Basic Information on closure

Task: closure
Purpose: Plot closure quantities
Categories: uv analysis, plotting

        CLOSURE is a Miriad task that plots closure quantities: the
        closure phase ("triple phase"), closure amplitude ("quad
        amplitude"), and two less well known quantities - the triple
        amplitude and quad phase.
 
        These are defined as
 
          Closure (triple) phase:    arg( V12*V23*conjg(V13) )
          Closure (quad) amplitude:  abs( (V12*V34)/(V14*conjg(V34)) )
          Triple amplitude:          abs( V12*V23*conjg(V13) )**0.3333
          Quad phase:                arg( (V12*V34)/(V14*conjg(V34)) )
 
        The closure phase, quad phase and closure amplitude should be
        independent of antenna-based errors, and for a point source
        should have values of zero phase or unit amplitude.  The triple
        amplitude is independent of antenna-based phase errors (but not
        amplitude errors), and for a point source is a measure of the
        flux density.
 
        The task works by averaging the quantites that are the argument
        of the abs or arg function.  These are always averaged over the
        selected frequency channels and over the parallel-hand
        polarizations. Optionally the averaging can also be done over
        time and over the different closure paths.
 
        CLOSURE also prints (and optionally plots) the theoretical error
        (as a result of thermal noise) of the quantities.  Note this
        assumes a point source model, and that the signal-to-noise ratio
        is appreciable (at least 10) in each averaged product.
 

Key: vis
        The input visibility datasets. Several datasets can be given.

Key: select
        Standard visibility selection. See help on select for more
        information.

Key: line
        Standard visibility linetype. See the help line for more
        information.

Key: stokes
        Normal Stokes/polaization selection. The default is to process
        all parallel-hand polarisations.

Key: device
        PGPLOT plotting device. The default is no plotting device (the
        program merely prints out some statistics).

Key: nxy
        The number of plots per page in x and y. The default depends on
        the number of plots.

Key: yrange
        The y range of the plots. The default is to autoscale.

Key: interval
        Time averaging interval. The default is no time averaging of the
        triple correlations.

Key: options
        Task enrichment parameters.  Several parameters can be given,
        separated by commas.  Minimum match is supported. Possible
        options are:
          amplitude Plot the amplitude quantity (the default is to plot
                    phase).
          quad      Plot the quad quantity (the default is to plot the
                    triple quantity).
          avall     Average all quantities from different triangles
                    together. Note the theoretical error estimates are
                    incorrect when using options=avall.
          notriple  Plot data from all quantities on a single plot.
          rms       Plot theoretical error bars on the points. The error
                    bars are +/- sigma.
          log       Make a log file closure_log.txt.
        The following give control over calibration to be applied to the
        visibility data before the triple correlations are formed. Note
        that applying phase calibrations does not affect the closure
        phase!
          nocal     Do not perform gain calibration.
          nopol     Do not perform polarisation calibration on the data.
          nopass    Do not perform bandpass calibration on the data.
 
Revision: 1.8, 2013/08/30 01:49:21 UTC