)=\frac{1}{W}\sum_k w_k V(u_k)e^{2\pi i u_k\theta}
=\sum_j I_jB(\theta-\theta_j).
$$
The final equality follows by substituting the point-source <complex visibility> and exchanging the finite sums. For continuous brightness it is a <convolution> with the <dirty beam>, with the <primary beam> included in the brightness when necessary. Negative values and subsidiary peaks can arise from that response even when every true source has positive strength.
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