 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.