4. Exporting visibilities

uvplot.io.export_uvtable(uvtable_filename, tb, vis='', split_args=None, split=None, channel='all', dualpol=True, fmt='%10.6e', datacolumn='CORRECTED_DATA', keep_tmp_ms=False, verbose=True)

Export visibilities from an MS Table to a uvtable. Requires execution inside CASA.

Currently the only uvtable format supported is ASCII.

Typicall call signature:

export_uvtable('uvtable_new.txt', tb, channel='all', split=split,
               split_args={'vis': 'sample.ms', 'datacolumn': 'DATA', 'spw':'0,1'},
               verbose=True)
Parameters

  • uvtable_filename (str) – Filename of the output uvtable, e.g. “uvtable.txt”

  • tb (CASA tb object) – As tb parameter you must pass the tb object that is defined in the CASA shell. Since tb cannot be accessed outside CASA, export_uvtable(‘uvtable.txt’, tb, …) can be executed only inside CASA.

  • vis (str, optional) – MS Table filename, e.g. mstable.ms

  • split_args (dict, optional) – Default is None. If provided, perform a split before exporting the uvtable. The split_args dictionary is passed to the CASA::split task. The CASA::split task must be provided in input as split.

  • split (optional) – CASA split task

  • channel (str, optional) – If ‘all’, all channels are exported; if ‘first’ only the first channel of each spectral window (spw) is exported. Number of channels in each spw must be equal, otherwise you will get a CASA error, e.g.: RuntimeError: ArrayColumn::getColumn cannot be done for column DATA; the array shapes vary: Table array conformance error Default is ‘all’.

  • dualpol (bool, optional) – If the MS Table contains dual polarisation data. Default is True.

  • fmt (str, optional) – Format of the output ASCII uvtable.

  • datacolumn (str, optional) – Data column to be extracted, e.g. “DATA”, “CORRECTED_DATA”, “MODEL_DATA”.

  • keep_tmp_ms (bool, optional) – If True, keeps the temporary outputvis created by the split command.

  • verbose (bool, optional) – If True, print informative messages. Default: True

Note

By default, all the spws and all the channels of the vis MS table are exported. To export only the first channel in each spw, set channel=’first’.

To export only some spws provide split_args, e.g.:

split_args = {'vis': 'input.ms', 'outputvis': 'input_tmp.ms', spw: '1,2'}

Flagged data can be removed from the .ms by providing split and the split_args dictionary, e.g.:

split_args = {'vis': 'input.ms', 'outputvis': 'input_tmp.ms', 'keepflags': False}

If you try exporting visibilites from an MS table with spws with different number of channels, you will get a CASA error (see requirement for the channel parameter above). To avoid that, you can either use the CASA split task to create a new MS table with only spws with same number of channels. Alternatively (or additionally) you can channel average all the spws to the same number of channels.

Example

From within CASA, to extract all the visibilities from an MS table:

export_uvtable('uvtable.txt', tb, vis='sample.ms', channel='all')

where tb is the CASA tb object (to inspect it type tb in the CASA shell). For more information on tb see https://casa.nrao.edu/docs/CasaRef/table-Module.html

From within CASA, to extract the visibilities in spectral windows 0 and 2 use the split_args parameter and the CASA split task:

export_uvtable('uvtable.txt', tb, channel='all', split=split,
 split_args={'vis': 'sample.ms' , 'datacolumn': 'DATA', 'spw':'0,2'})

To perform these operations without running CASA interactively:

casa --nologger --nogui -c "from uvplot import export_uvtable; export_uvtable(...)"

ensuring that the strings are inside the ‘..’ characters and not the “…” one.