UV plane analysis

IMAGER 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:

• READ UV
• READ MODEL
• SHOW UV_FIT
• SHOW UV
• UV_CIRCLE
• UV_CONTINUUM
• UV_DEPROJECT
• UV_FIT
• UV_RESIDUAL
• UV_RADIAL
• 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.

• 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 ¹¹.

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

  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.

• UV_CONTINUUM converts a spectral line uv table into a bandwidth synthesis continuum uv table. 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 ( MAP_FIELD, or the product of MAP_SIZE by MAP_CELL) and subroutines as for commands UV_STAT SETUP and 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 \(\lambda/B\) . In practice, it is only useful for UV_FIT, but not suitable for self-calibration for example.

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

• 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 !

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

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

• Similarly to command MODEL, UV_RESIDUAL computes the residual visibilities (the UV_RESIDUAL data set).

• Commands UV_CIRCLE UV_DEPROJECT UV_RADIAL can be useful in conjunction to 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 UV_FIT that use the PA of the major axis: they thus differ by 90 degrees.

• Finally, command UV_SELECT allow to select which data set is displayed by SHOW UV among the UV data, the UV_MODELor the UV_RESIDUAL.

Try @ gag_demo:demo-uvfit for some examples.