Specifying Observation

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Simulating Observations in CASA

Specifying what you want to observe and how

  • project is the root file name for simdata output (ms, images, will all start with that root)

  • refdate, totaltime, integration set when you want to observe and for how long.
  • simdata will recenter the track at transit i.e. the maximum altitude. A long track will see the source rise and set, and placing your model at the appropriate declination can ensure that the source is always low. More flexibility e.g. for snapshot observations through a range of LST, are possible at the tool level. Contact remy at nrao.edu for more information.
  • Mosaic observations will rotate through the mosaic fields during the track defined by totaltime, remaining at each pointing for scanlength integrations each of duration integration. No slew time is inserted.

  • direction, pointingspacing, and relmargin specify where you want to observe, and how to space the pointings of the mosaic, since you'll probably do a mosaic.
  • mosaicsize is used to set the field of view and lay out mosaic pointings.

  • startfreq, chanwidth, nchan specify the frequency information for the output which will be interpolated from the input. There is no "continuum" mode in ALMA as there was on the old VLA, so you just specify nchan =1 if you want a continuum simulation.

Checking what you're doing (without waiting a long time)

  • checkinputs="yes" produces a graphical output with your scaled input model and the primary beams of your desired mosaic overlaid (and the elevation of the source as a function of time and coming in the future a spectral window with atmospheric transmission).
  • checkinputs="only" produces the same window, and then stops before running the time-consuming visibillity calculation and deconvolution. This is recommended if you are trying a new image size, or mosaic parameters, etc. Get it all right and then run the actual simulation.