Flag dataΒΆ

[relevant workers: flagging]

The flagging worker can run on the input .MS files or on .MS files created by MeerKATHI at various stages of the pipeline (e.g., by the split_target worker). In the latter case the name of the .MS files to be flagged is based on that of the input .MS files, with a label added before the extension. Users can set the label with the flagging: label parameter in this worker.

The flagging worker allows users to flag the data in a variety of ways. Unless otherwise stated below, flagging is done with the CASA task FLAGDATA. Follow the links below for a detailed documentation of the individual flagging modes.

  • Flag on autocorrelations to catch antennas with obvious problems using the custom program POLITSIYAKAT (flagging: autoflag_autocorr_powerspectra). Individual scans are compared to the median of all scans per field and channel; and individual antennas are compared to the median of all antennas per scan, field and channel. Both methods have their own flagging threshold, which users can tune. Users can also set which column and which fields to flag.
  • Flag all autocorrelations (flagging: flag_autocorr).
  • Flag specific portions of the beginning and/or end of each scan (flagging: quack_flagging). As in the CASA task FLAGDATA, users can set the time interval that should be flagged and the quackmode.
  • Flag shadowed antennas (flagging: flag_shadow). Users can tune the amount of shadowing allowed before flagging an antenna. For observations obtained with a MeerKAT subarray it is possible to include offline antennas in the shadowing calculation.
  • Flag selected channel ranges (flagging: flag_spw).
  • Flag selected time ranges (flagging: flag_time).
  • Flag selected antennas (flagging: flag_antennas). Within this task, users can limit the flagging of selected antennas to a selected timerange.
  • Flag selected scans (flagging: flag_scan).
  • Flag according to a static mask of bad frequency ranges using the custom program RFIMASKER (flagging: static_mask). The mask file should be located in the input directory set by general: input. Users can decide to limit the flagging to a selected UV range. This could be useful to flag short baselines only.
  • Flag with AOFlagger (flagging: autoflag_rfi). The AOFlagger strategy file should be located in the input directory set by general: input. MeerKATHI comes with a number of strategy files, which are located in the meerkathi/data/meerkat_files directory and are copied to the input directory by the general worker. However, users can copy their own strategy file to the same input directory and use it within MeerKATHI. Additional parameters allow users to limit the execution of AOFlagger to selected columns, fields or frequency bands of the .MS files. AOFlagger is described by Offringa et al. (2012), A&A, 539, A95.

Finally, a summary of the flags can be obtained with flagging: flagging_summary. The summary is available at the relevant log file (see Data products).