Interactive Baseline-Based Flagging

We now flag the bad integrations indentified in uvplt or smauvplt. The task blflag is a baseline-based interactive flagger. For non-polarization data, smablflag in which the sources are coded with colors can be used. This task plots the visibilities (e.g. time vs. amplitude) either a baseline at a time or all at once, and allows users to flag discrepant points using the plotting cursor. The task blflag / smablflag appears to be very useful in identifying various bad correlations which hide in the spectra. For example, the bad correlations due to poor telescope pointing in the beginning integration scans can be easily picked up in the visibility plot with color-coded data points. The setup of the input parameters is given below:

smablflag% inp
 Task:   smablflag
  vis      = gc_rx1.lsb.tsys
  line     =
  device   = /xs
  stokes   = xx
  select   =
  axis     = time,ampl
  options  =

Figure: A visibility plot (time vs. amplitude) on baseline 1-3 for the five sources observed. The bad correlations in the beginning scans are obvious.

The defaults in line and select lead to average all channels in all the spectral windows and plot the visibilities (time vs. amplitude) on each baseline. Fig.2.9. shows a visibility plot (time vs. amplitude) on baseline 1-3 for the five sources observed in the program. The bad correlations in the beginning integration during repointing the array from one source to another are obvious. For example, the relatively higher amplitude in SgrA* and relatively lower amplitude in SgrB2 are clearly bad correlations. Users can flag bad data by moving the plotting cursor to the problematic visibility points and clicking the ''left button'' of the computer mouse. After marking all the bad visibilities (the red points) on one baseline (see the bottom panel of Fig.2.9), users can move to the next baseline by clicking the ''right button'' of the mouse. Users can go through all the baselines to flag bad correlations. The task smablflag only changes the flag status for the bad correlations (red points; see Fig.2.10) and no data points have been removed from the uv file in the flagging process. Any mistakes in the flagging process can be corrected and flagged data can be recovered.

Figure: The bad correlations in the beginning scans are flagged (red points).