When are short-spacing information needed?

• If the source size is smaller than 1/3 the primary beam size, short-spacing information is superfluous.
• If the source size is between 1/3 and 1/2 the primary beam size of PdBI antennae, a single spectra obtained at the IRAM-30m telescope in the direction of the source can be used to produce the zero spacing information with the UV_ZERO task. Indeed, the IRAM-30m diameter being twice the diameter of the PdBI antenna, all the flux of the source will be measured by a single IRAM-30m spectrum only if the size of the source is smaller than 1/2 the primary beam size of PdBI antennae.
• If the source size is larger than 1/2 the primary beam size of PdBI antennae, short-spacing information under the form of an IRAM-30m map is almost always mandatory. The only exception could be wide-field imaging of a region made of unresolved or small (compared to the primary beam size) sources as it may happen when mapping close-by external galaxies for instance. However, adding short-spacing will anyway help the deconvolution. In case of doubt, always add short-spacing information.