IMAGER versus CASA: controlling the number of synthesized beams, blueBEAM_STEP
Keywords: imager

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IMAGER versus CASA: controlling the number of synthesized beams, blue`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 [rgb]1,0,0IMAGER 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 [rgb]1,0,0IMAGER is different, and can be controlled by variable blue`BEAM_STEP`.

- blue`BEAM_STEP` = 0 instructs [rgb]1,0,0IMAGER 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.
- blue`BEAM_STEP` = N instructs [rgb]1,0,0IMAGER 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.
- blue`BEAM_STEP` = -1 instructs [rgb]1,0,0IMAGER 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 blue`MAP_PRECIS`. The derived N will depend of this number, on the relative bandwidth and on the size of the synthesized field (hence blue`MAP_FIELD` or blue`MAP_CELL` blue`MAP_SIZE`).
- blue`BEAM_STEP` = 1 mimicks to some extent the behaviour of CASA.

However, contrary to CASA, [rgb]1,0,0IMAGER 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 magenta`UV_RESTORE` command.

blue`BEAM_STEP` is a key control parameter for [rgb]1,0,0IMAGER. The [rgb]1,0,0IMAGER pipelines (see Section use sensible default for each telescope, but users should be aware of this possible difference when comparing with CASA results.

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