Generate a gain table based on Water Vapor Radiometer data wvrgcal

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4.3.7 Generate a gain table based on Water Vapor Radiometer data wvrgcal

#  wvrgcal :: Generate a gain table based on Water Vapour Radiometer data
vis                 =         ’’        #  Name of input visibility file
caltable            =         ’’        #  Name of output gain calibration table
toffset             =         -1        #  Time offset (sec) between interferometric and WVR data
segsource           =       True        #  Do a new coefficient calculation for each source
     tie            =         []        #  Prioritise tieing the phase of these sources
                                        #   as well as possible
     sourceflag     =         []        #  Flag the WVR data for these source(s) as bad
                                        #   and do not produce corrections for it
disperse            =      False        #  Apply correction for dispersion
wvrflag             =       [’’]        #  Flag the WVR data for these antenna(s) as bad
                                        #   and replace its data with interpolated values
statfield           =         ’’        #  Compute the statistics (Phase RMS, Disc) on
                                        #   this field only
statsource          =         ’’        #  Compute the statistics (Phase RMS, Disc) on
                                        #   this source only
smooth              =          1        #  Smooth WVR data by this many samples before
                                        #   applying the correction
scale               =        1.0        #  Scale the entire phase correction by this factor
reversespw          =         ’’        #  Reverse the sign of the correction for the
                                        #   listed SPWs (only needed for early ALMA data
                                        #   before Cycle 0)
cont                =      False        #  Estimate the continuum (e.g., due to clouds)
                                        #   (experimental)
async               =      False        #  If true the taskname must be started using
                                        #   wvrgcal(...)

The task wvrgcal generates a gain table based on Water Vapor Radiometer (WVR) data and is used for ALMA data reduction. It is an interface to the executable “wvrgcal” which is part of the CASA 3.4 distribution and can also be called from outside CASA. The wvrgcal software is based on the libair and libbnmin libraries which were developed by Bojan Nikolic at the University of Cambridge as part of EU FP6 ALMA Enhancement program.

CASA 3.4 contains version 1.1.1 of wvrgcal. Source code of the stand-alone package and links to documentation can be found at http://www.mrao.cam.ac.uk/\~bn204/alma/wvrsoft.html. In particular, there are two ALMA memos which describe the algorithms implemented in the software. They can be found at http://www.mrao.cam.ac.uk/\~bn204/publications/2009/ALMAMemo587.pdf and http://www.mrao.cam.ac.uk/\~bn204/publications/2009/ALMAMemo588.pdf.

Briefly, wvrgcal follows a Bayesian approach to calculate the coefficients that convert the outputs of the ALMA 183 GHz water-vapor radiometers (mounted on each antenna) into estimates of path fluctuations which can then be used to correct the observed interferometric visibilities.

The CASA task interface to wvrgcal follows closely the interface of the shell executable at the same time staying within the CASA task parameter conventions.

In ALMA data, the WVR measurements belonging to a given observation are contained in the ASDM for that observation. After conversion to an MS using importasdm, the WVR information can be found in separate spectral windows (as of May 2012, it is still spectral window id 0). This spectral window must be present in the MS for wvrgcal to work.

The various features of wvrgcal are then controlled by a number of task parameters (see the list above). They have default values which will work for ALMA data. An example for a typical wvrgcal call can be found in the ALMA CASA guide for the NGC 3256 analysis:

wvrgcal(vis=’uid___A002_X1d54a1_X5.ms’, caltable=’cal-wvr-uid___A002_X1d54a1_X5.W’,
toffset=-1, segsource=True, tie=["Titan,1037-295,NGC3256"], statsource="1037-295")

Here, vis is the name of input visibility file (which as mentioned above also contains the WVR data in spectral window 0) and caltable is the name of the output gain calibration table.

toffset is the known time offset in seconds between the WVR measurements and the visibility integrations they are valid for. For ALMA, this offset is presently -1 s (which is also the default value).

The parameter segsource (segregate source) controls whether separate coefficients are calculated for each source. The default value True is the recommended one for ALMA. When segsource is True, the subparameter tie is available. It permits to form groups of sources for which common coefficients are calculated as well as possible.

Finally, the parameter statsource controls for which sources statistics are calculated and displayed in the logger. This has no influence on the generated calibration table. See the online help of wvrgcal for a brief explanation of the other parameters.

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