Restoring step: magentaUV_RESTORE
Keywords: imager

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Restoring step: magenta`UV_RESTORE`

magenta`CLEAN` (except for magenta`MRC`) has three outputs: the blue`CLEAN` image itself, the blue`RESIDUAL` image, and the list of point sources (the Clean Components) that reproduce the observed visibilities, the blue`CCT` table.

An aesthetically better results, with better noise properties, can be obtained after magenta`CLEAN` by removing the Clean Components from the measured visibilities, and re-imaging this to produce the blue`RESIDUAL` 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 magenta`MX` instead of magenta`CLEAN`. It can also be done after magenta`CLEAN` using command magenta`UV_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 blue`JvM factor`. This factor, introduced in (), estimates the ratio of clean beam area to dirty beam area, and is stored in variable blue`BEAM_JVM` and (on a per beam basis) blue`BEAM_VALUES[4]` which are computed by magenta`FIT /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 blue`JvM factor` can be of order 0.5 to 0.6.

- Caveat: This scaling is only meaningful to first order for (reasonably) extended sources and well-behaved beams. It should NOT be used to analyze data for point sources.

magenta`UV_RESTORE` uses the `JvM factor` if magenta`FIT /JVM_FACTOR` has been used before. It also uses the mean Clean beam geometry defined by blue`BEAM_FITTED`.

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