magentaCLEAN (except for magentaMRC) has three outputs: the blueCLEAN image itself, the blueRESIDUAL image, and the list of point sources (the Clean Components) that reproduce the observed visibilities, the blueCCT table.
An aesthetically better results, with better noise properties, can be obtained after magentaCLEAN by removing the Clean Components from the measured visibilities, and re-imaging this to produce the blueRESIDUAL image. Aliasing at map edges is then minimized, as it only concerns noise if the deconvolution was reasonable. As mentioned previously, this is implicitely done when using magentaMX instead of magentaCLEAN. It can also be done after magentaCLEAN using command magentaUV_RESTORE.
This step can also be used to properly scale the residuals when the synthesized beam
is not well fit by a Gaussian, or when all channels do not share the same beam (see Section ). This
is done through the use of the so-called blueJvM factor. This factor, introduced in (),
estimates the ratio of clean beam area to dirty beam area, and is stored in variable
blueBEAM_JVM and (on a per beam basis) blueBEAM_VALUES[4] which are computed by magentaFIT /JVM_FACTOR.
Residuals are multiplied by this factor (on a channel per channel basis if needed), allowing
to first order to scale the residuals to the same unit as the Cleaned data, i.e. Jy/beam area.
In general, this number is fairly close to 1. However, when several observing configurations have been merged together, the synthesized beam often exhibits a central peak over a larger plateau, because of the higher weights of short baselines. Robust weighting limits the effect, but is not able to suppress it totally. In such cases, the blueJvM factor can be of order 0.5 to 0.6.
magentaUV_RESTORE uses the JvM factor if magentaFIT /JVM_FACTOR has been used before.
It also uses the mean Clean beam geometry defined by blueBEAM_FITTED.
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2023-06-01