Interactive Flagging - BLFLAG

The task blflag plots visibility data (usually baseline at a time, with axes such as amplitude, phase, real part, imaginary part, time, etc), and allows the user to interactively flag that data.

The user positions the plot cursor, and uses the mouse buttons or single keyboard characters to perform an operation. The interactive commands are as follows:

Left-Button:

Pressing the left mouse button flags the nearest visibility.

Right-Button:

Pressing the right mouse button causes blflag to precede to the next baseline (or to finish up, if all baselines have been cycled through).

Carriage-return:

This gives a brief help message.

a:

Flag nearest visibility (same as left mouse button).

c:

Clear the flagging for this baseline, and redraw plot.

h:

Give help (same as carriage return).

p:

Define a polygonal region, and flag visibilities within this region. You define the vertices of the polygon by moving the cursor and then hitting the left mouse button (or a). You finish defining the polygon by hitting the right mouse button (or x). You can delete vertices with the middle mouse button (or d).

q:

Abort completely. This exist without apply the flagging.

r:

Redraw plot.

x:

Move to next baseline (same as right mouse button).

The inputs to blflag are straightforward.

• vis: The name of the visibility data-set to be flagged.
• device: The PGPLOT device to use for plotting. This needs to be an interactive device.
• line: This gives the normal linetype parameter (see Section 5.4). blflag averages together all the selected channels before plotting it. The default is to use all channels. If you flag a data point on a plot, you flag all selected channels.
• stokes: Polarization/Stokes selection - see Section 5.8. If several are given, these are averaged together before plotting. So normally you only select one. The default is `ii'. If you flag a point, you flag all polarizations (even if you did not select that polarization).
• axis: This gives two values, being the X and Y axes of the plot. Possible values are time, lst, uvdistance ( $\sqrt{u^2+v^2}$), hangle (hour angle), amplitude, phase, real and imaginary. The default is time vs amplitude.
• options: The options keyword give extra processing options. These include:

  nobase

  Normally blflag plots each baseline separately, and cycles over all baselines. The nobase option allows all baselines to be displayed simultaneously. This is particularly useful for point sources or when the visibilities are purely noise-like.

  noapply

  This causes the flagging operations not to be actually applied to the dataset.

  selgen

  This option generates a text file, blflag.select. This file contains visibility data selection commands to select the flagged data.

The inputs are normally very simple:

BLFLAG
vis=vela.line	Specify visibility dataset.
device=/xs	Specify an interactive PGPLOT device.
stokes	Defaults to `ii'.
line	Defaults to all channels.
axis	Defaults to time vs amplitude.
select	Defaults to selecting all data.

For a point source (e.g. calibrator), a particularly useful way of flagging, at least after initial calibration, is to plot the ``scatter diagram'' - real vs imaginary parts of the visibility, with all baselines simultaneously. This should show points scattered around the true flux of the source. An arc in such a plot indicates poor phase stability. Typical inputs in this mode are

BLFLAG
vis=cal.uv	Specify visibility dataset.
device=/xs	Specify an interactive PGPLOT device.
stokes	Defaults to `ii'.
line	Defaults to all channels.
axis=real,imag	Plot real vs imaginary.
opptions=nobase	Plot all baselines simultaneously.