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The CASA and IMAGER approaches to this spatial resolution problem differ. CASA systematically produces one synthesized beam per spectral channel, leading to a frequency variable spatial resolution. In this approach, CASA can handle different weighting for individual channels, optimizing signal to noise, but the interpretation of the data becomes more difficult since there is no unique angular resolution after deconvolution.
IMAGER can mimick the CASA behaviour by using BEAM_STEP = 1, but IMAGER uses a common clean beam in all cases. This is achieved through the use of the so-called JvM factor, which approximately scales the undeconvolved residuals to match the flux scale of the Clean components. This behaviour is only obtained through the UV_RESTORE command.
BEAM_STEP is a key control parameter for IMAGER. The IMAGER pipelines (see Section13) use sensible default for each telescope, but users should be aware of this possible difference when comparing with CASA results.