Plotting Visibilities

  1. You may tire of all this listing of numbers, and would prefer to make some plots. The task uvplt provides a variety of choices when plotting visibilities, and ancillary information associated with the visibilities. It has rather a lot of inputs, so read the help file first for the details. We discuss some of the keywords below and then give a couple of examples.

    The following example plots u versus v, and -u versus -v, selecting channels at the start, middle and end of a band of 32 channels (so that the u-v coverage benefit obtained across the band can be seen).

    UVPLT
    vis=vela*.uv Specify visibility dataset(s)
    line=channel,3,2,1,15 Plot channels 2,17,32
    select Leave unset for all data
    stokes=xx u-v coverage same for all polarizations
    axis=uc,vc Plot u-v and conjugate u-v plane
    xrange Auto-scale
    yrange Auto-scale
    average No averaging
    hann No Hanning
    inc=1 Plot every point
    options=nobase All baselines on one plot
    device=/xs Plot on local X-window
    nxy=1 One sub-plot only please
    size Default character sizes

    The next example shows how to plot a scatter diagram (real vs imaginary) from a single-channel dataset for the Q, U and V. polarizations, with all baselines plotted on the one plot. This is a quite useful plot to make of calibrated calibrator data. Because you are asking for Stokes parameters, it is assumed that a calibration has been made (the calibration tables will be applied by uvplt and a reminder issued) or that you converted to Stokes parameters along the way (e.g. with the AIPS task ATLOD or with uvaver).

    UVPLT
    vis=vela.uv Specify visibility dataset
    line Plot the first channel only
    select Leave unset for all data
    stokes=q,u,v Select Stokes Q, U and V
    axis=real,imag Is the default
    xrange Auto-scale
    yrange Auto-scale
    hann No Hanning
    inc Plot all points
    options=nobase Scalar averaging
    device=/xs Plot on local X-window
    nxy=1 1 plot per page
    size Default character sizes

  2. Task closure is another useful plotting task. It is most useful for plotting the closure phases of point sources. The closure phase of an object is the sum of the baseline phases around a triangle. For example, for antennas 1, 2 and 3 we measure three baseline phases $\phi_{12}$, $\phi_{23}$ and $\phi_{31}$. It can be shown that the closure phase, which is the sum of these three baseline phases

    \begin{displaymath}
\phi_{\rm closure} = \phi_{12} + \phi_{23} + \phi_{31}
\end{displaymath}

    will be independent of any antenna-based gain errors (atmosphere and instrumental). This quantity will not be affected by Miriad's antenna calibration process. Additionally for a point source (or any source with 180 degree rotational symmetry) the closure phase should be zero. Plotting the closure phase of a calibrator is thus a way of checking the quality of the data. If the closure phase is large, the data are probably bad, or there is some calibration error that is not accounted for in Miriad's antenna-based model.

    Task closure plots averages of closure phases (actually it averages together triple products and then takes the phase of this). The averages are taken over the the different correlator channels and polarizations, and optionally over time.

    The inputs to closure are simple enough. See the help file for more information. Typical inputs to closure are:

    CLOSURE
    vis=vela*.uv Specify visibility dataset(s)
    line= Set the channels of interest.
    select Leave unset for all data
    stokes=xx Closure phase for XX
    yrange Auto-scale
    options=notrip Plot all closure phases on one plot
    device=/xs Plot on local X-window
    nxy=1 One sub-plot only please

  3. If you are working with spectral data (line or continuum) you may want to look at some spectra. Use task uvspec for this. The inputs to uvspec are pretty much self-explanatory. Here is an example of how to plot the spectra (phase) of the XX and YY correlations averaged over 15 minutes for all baselines involving antenna 1 with all calibration turned off. uvspec makes a new sub-plot every time it begins a new averaging interval. Assuming that there are five antennas in this dataset, we have arranged for each baseline to occupy one column of the plot page with time increasing down the page.

    UVSPEC
    vis=vela.line Specify visibility dataset
    select=ant(1) Baselines involving antenna 1
    line Unset for all channels
    stokes=xx,yy XX and YY correlations
    interval=15 15 minute averages
    options=nocal,nopol,nopass Turn off calibration,
    axis=freq,phase Plot phase versus frequency
    yrange=-185,185 Specify y-range for plot
    device=/xs Plot on local X-window
    nxy=5,6 6 time intervals in y directions

Miriad manager
2016-06-21