IMAGER versus CASA: controlling the number of synthesized beams, BEAM_STEP

In aperture synthesis, the angular resolution scales with the frequency: in fact the whole Fourier plane scales with this frequency, so that the angular scaled defined by the baselines varies across the frequency coverage. Accordingly, when using sufficiently wide bandwidths (and/or imaging sufficiently large areas), it becomes important to account correctly for this effect.

The CASA and IMAGER approaches to this 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.

The approach offered by IMAGER is different, and can be controlled by variable BEAM_STEP.

However, contrary to CASA, IMAGER can use 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 Section[*] use sensible default for each telescope, but users should be aware of this possible difference when comparing with CASA results.