Usage

Sky Simulations

The simplest way to run a simulation

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

Let’s break down what this command does:

  • --ascii-sky skymodel.txt: A catalogue of parametrised sources

  • --column DATA : The column where simulated visibilities will be written

  • visdata.ms : The MS where visibilities will be stored. The MS must exist (e.g., from an observation or created by simms telsim)

ASCII sky model schema

  1. Point Sources

    • The simplest source that can be simulated is a point source, which only requires the RA, Dec, and intensity (stokes_i)

  2. Extended Sources

    • The only resolved sources supported are 2D Gaussian sources. These sources are parametrised via the FHWM values for the major and minor axes (emaj and emin) and the position angle (pa).

  3. Spectral Line Sources - For spectral line sources, the frequency where the line peaks (line_peak) needs to be specified along with the width of the line (line_width). Double-horn profiles are not supported at this point.

  4. Continuum Sources

    • Continuum sources require the reference frequency (cont_reffreq) and at least one (of three) power-law coefficient (cont_coeff_<number>). Where, cont_coeff_1 is the spectral index, cont_coeff_2 is the spectral curvature, and so on.

Add thermal noise

There are to ways to add thermal noise to the generated visibilities. You can either provide a System Equivalent Flux Density (SEFD; in Jy) via the --sefd option, or via the --tsys-over-eta option which specifies \(T_\mathrm{sys}/\eta\), where \(T_\mathrm{sys}\) is the antenna system temperature and \(\eta\) is the antenna efficiency.

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

Custom ASCII sky models

simms skysim --ascii-sky pybdsf.txt --cat-species bdsf_gaul --column DATA visdata.ms

  • --cat-species specifies the catalogue type. This automatically uses the correct mapping for a PyBDSF catalogue. You have to specify --cat-species if the catalogue is different from the default.

2. FITS sky models

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

3. Subtracting from or adding to an existing column

You can subtract or add simulated data (or any data you already have) from/to an existing column in the MS.

Steps:

  1. Simulate the data using the basic simulation command.

  2. Add or Subtract to Existing Data

    You only need to specify the simulated column and the target column to add or subtract, assuming both already exist.

    Adding:

    simm skysim --ic DATA --column MODEL_DATA --mode add visdata.ms

    Subtracting:

    simm skysim --ic DATA --column MODEL_DATA --mode subtract visdata.ms

    Where:

    • --ic: Input column you want to subtract/add from

    • --column: In the add/subtract case, this is the column where your residuals will be saved

    • --mode: Specifies whether to add/subtract or simulate (the default is simulate: skysim will just simulate if mode is not specified)

4. Chunking for Large MS Data

simms skysim --ascii-sky skymodel.txt --column SIMULATED_DATA --rcs 5000 largevis.ms

Where --rcs is the row chunk size. Default is 1000.

4. FITS-Specific Cases

simms skysim --fits-sky model.fits --column DATA --pixel-tol 1e-6 --fft-precision single --no-do-wstacking largevis.ms

Where:

  • --pixel-tol: Minimum brightness for a pixel to be considered in DFT. Default is 1e-7

  • --fft-precision: Precision for FFT calculations (single or double). Default is double

  • --no-do-wstacking: Disables w-stacking for FFT-based simulation. Default is enabled