UV plane analysis
Keywords: imager

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UV plane analysis

[rgb]1,0,0IMAGER is not only a tool to produce images. It also allows direct analysis by model fitting to the UV data.

A number of commands are directly related to these possibilities:

- magenta`READ UV`
- magenta`READ MODEL`
- magenta`SHOW UV_FIT`
- magenta`SHOW UV`
- magenta`UV_CIRCLE`
- magenta`UV_CONTINUUM`
- magenta`UV_DEPROJECT`
- magenta`UV_FIT`
- magenta`UV_RESIDUAL`
- magenta`UV_RADIAL`
- magenta`UV_SELECT`

The overall intent of these commands is described below. Please refer to the corresponding HELP for more details about the command syntax, options and control values.

- magenta`UV_FIT` is the primary fitting command. It allows to adjust simple, analytic models of source brightness to the current UV data. Up to 8 different source models can be combined in a single fit 8.

  The UV_FIT results can be displayed using command magenta`SHOW UV_FIT`. The visibilities modeled by UV_FIT (or the fit residuals) can be displayed using command magenta`SHOW UV`, either alone, or superimposed to the current UV data.

  magenta`UV_FIT` works on spectral line data. To optimally fit continuum data, it is best to convert the spectrally resolved information into a bandwidth synthesis continuum uv table.

- magenta`UV_CONTINUUM` converts a spectral line uv table into a bandwidth synthesis continuum uv table. magenta`UV_CONTINUUM` requires some knowledge of the field of view to evaluate how many channels should be averaged together. This is done using the same parameters (blue`MAP_FIELD`, or the product of blue`MAP_SIZE` by blue`MAP_CELL`) and subroutines as for commands magenta`UV_STAT SETUP` and magenta`UV_MAP`.

  The bandwidth synthesis continuum uv table is peculiar in the sense that it has several baselines for the same pair of antennas at the same observing time: one baseline per independent frequency point, in order to preserve optimally the angular resolution that goes as . In practice, it is only useful for magenta`UV_FIT`, but not suitable for self-calibration for example.

- magenta`SHOW UV` will display the best fit visibilities (stored in the blue`UV_MODEL` data set) if (and only if) the blue`UVSHOW%FIT` variable is set to `MODEL, POINT` or `CURVE`. When blue`UVSHOW%FIT` variable is set to `MODEL`, the blue`UV_MODEL` data set is assumed to exist, and not re-computed. It is displayed as data points (not as a curve).

- magenta`SHOW UV_FIT` is intended to display the fit results (the best fit parameter values and their errors) as a function of channels (or velocity, or frequency). This is often a nice way to show the detection of a signal when the signal to noise is limited, but of course is useless for single channel data !

- magenta`READ MODEL` can import the visibilities from an outside model, so that these can be overlaid to the current UV data by magenta`SHOW UV` with blue`UVSHOW%FIT` variable set to `MODEL`.

- Command magenta`MODEL` computes the visibilities (the blue`UV_MODEL` dataset) for the best fit parameters found by magenta`UV_FIT` or for the selected Clean components, depending whether magenta`UV_FIT` or magenta`CLEAN` was used last.

- Similarly to command magenta`MODEL`, magenta`UV_RESIDUAL` computes the residual visibilities (the blue`UV_RESIDUAL` data set).

- Commands magenta`UV_CIRCLE` magenta`UV_DEPROJECT` magenta`UV_RADIAL` can be useful in conjunction to magenta`UV_FIT`, as they can provide a (deprojected, optionally azimutally averaged) visibility data set which can the be much more easily compared to simple, circularly symmetric models.

  Beware that the convention for angle are different in these commands (that use the PA of the axis) and magenta`UV_FIT t`hat use the PA of the major axis: they thus differ by 90 degrees.

- Finally, command magenta`UV_SELECT` allow to select which data set is displayed by magenta`SHOW UV` among the UV data, the blue`UV_MODEL` or the blue`UV_RESIDUAL`.

Try magenta`@` cyan`gag_demo:demo-uvfit` for some examples.

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