Visibility coverage from a six-hour observation of Cygnus A (McKean et al. 2010). Visibilities are plotted in the Fourier space with the U (x-axis) and V (y-axis) being the spatial frequencies in wavelength unit, λ (here f = 151 MHz, λ ≈ 2 m) determined by the baseline projection on the sky. Each (u,v) point (red) has its symmetric (−u, − v) point (blue) corresponding to the same baseline. The lines indicate (u,v) points where a visibility was recorded. The arcs are built from varying the baseline projections with the rotation of Earth during the observation.