Plot Field

Plot Field

Below is an introduction to the plot_field.py script which should be run at the beginning of the LOFAR-VLBI pipeline. The script downloads the necessary catalogues needed to continue with the pipeline, plots relevant information regarding the field and provides some summary information in the command line. It also has some optional arguments which can provide additional diagnostic information regarding the field and potential delay calibrators.

Running the script

The script requires a direction in which to query various catalogues. This can either provided by the --RA and --DEC command line arguments or by providing a measurement set with a phase center of your field of interest via the --MS argument.

I will present various options using the pointing P038+36 from the LoTSS DR2 release.

The minimal version of the script to run would be:

python3 plot_field.py --RA 38.599 --DEC 36.276

This will download the LBCS catalogue within 1.5 degrees (lbcs_catalogue.csv), the LoTSS catalogue within 1.5 degrees (lotss_catalogue.csv) and any sources greater than 5Jy within 10 degrees (extreme_catalogue.csv).

After this, it cross-matches LBCS and LoTSS information to produce the delay_calibrators.csv file and filters all LoTSS sources within 1.24 degrees and above 10mJy for the image_catalogue.csv.

All of the previous radius limits and flux limits can be changed via command line options specific to your need. The defaults are chosen to align with the goals of the high-resolution reprocessing of LoTSS.

The script will print some summary information to the command line. An example is shown below.

There are 10 delay calibrators within 1.24 degrees of the pointing centre
There are 153.0 sources above 10.0 mJy within 1.24 degrees of the phase centre.
Assumed averaging - nchannels: 16; time averaging: 1.0

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Standard Plot

With the creation of these catalogues, the script continues by plotting a summary figure displaying information about the field, delay calibrators, bright sources and the direction of extreme sources outside the field that may affect results.

The plot also indicates the distance between the pointing centre and the closest potential delay calibrator. The shaded regions indicate the loss of flux with radius based on standard frequency and time averaging used in the pipeline.

If you are interested in seeing information regarding a different delay calibrator, you can simply place the calibrator of interest as the first entry in delay_calibrators.csv and re-run the script. It will ask you whether you intend to re-download the catalogues, if you enter n the plot should update with your change.

Adding a Target

Depending on your science case, you may either be interested in several sources within a field or just one in particular. In the latter case, there is an option to view the relation between your target, pointing centre and the delay calibrator within the image.

For example, if one runs the following:

python3 plot_field.py --RA 38.599 --DEC 36.276 --targRA 39.2 --targDEC 36.9

Then the plot will update with the target direction

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VLASS Images

Using a script developed by R. Timmerman, you can now also optionally download VLASS fits cutouts and convert to png for inspection. This may be helpful in identifying suitable calibrators as typically the brightest but also compact sources are best for calibration.

This can be done via:

python3 plot_field.py --RA 38.599 --DEC 36.276 --vlass

In this case, cutouts will be downloaded for all 8 calibrators in the delay_calibrators.csv file. The quick look image for the primary delay calibrator L449628 is shown below

The second closest calibrator the pointing centre is shown below. This source is far brighter which may make it appealing but it does show some structure in VLASS. If we select it instead, having this VLASS image can verify that our LOFAR calibration is proceeding well as we expect to observe a similar structure.