Basic Information on gpscal

Task: gpscal
Purpose: Self-calibration for arrays with linear feeds
Categories: 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.

Key: vis
Name of input visibility data file. No default.

Key: select
Standard uv data selection criteria. The default is all data.

Key: model
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).

Key: clip
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.

Key: flux
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.

Key: offset
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.

Key: interval
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.

Key: options
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.

Key: minants
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.

Key: refant
This sets the reference antenna, which is given a phase angle of zero.
The default is antenna 3.

Key: line
The visibility linetype to use, in the standard form, viz:
  type,nchan,start,width,step
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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