s_fov as an instantaneous concept

[alle-ira]

The interpretation of the spatial field of view as based on a mid/representative value for the frequency or wavelength should be better clarified in the ObsCore definition of this parameter.
We also point out that in the radio domain the field of view is an instantaneous concept, i.e. the instantaneous footprint (a circular primary beam in the case of mono-feed receivers, a more complex shape in the case of multi-feed/beamforming/PAFs), while in ObsCore it is defined as the approximate size of the region covered by the data product.
This means that the region covered by an observation can be larger than the instantaneous field of view. This happens for instance in a SD radio map, in an interferometric map/mosaic, etc.