- 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
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.
