Content-type: text/html Manpage of gpscal


Section: User Commands (1)
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gpscal - Self-calibration for arrays with linear feeds  




calibration, map making  


GPSCAL is a MIRIAD task to perform self-calibration of visibility data, where the visibility data is produced by linear feeds, and the source is moderately or strongly polarised. Either phase only or amplitude and phase calibration can be performed. The inputs to GPSCAL include a visibility data file, and model images. The models should be of differing polarisations of the same pointing and frequencies. GPSCAL then calculates the visibilities corresponding to the model, accumulates the statistics needed to determine the antennae solutions, and then calculates the self-cal solutions.

Note that while GPSCAL can be used for self-calibrating with a point source model, task GPCAL is more flexible (and efficient) at doing this. For a point source model, GPSCAL is preferable only if the data-set is not in time order (GPCAL insists that it is) or the point source is not at the observing center.  


Name of input visibility data file. No default.
Standard uv data selection criteria. The default is all data.
Name of the input models. Several models can be given, which differ in polarisation type. Polarisation types can be I,Q,U or V. Any polarisation types that are not given are assumed to be zero. The different models should correspond to the same pointing and range of channels. Generally the model should be derived (by mapping and deconvolution) from the input visibility file, so that the channels in the model correspond to channels in the visibility file. Though the maps can be made using any linetype, generally "channel" linetype will give best results (??). The units of the model MUST be JY/PIXEL, rather than JY/BEAM. It should be weighted by the primary beam. The task DEMOS can be used to extract primary beam weighted models from a mosaiced image. If no models are given, a point source model is assumed (see FLUX keyword).
Clip level. For models of intensity, any pixels below the clip level are set to zero. For models of Stokes Q,U,V, or MFS I*alpha models, any pixels whose absolute value is below the clip level are set to zero. Default is 0.
If not model is given, then a point source model is assumed. This keyword gives the flux of the point source model. Four values can be given, corresponding to I,Q,U and V respectively. NOTE: The flux of the model is not adjusted to match the flux of the data, so if amplitude selfcalibration is being performed, the fluxes given should accurately portray the fluxes of the source. The default is 1,0,0,0.
This gives the offset in arcseconds of a point source model (the offset is positive to the north and to the east). This parameter is used if the MODEL parameter is blank. The default is 0,0. The amplitude of the point source is chosen so that flux in the model is the same as the visibility flux.
The length of time, in minutes, of a gain solution. Default is 5, but use a larger value in cases of poor signal to noise, or if the atmosphere and instrument is fairly stable.
This gives several processing options. Possible values are:
  amplitude  Perform amplitude and phase self-cal.
  phase      Perform phase only self-cal.
  noxy       Do not attempt to solve for the XY phase difference.
             Normally GPSCAL attempts to solve for the XY phase
             on all antennae except for the reference antenna.
             The XY phase difference is assumed to be constant
             throughout the observation.
  xyvary     Allow the XY phase difference to vary from integration
             to integration. If not specified, the XY phase is
             constrained to be constant throughout the data-set.
  xyref      Solve for the XY phase of the reference antenna. To
             do this, the source should be strongly polarized (at
             least 5%) and Q or U models should be provided. This
             option cannot be used with ``noxy''. 
  mfs        This is used if there is a single plane in the input
             model, which is assumed to represent the image at all
             frequencies. This should also be used if the model has
             been derived from MFCLEAN. You should specify the
             LINE keyword if you use the mfs option.
  noscale    Do not scale the gains. By default the gains are scaled
             so that the rms gain amplitude is 1. Generally this
             should be used if the model is believed to have the
             correct flux density scale.
Note that "amplitude" and "phase" are mutually exclusive. The default is options=phase.
Data at a given solution interval is deleted if there are fewer than MinAnts antennae operative during the solution interval. The default is 3 for options=phase and 4 for options=amplitude.
This sets the reference antenna, which is given a phase angle of zero. The default is antenna 3.
The visibility linetype to use, in the standard form, viz:
Generally if there is an input model, this parameter defaults to the linetype parameters used to construct the map. If you wish to override this, or if the info is not in the header, or if you are using a point source model, this parameter can be useful.




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Time: 18:35:38 GMT, July 05, 2011