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

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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.

- BEAM_STEP = 0 instructs IMAGER to produce a single synthesized beam for all channels. This is appropriate for narrow bands and spectral channels with very similar noise (ideally identical). Unavoidably, the synthesized beam is only an approximation, exact only at the reference frequency.
- BEAM_STEP = N instructs IMAGER to ensure that N consecutive channels share the same synthesized beam. This can be used when channels have the same weights (same effective UV coverage) but the overall bandwidth is large.
- BEAM_STEP = -1 instructs IMAGER to derive the numnber N of consecutive channels that can shared the same beam to within a certain precision in the imaged field of view. This precision is given by MAP_PRECIS. The derived N will depend of this number, on the relative bandwidth and on the size of the synthesized field (hence MAP_FIELD or MAP_CELL MAP_SIZE).
- BEAM_STEP = 1 mimicks to some extent the behaviour of CASA.

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.