Interactive Channel/Baseline Flagging -- TVFLAG

Before using tvflag , you will need to start up the TV server, xmtv. Although this is not essential, you will probably also want to use the control panel server, xpanel. To start these, you should give the commands

     % xmtv &
     % xpanel &

The input parameters to tvflag are pretty straightforward. We mention only the more useful ones:

• vis: The name of the visibility data-set to be flagged.
• server: The TV server parameter (see Chapter 3).
• select: The usual visibility data selection parameter. One of tvflag 's odd characteristics is in its handling of different polarisations. Task tvflag can deal with only a single polarisation per run. If it finds that the input data-set contains several polarisations, it will average these together when forming its display. While unusual, this is moderately useful. Assuming the data-set contains all four linear polarisations,

         select=pol(xx,yy)
  

  will produce a Stokes-I display. On the other hand,

         select=pol(xy,yx)
  

  will produce a quantity which is not physically meaningful, but will still allow you to make a quick check to interference and outliers (interference is often much more apparent in the cross-hand polarisations than the parallel-hand ones).
• line: The normal visibility linetype parameter. Task tvflag

  does not support spectral averaging.

• mode: This determines the qunaity to be displayed. Possibilities are amplitude, phase, real and imaginary.
• taver: This gives a time interval used for the running mean calculation when displaying in ``DIFF'' mode (see below). This parameter takes two values, both in minutes. The first gives the maximum amount of time in an averaging interval, whereas the second gives the time gap between records which causes an averaging interval to be ended. The default is 5 minutes for both of these.

Typical inputs are:

After invoking tvflag , a control panel will pop up (assuming you are running the control panel server) and a baseline of data will be displayed on the TV. The display shows channels along the x-direction and time along the y-direction. A gap in time in the data is shown by a dark gap. A wedge is displayed both above and to the right of the data. The top wedge is the data averaged over time, whereas the wedge to the right is the data averaged over the channels.

You now perform flagging operations by pressing ``buttons'' on the control panel. We review the meaning of some of the buttons:

• SELECT, SLCT CHAN and SLCT TIME: The first thing that you will normally do is to select some set of data, with one of the three ``select'' buttons. SELECT selects a rectangular region on the display (i.e. some range of times and channels), whereas SLCT CHAN selects all times for a particular range of channels, and SLCT TIME selects all channels for a particular range of times. After pressing the appropriate ``select'' button,
  • move the mouse so that the cursor is at one corner of the region to be selected,
  • depress the left mouse button,
  • drag the cursor to the other corner of the region to be selected,
  • release the mouse button.
• FLAGOOD and FLAGBAD: Having selected some set of data with the previous commands, this data can be either flagged as good or bad with the FLAGOOD and FLAGBAD buttons respectively.
• UNDO: The undo button causes the last flagging command to be undone and discarded.
• RESCALE: The rescale button changes the display scaling, so that the data occupies the full greyscale or colour range. If you have some bad data which has an extremely high amplitude, for example, you will want to flag this and then rescale the display range so that you can see the detail in the remaining data.
• DIFF: The ``diff'' button causes a running mean to be subtracted off the data.
• EXIT, QUIT and ABORT: The ``exit'' button causes the next baseline to be loaded. The ``quit'' button also causes the next baseline to be loaded, but discards any flagging commands performed on the current baseline. Normally tvflag will terminate when it has gone through all the baselines. It is only at this stage that it applies the flagging operations to the actual data-set. Althernately the ``abort'' button causes tvflag to terminate at any time, and not to apply any of the flagging operations to the data-set.
• LIST: This button lists the flagging operations that have been performed to the current baseline.
• VALUE: This button allows you to find the value of pixels on the display. It is somewhat cumbersome -- after pressing the ``value'' button, you move the mouse cursor to the pixel of interest, and press the [F3] function key. You continue to move the cursor and press [F3] until you have exhuasted the pixels of interest. Then press [F6] to return back to tvflag 's normal mode.

If you are not running the control panel server, you will be prompted for flagging operations at the command line. In this command-line mode, the flagging commands fairly similar names -- use the ``help'' command to get more information.

One shortcoming of tvflag is that it averages the data in time so that it will fit on the display. Unfortunately full time resolution display cannot be recovered. To display the data at full time resolution, you may need to select the data in chunks (with select=time()).