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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:

  • magentaREAD UV
  • magentaREAD MODEL
  • magentaSHOW UV_FIT
  • magentaSHOW UV
  • magentaUV_CIRCLE
  • magentaUV_CONTINUUM
  • magentaUV_DEPROJECT
  • magentaUV_FIT
  • magentaUV_RESIDUAL
  • magentaUV_RADIAL
  • magentaUV_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.

  • magentaUV_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 magentaSHOW UV_FIT. The visibilities modeled by UV_FIT (or the fit residuals) can be displayed using command magentaSHOW UV, either alone, or superimposed to the current UV data.

    magentaUV_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.

  • magentaUV_CONTINUUM converts a spectral line uv table into a bandwidth synthesis continuum uv table. magentaUV_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 (blueMAP_FIELD, or the product of blueMAP_SIZE by blueMAP_CELL) and subroutines as for commands magentaUV_STAT SETUP and magentaUV_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 $\lambda/B$. In practice, it is only useful for magentaUV_FIT, but not suitable for self-calibration for example.

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

  • magentaSHOW 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 !

  • magentaREAD MODEL can import the visibilities from an outside model, so that these can be overlaid to the current UV data by magentaSHOW UV with blueUVSHOW%FIT variable set to MODEL.

  • Command magentaMODEL computes the visibilities (the blueUV_MODEL dataset) for the best fit parameters found by magentaUV_FIT or for the selected Clean components, depending whether magentaUV_FIT or magentaCLEAN was used last.

  • Similarly to command magentaMODEL, magentaUV_RESIDUAL computes the residual visibilities (the blueUV_RESIDUAL data set).

  • Commands magentaUV_CIRCLE magentaUV_DEPROJECT magentaUV_RADIAL can be useful in conjunction to magentaUV_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 magentaUV_FIT that use the PA of the major axis: they thus differ by 90 degrees.

  • Finally, command magentaUV_SELECT allow to select which data set is displayed by magentaSHOW UV among the UV data, the blueUV_MODEL or the blueUV_RESIDUAL.

Try magenta@ cyangag_demo:demo-uvfit for some examples.


next up previous contents index
Next: Continuum emission Up: imager Previous: Data flagging   Contents   Index
Gildas manager 2023-06-01