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
\(d\)
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
\(\sim d\)
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 d meters, the diameter of single-dish antenna must be
larger or equal to the diameter of the interferometer antennae:
\(D \ge d\)
.
