Continuum imaging

These scripts simulate MID continuum imaging observations of multiple point sources.

  • The sky model is constructed from Oxford S3-SEX catalog. These are unpolarised point sources.

  • The observation is by MID or MEERKAT+ over a range of hour angles.

  • The visibility is calculated by Direct Fourier transform after application of the gaintable for each source.

  • Dask is used to distribute the processing over a number of workers.

  • Processing can be divided into chunks of time (default 1800s).

The core simulation functions reside in RASCIL. The RASCIL driver script for simulation in this repository is direction_dependent/src/mid_simulation.py. This driver scripts can be run directly using the command line arguments listed below. Canonical bash scripts have been provided in continuum_imaging/scripts. The scripts will need to be altered for location of the various files needed. We recommend use of the bash scripts at first.

These simulations typically requiring a cluster to run. In Cambridge They work well on the P3 cluster using 16 nodes of 128GB each.

There are types of output: MeasurementSets and Images. Each are calculated for actual, nominal, and difference. The time consumed in the calculation of the MeasurementSets is small compared to the time in writing the MeasurementSet and the time to make the images. Images may be calculated using the dist_imager.

The fits images can be viewed using the casaviewer or carta. The MeasurementSets can be viewed using casaviewer.

If you are running on your own machine, make sure you have the below environment variables set up:

SSMROOT : location where the ska-sim-mid repository is SSMRESOURCES : location of the resources e.g. beam models SSMRESULTS: location of results folder

Effects simulated

Nominal

  • A set of point sources is simulated and the relevant nominal voltage pattern for each end of interferometer is applied before Fourier transform. The nominal pattern is constructed from a tapered symmetric illumination pattern, with the diameter of the SKA dishes.

  • stokesIQUV, band B2

Heterogenous

  • A set of point sources is simulated and the relevant voltage pattern for each end of interferometer is applied before Fourier transform.

  • Simulates observations with all SKA and MEERKAT dishes (on state) and all SKA dishes (off state).

  • stokesIQUV, band B2

Heterogenous_meerkat+

  • A set of point sources is simulated and the relevant voltage pattern for each end of interferometer is applied before Fourier transform.

  • Simulates observations with MEERKAT+ configuration, all SKA and MEERKAT dishes (on state) and all SKA dishes (off state).

  • stokesIQUV, band B2

Ionosphere

  • A set of point sources is simulated and the phases calculated using a thin screen model for the ionosphere. The screen has units of meters of Total Electron Content. The phase is evaluated at places in the screen where the line of sight from source to a dish pierces the screen.

  • Simulates observations with ionospheric screens on and off.

  • This requires the screens to be calculated first or downloaded: see ../screens/README.md

  • stokesI, band B1LOW

Polarisation

  • Simulates observations with SKA and EMSS calculated primary beam models with cross pol (on) and no cross pol set to zero (off)

  • Polarisation stokesIQUV, linear, band B2

Surface sag

  • Simulates observations with sagging dish (on) and nominal dish (off).

  • Models of the voltage pattern are available at +15, +45, +90 deg elevation.

  • We interpolate between those to 5 degrees.

  • Stokes I, band B2

For more details see: https://confluence.skatelescope.org/display/SE/Dish+deformation+simulations

Troposphere

  • Simulates observations with tropospheric screens on and off.

  • This requires the screens to be calculated first or downloaded: see ../screens/README.md

  • stokesI, bands B2 and B5

  • A set of point sources is simulated and the phases calculated using a thin screen model for the atmosphere. The phase is evaluated at places in the screen where the line of sight from source to dish pierces the screen. The screen has units of meters of delay.

Wind pointing

  • Simulates observations with wind buffeting of dishes (on) and without (off)

  • Stokes I, bands B2 and B5

For more details see: https://confluence.skatelescope.org/display/SE/MID+pointing+error+simulations