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Basic Information on uvflux
Purpose: Determine some statistics about visibilities.
UvFlux is a MIRIAD task which gives a mean visibility value.
The visibilities are averaged together regardless of time, (u,v)
coordinate, frequency or baseline. Distinct polarisations
and sources, however, are averaged separately. This task will be
most useful when examining the fluxes of point sources.
Both vector and scalar averages are formed. It also prints out
the rms scatter around the vector mean, and the RMS variation
in the amplitude.
Definitions in Detail: UvFlux prints out six quantities.
Theoretical RMS -- This is the rms error, resulting from
receiver thermal noise, that would be expected in the real or
imaginary part of each visibility.
Vector Average -- This is simply the normal average (mean)
of the real and imaginary parts of the visibility data. For a
point source, the real part should give the point source flux
density, and the imaginary part should be noise.
RMS Scatter -- This is the RMS scatter of real and imaginary
parts of the actual visibilities around their mean value. If the
data do represent a point source, and it is well calibrated, this
should be the same number as the ``Theoretical RMS''.
Average Amplitude -- This gives the mean value of the visibilty
amplitude (the so-call amplitude scalar average). If the S/N
ratio is much less than 1, this average amplitude will be dominated
by thermal noise bias, and should be 1.25 times larger than the
``Theoretical RMS''. If the S/N ratio is much greater than 1, then
this should be a good estimate of the point-source's flux density.
This will be unaffected by phase calibration errors (unlike the
``Vector Average'', which decorrelates).
RMS Amp Scatter -- This gives the RMS scsatter of the visibility
amplitudes around the mean visibility amplitude. If the S/N ratio
is much less than 1, then this should be 0.65 times the ``Theoretical
RMS'. If the S/N ratio is much greater than 1, this should be the
same as ``Theoretical RMS''.
Number Corrs -- This gives the number of correlations, N, used in
forming each mean. NOTE that the three RMS values printed are RMS
difference between the correlations and some mean -- it is not
an error-of-the-mean. To convert an RMS to an error-in-the-mean,
divide by sqrt(N).
The name of the input visibility data-set. Several files can be
given, wildcarding is supported. No default.
Standard uv selection. The default is all data.
Standard line-type specification. When there are multiple channels
selected, uvflux averages them all together.
Normal Stokes processing. You can select several Stokes or
polarisation parameters, which will be be averaged independently.
An offset (arcsec) to shift the uv data. The sign convention is the
same as INVERT, MAXFIT, UVFIT, etc. The default is 0,0 (no shift).
Extra processing options. Several can be given, separated by
commas. Minimum match is used. Possible values are:
nocal Do not apply any antenna gain calibration corrections.
By default these are applied if they exist.
nopol Do not apply polarisation leakage corrections. By default
these are applied if they exist.
nopass Do not apply bandpass corrections. By default these
are applied if they exist.
uvpol Print out fractional linear polarisation and
polarisation position angle (provided Stokes I,Q,U are
vlbi Average amplitude of vector average for each integration.
data are vector averaged over channel and baseline,
and amplitude averaged for each integration interval.
long Write long lines (>80 characters wide) with results in
more significant figures.
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