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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 no 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: spec The reference frequency (GHz), spectral index and up to two higher order terms. The spectral index terms default to zero. 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.