Keywords: imager

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### 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 NOEMA antennas, 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 magenta`UV_SHORT` command. Indeed, the IRAM-30m diameter being twice the diameter of the NOEMA 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 NOEMA antennae.
- If the source size is larger than 1/2 the primary beam size of NOEMA antennas, 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.
- Short-spacing information is only useful if the brightness of the extended component is above the noise level. This requires a prior knowledge of the total flux in the imaged area to be determined. However, this information may be available from previous low-sensitivity single-dish observations. Checking this can avoid wasting a lot of telescope (and astronomer) time.

A generalization to ALMA (12 m antenna) and ACA (7 m antennas) is straightforward.

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