Virtual Model Visibilities

The tasks that generate model visibilities (clean, tclean, ft, and setjy) can either (in most cases) save the data in a MODEL_DATA column inside of the MeasurementSet (MS) or it can save it in a virtual one. In the latter case the model visibilities are generated on demand when it is requested and the data necessary to generate that is stored (usually the Fourier transform of the model images or a component list). More detailed descriptions of the structure of an MS can be found on the CASA Fundamentals pages. 

The tasks that can read and make use of the virtual model columns include calibration tasks, mstransform tasks (including uvsubtraction), plotms.

Advantages of virtual model column over the real one:

  • Speed of serving visibilities (in most cases because calculating models visibilities is faster than disk IO)
  • Disk space saving (a full size of the original data size is saved)

When not to use virtual model

  • model image size is a significant fraction of the visibility data size (e.g large cube from a small data set). Virtual model column serving might be slower than real one
  • when the user wants to edit the model physically via the table tool for e.g
  • when using an FTMachine that does not support virtual model saving when imaging (AWProjectFT for e.g)

Additional Information

  • When both a physical model column exists along with a virtual model, then the virtual model is the one that gets served by tasks that uses the visbuffer (e.g calibration tasks)
  • Use delmod task to manage your MODEL_DATA column and virtual model
  • If  model data is written for a subset of the MS (say the user used field , spw and/or intent selection in tclean) then the model visibilities will be served properly for the subset in question the other part of the MS will have 1 served for parallel hand visibilities and 0 for crosshand visibilities. So be careful when doing calibration or uvsub after writing model visibilities only for a subset of the MS (this applies to using the physical scratch column MODEL_DATA too)
  • The virtual model info is written in the SOURCE table of the MS usually (and in the main table if the SOURCE table does not exist)
  • FTMachines (or imaging gridding mode) supporting virtual model data are:
    • GridFT: Standard gridder (including mutiterm and multi fields or cube),
    • WProjectFT: widefield wterm  (including mutiterm and multi fields or cube),
    • MosaicFT: mosaic imaging (including mutiterm or cube),
    • ComponentLists