Processing Chunks of Visibilities

The pipeline provides users the option to use chunks of the visibilities by selecting a specified frequency range, splitting the full range into smaller frequency chunks, and applying the static RFI mask. Any combination of these are allowed. The order of operation in the pipeline is as follows:

• Select visibilities for some frequency range with the --start_freq and --end_freq command line arguments in MHz. Selecting visibilities by channel numbers from the command line is not supported, and we do not intend to provide it unless many users of this pipeline have the need for it.

• Split the visibilities into frequency chunks with the --num_chunks command line argument. Note that the first and last chunk in the band are always discarded. Therefore, the pipeline does not allow the use of --num_chunks less or equal 2. Currently we do not provide the option to override discarding the first and last chunk in the band.

• The static RFI mask is then applied within each frequency chunk. If all channels in a frequency chunk are flagged, the chunk is discarded and not used further in the calibration of the pointing offsets.

Frequency Range Selection

The user is allowed to select visibilities by specifying the frequency range in MHz, but the Measurement Set reader used in the pipeline only has access to channel numbers. It infers the frequency of each visibility based on the channel numbers. Therefore, we first determine the channel numbers corresponding to the user supplied frequencies before passing this information to the Measurement Set reader which extracts the visibilities in the selected channels. The pipeline does not support selecting visibilities based on time ranges, and we do not intend to add such functionality, unless many users have a need for it.

Splitting into Frequency Chunks

The visibilities from a frequency selection or full band can be split into multiple frequency chunks. As described above, the first and last chunks in the band are discarded so --num_chunks, if given, should be larger than 2. Otherwise, any number of chunks is allowed if and only if the available frequencies can be divided (either equally or unequally) by the requested number of chunks. For example, a request to divide a 32-channel dataset into 64 chunks will results in an error since it is impossible to do so. Hence the maximum number of chunks in this case would be 32 and the minimum is 1.

Removing Bad Frequency Channels

A detailed step-by-step procedure showing how to apply a static RFI mask to the visibilities is described in the Applying static RFI mask section. The mask can be applied to the full-band, or to chunks of visibilities. When applied to the latter, if all channels in a frequency chunk are flagged, that chunk is discarded. The remaining chunks are then used to determine the pointing offsets of interest.