Complex Gain Calibration (gaincal)

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4.4.3 Complex Gain Calibration (gaincal)

The fundamental calibration to be done on your interferometer data is to calibrate the antenna-based gains as a function of time in the various frequency channels and polarizations. Some of these calibrations are known beforehand (“a priori”) and others must be determined from observations of calibrators, or from observations of the target itself (“self-calibration”).

It is best to have removed a (slowly-varying) “bandpass” from the frequency channels by solving for the bandpass (see above). Thus, the bandpass calibration table would be input to gaincal via the gaintable parameter (see below).

The gaincal task has the following inputs:

  gaincal :: Determine temporal gains from calibrator observations
vis                 =         ’’        #  Name of input visibility file
caltable            =         ’’        #  Name of output gain calibration table
field               =         ’’        #  Select field using field id(s) or field name(s)
spw                 =         ’’        #  Select spectral window/channels
intent              =         ’’        #  Select observing intent
selectdata          =      False        #  Other data selection parameters
solint              =      ’inf’        #  Solution interval: egs. ’inf’, ’60s’ (see help)
combine             =     ’scan’        #  Data axes which to combine for solve (scan, spw,
                                        #   and/or field)
preavg              =       -1.0        #  Pre-averaging interval (sec) (rarely needed)
refant              =         ’’        #  Reference antenna name(s)
minblperant         =          4        #  Minimum baselines _per antenna_ required for solve
minsnr              =        3.0        #  Reject solutions below this SNR
solnorm             =      False        #  Normalize average solution amplitudes to 1.0 (G, T
                                        #   only)
gaintype            =        ’G’        #  Type of gain solution (G,T,GSPLINE,K,KCROSS)
smodel              =         []        #  Point source Stokes parameters for source model.
calmode             =       ’ap’        #  Type of solution: (’ap’, ’p’, ’a’)
append              =      False        #  Append solutions to the (existing) table
gaintable           =       [’’]        #  Gain calibration table(s) to apply on the fly
gainfield           =       [’’]        #  Select a subset of calibrators from gaintable(s)
interp              =       [’’]        #  Temporal interpolation for each gaintable (=linear)
spwmap              =         []        #  Spectral windows combinations to form for
                                        #   gaintables(s)
gaincurve           =      False        #  Apply internal VLA antenna gain curve correction
opacity             =         []        #  Opacity correction to apply (nepers), per spw
parang              =      False        #  Apply parallactic angle correction on the fly
async               =      False        #  If true the taskname must be started using
                                        #   gaincal(...)

Data selection is done through the standard field, spw, intent, and selectdata expandable sub-parameters (see § 2.3). The bulk of the other parameters are the standard solver parameters. See § 4.4.1 above for a description of these.

The gaintype parameter selects the type of gain solution to compute. The choices are ’T’, ’G’, and ’GSPLINE’. The ’G’ and ’T’ options solve for independent complex gains in each solution interval (classic AIPS style), with ’T’ enforcing a single polarization-independent gain for each co-polar correlation (e.g. RR and LL, or XX and YY) and ’G’ having independent gains for these. See § 4.4.3.1 for a more detailed description of ’G’ solutions, and § 4.4.3.2 for more on ’T’. The ’GSPLINE’ fits cubic splines to the gain as a function of time. See § 4.4.3.3 for more on this option.

   4.4.3.1 Polarization-dependent Gain (G)
   4.4.3.2 Polarization-independent Gain (T)
   4.4.3.3 GSPLINE solutions
   4.4.3.4 Antenna Delays — ’K’ solutions
   4.4.3.5 Cross-Hand Delays — ’KCROSS’ solutions

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