Simulating Observations

IMAGER offers a simple, but powerful, simulation tool that allows users to process an input data cube (from a model, or other observations) as if it was observed by an array.

This is done through the SIMULATE command, that uses commands in the SIMULATE\ language to mimick the whole observation process. This involves

• Specifying the array. SIMULATE uses command ARRAY from the SIMULATE\ language for this.
• Specifying the configuration(s) used. SIMULATE uses command UV_TRACKS for this purpose.
• Specifying the model cube (or single plane). This is done through a call to the command MODEL of the CALIBRATE\ language.
• Specifying the (virtual) source declination, observing frequency through the arguments or options of the SIMULATE command, and the required field of view through the SIZE variable.
• Specifying the sensitivity, through the SIMULATE /NOISE option.

The simulator estimates whether a Mosaic is required and the corresponding fields layout. A $uv$ coverage is derived from the source declination, array configuration and hour angle coverage. Model visibilities are then computed from the model plane or cube. Noise is added to the model visibilities, and ultimately the produced simulated UV data can be imaged.

Up to now, the Simulator has no tool to predict the expected noise level from the observation characteristics. It is the user responsibility to specify the sensitivity in accordance to the expected array performance. The ASTRO program in the GILDAS suite, or the on-line time/sensitivity estimators that are available for most arrays (in particular NOEMA and ALMA) are possible tools make this estimation. Just beware of being consistent between the required Hour angle coverage, Bandwidth and the Noise.

Fig.19 shows an example of Mosaicing pattern and UV coverage using two ALMA configurations. Note the change of declination between the model source and the `observed” direction.

Figure: Windows created by the SIMULATE command.

Try the @ gag_demo:demo-simulate for a demonstration on the Simulator usage. Use HELP SIMULATE for further details.