Reweighing visibilities based on their scatter (statwgt)

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4.7.8 Reweighing visibilities based on their scatter (statwgt)

Alert: statwgt is still an experimental task. Please check the results carefully and report any problems to the NRAO CASA helpdesk.

I most cases, the data that comes from the telescopes have the correct absolute or relative weights associated (absolute weights will be supplied once the JVLA switched power application becomes standard; for ALMA the Tsys application is already in place). However, there are data sets where one would like to adjust the weights based on the scatter of the visibilities (typically as a function of time, antenna, and/or baseline). This calculation is performed by the task statwt that updates the WEIGHT and SIGMA columns of the measurement set. statwt inputs are:

#  statwt :: Reweight visibilities according to their scatter
vis                 =         ’’        #  Name of measurement set
dorms               =      False        #  Use rms instead of stddev?
byantenna           =      False        #  Estimate the noise per antenna -not
                                        #   implemented (vs. per baseline)
fitspw              =         ’’        #  The signal-free spectral window:channels
                                        #   to estimate the scatter from
fitcorr             =         ’’        #  The signal-free correlation(s) to estimate
                                        #   the scatter from (not implemented)
combine             =         ’’        #  Let estimates span changes in spw, corr,
                                        #   scan and/or state
timebin             =       ’0s’        #  Bin length for estimates (not implemented)
minsamp             =          2        #  Minimum number of unflagged visibilities
                                        #   for estimating the scatter
field               =         ’’        #  Select field using ID(s) or name(s)
spw                 =         ’’        #  Select spectral window/channels
antenna             =         ’’        #  Select data based on antenna/baseline
timerange           =         ’’        #  Select data by time range
scan                =         ’’        #  Select data by scan numbers
intent              =         ’’        #  Select data by scan intents
array               =         ’’        #  Select (sub)array(s) by array ID number
correlation         =         ’’        #  Select correlations to reweight
observation         =         ’’        #  Select by observation ID(s)
datacolumn          = ’corrected’       #  Which data column to calculate the scatter
                                        #   from
async               =      False        #  If true the taskname must be started using
                                        #   statwt(...)

statwt should only be run after all calibration steps have been performed. The parameter dorms switches from a scatter standard deviation to a root mean square scatter estimator. datacolumn specifies the column on which the task operates and the usual data selection parameters apply. Channels with strong RFI or a spectral line should be avoided for the calculation and good channel range should be specified via fitspw. In its current implementation, statwgt uses data samples of an integration time interval but eventually wider sample intervals can be specified by the timebin parameter. Those samples are contained within a scan, spw, and polarization product but using the combine can relax this restriction. minsamp sets the minimum number of unflagged visibilities used for the calculation.

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