Extended structure are missed, attenuated or distorted in interferometric maps by lack of short uv spacing information. While this effect may be negligible for some astronomical problems, it could also be essential in a proper analysis. Deconvolution recovers some of them, but under-estimate the total flux because the integral of the dirty beam is zero (the integral of the dirty beam is the weight of the (0,0) uv cell in the uv data set).
Constructing a beam with a non zero integral can help deconvolution. This can be done by incorporating the Zero spacing flux or spectrum.
Short spacings provides even more information, because they give information on the spatial distribution of this flux on scales between half the primary beam and the primary beam itself. Short spacings can be provided by a smaller interferometer (e.g. BIMA) or a large single dish (e.g. 30-m). In theory, short spacings can also be provided by the interferometer antennas used in single-dish mode. However, because most interferometer have not been designed with total power stability as a goal, this has not been practiced so far.
Incorporating short spacings into interferometer data is a two step process. Task UV_SINGLE extracts short spacing information from single dish data (spectra) and creates a uv table. Task UV_MERGE merges the single-dish and interferometer tables. Coordinates system should be consistent and checked before (coordinates are always J2000.0 at Plateau de Bure, often B1950.0 at the other observatories...).