Keywords: imager

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## Principle

Let us note D the diameter of the single-dish antenna (D=30m for the IRAM-30m telescope) used to produce the short-spacing information and the diameter of the interferometer antennas (d=15m for NOEMA). We already mentioned that a multiplicative interferometer filters out all the spatial frequencies smaller than 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 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 d meters, the diameter of single-dish antenna must be larger or equal to the diameter of the interferometer antennae: D d.