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Basic Information on closure
Task: closure
Categories: uv analysis, plotting
CLOSURE is a Miriad task which plots some closure quantities.
The these include the closure phase (the "triple phase"), the
closure amplitude (the "quad amplitude"), and two less well known
quantities -- the triple amplitude and quad phase.
These are gefined as
Closure phase (triple phase): arg( V12*V23*conjg(V13) )
Closure amplitude (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.
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.