Interpreting the pipeline results (part I)
Plots of visibility data
The pipeline produces a range of plots so that you can assess the quality of the raw data, and the effectiveness of the calibration. The plots are generated in the Editing stages before and after calibration, however they are not available to download until after the pipeline has finished its processing.
Each plot file has a name which uniquely describes it using the following scheme
freq-nchan-npol-id-[apst]-[c-][f-].type.ps where the first segment freq-nchan-npol-id describes the observation (frequency, number of channels and number of polarisations, followed by a unique identifier). The next segment [apst]-[c-][f-] consists of three letters or dashes with the following meanings
- [apst]: a code saying which sources are plotted; a = All, p = Primary, s = Secondaries, t = Targets
- [c-]: a code saying whether the data plotted is calibrated; c = calibrated, - = not calibrated.
- [f-]: a code saying whether the data plotted has been editing; f = flagged, - = not flagged.
Finally there is a two letter code which gives the type of plot in that file. This code can be one of
- uv: u-v Coverage plot
- at: Amplitude vs. Time plot
- pt: Phase vs. Time plot
- ad: Amplitude vs. u-v Distance plot
- ri: A plot of Real vs. Imaginary components
So for example the plot with the name 2378-14-4-1-s-f.pt.ps is a plot of Phase vs. Time (pt) for Secondary sources from the observation at 2378 MHz. It consists of flagged, but uncalibrated data (-f).
Each plot is described briefly below (click on the title to see an example plot).
- u-v Coverage: This plot shows the coverage of the u-v plane for the target sources. A long observation will show a more complete circle. The different colours represent each half of the plane.
- Amplitude vs. Time: This plot shows the amplitude of the data as it varies with time. The four colours are the XX, YY, XY and YX components (or Stokes IQUV parameters for calibrated data). A separate plot is produced for the primary calibrator, secondary calibrators and target sources.
- Phase vs. Time: This plot shows the phase of the data as it varies with time. The plot is produced for the secondary calibrators, after calibration, using the Stokes I component only. If calibration has been effective, the phase should be close to zero.
- Amplitude vs. u-v Distance: This plot shows how the amplitude of the data varies with u-v Distance. Like the Amplitude vs. Time plots it can help you identify bad data and judge the effectiveness of the editing stage. The four colours are the XX, YY, XY and YX components (or Stokes IQUV parameters for calibrated data). A separate plot is produced for the primary calibrator, secondary calibrators and target sources.
- Real vs. Imaginary: This plot shows the Real versus the Imaginary components of the visibilities for the primary calibrator, after calibration. The four colours are the Stokes IQUV parameters. If calibration has been effective, you should see 3 clumps of data (one for each of Q, U, V), clustered around (0,0) and a 4th clump at the intensity of the primary calibrator. In the case shown here, the arcs around the Stokes I clump (black) suggest the calibration has not been completely successful.
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