Short spacings (RUN UV_SHORT and RUN UV_ZERO)

Let's note $D$ the diameter of the single-dish antenna ($D = 30$m for the IRAM-30m telescope) used to produce the short-spacing information and $d$ the diameter of the interferometer antennas ($d = 15$m for PdBI). We already mentioned that a multiplicative interferometer filters out all the spatial frequencies smaller than $\sim d$ meters. When this information is needed to get reliable results, the source should also be observed with a single-dish antenna to produce the missing information. The single-dish antenna furnishes information about all spatial frequencies up to $\sim D$ meters (but this information is weighted by the single-dish beam shape, i.e. high frequencies are measured with a worse signal-to-noise ratio than low frequencies). To recover all the information at spatial frequencies smaller than $\sim d$ meters, the diameter of single-dish antenna must be larger or equal to the diameter of the interferometer antennae: $D \ge d$.