= Radio interferometer
A <radio interferometer> combines voltage signals from separated <antennas> to measure the <spatial coherence> of <radio waves>. A correlator compares signals after correcting for the geometrical delay associated with an observing direction. The resulting <complex visibility> retains relative <wave phase> information that independent power measurements lose.
For a baseline vector $\mathbf b$ and source direction $\mathbf s$, the geometrical delay is $\tau=\mathbf b\cdot\mathbf s/c$. The angular resolution scale is roughly $\lambda/B$, where $B$ is the longest relevant baseline. Multiple baselines and the changing orientation supplied by Earth's rotation sample different spatial frequencies for <aperture synthesis>; a single pair does not supply a complete image.
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