skysim – predicting visibilities

skysim predicts model visibilities from a sky model (ASCII catalogue, FITS image(s), or a WSClean component list) into an existing MS, writing them to a chosen data column. It can also add or subtract an existing column in-place instead of simulating from a sky model.

Basic usage

$ simms skysim --ascii-sky skymodel.txt --column DATA visdata.ms

  • --ascii-sky skymodel.txt – a catalogue of parametrised sources.

  • --column DATA – the column where simulated visibilities are written.

  • visdata.ms – the target MS, which must already exist (from an observation, or created with telsim – building a Measurement Set).

ASCII sky model schema

  • Point sources need only RA, Dec, and intensity (stokes_i).

  • Extended sources are 2D Gaussians, parametrised by FWHM major/minor axes (emaj/emin) and position angle (pa); double-horn profiles are not supported.

  • Spectral line sources need the line centre and width (line_width, an observed-frame FWHM). The centre is either the observed peak frequency (line_peak), or a rest frequency (line_restfreq) optionally shifted by a redshift (line_redshift, default 0). line_peak wins when both are given.

  • Continuum sources need a reference frequency (cont_reffreq) and at least one power-law coefficient (cont_coeff_1 = spectral index, cont_coeff_2 = curvature, …).

See Schemas for the full schema, and use --ascii-species to select a non-default catalogue mapping (e.g. bdsf_gaul for a PyBDSF catalogue).

FITS sky models

$ simms skysim --fits-sky skymodel.fits --column DATA visdata.ms

Provide separate FITS files per Stokes when simulating polarised sources. Tune the prediction with --pixel-tol (minimum pixel brightness considered, default 1e-7), --fft-precision (single/double), and --no-do-wstacking to disable w-stacking.

Adding or subtracting an existing column

Once visibilities are simulated into one column, add or subtract them against another:

$ simms skysim --ic DATA --column MODEL_DATA --mode add visdata.ms
$ simms skysim --ic DATA --column MODEL_DATA --mode subtract visdata.ms

--ic/--input-column is the source column; --column is where the result is written; --mode defaults to simulate.

Thermal noise

$ simms skysim --ascii-sky skymodel.txt --column SIMULATED_DATA --sefd 421 visdata.ms

Provide either --sefd (System Equivalent Flux Density, in Jy) or --tsys-over-eta (\(T_\mathrm{sys}/\eta\)).

Chunking large MSs

$ simms skysim --ascii-sky skymodel.txt --column SIMULATED_DATA --row-chunks 5000 largevis.ms

--row-chunks controls the row-wise task/memory granularity (default 10000).

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