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IMAGER versus CASA: controlling the number of synthesized beams, blueBEAM_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 blueBEAM_STEP.

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 magentaUV_RESTORE command.

blueBEAM_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.


next up previous contents index
Next: The input data: UV Up: IMAGER principles Previous: Getting Help: the HELP   Contents   Index
Gildas manager 2023-06-01