Content-type: text/html Manpage of gpcal


Section: User Commands (1)
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gpcal - Gain/phase/polarization calibration of dual feed data.  






Gpcal is a MIRIAD task which determines calibration corrections (both antenna gains and instrumental polarisation characteristics) for an array with dual feeds, from an observation of a point source. The source can be polarised, with unknown polarisation characteristics. Though the source may be strongly polarized, the instrumental polarisation errors are assumed to be small (of order at most a few percent).

Normally GPCAL writes the solutions as a gains table (item `gains') and a polarization leakage table (item `leakage').

GPCAL can handle either dual linear or dual circular feeds. However by default it expects dual linears -- you must use options=circular so switch it to circular mode. Also the terminology in this document is aimed at linears (e.g. we talk of ``XY phase'' -- for dual circulars this is really ``RL phase'').

Note that the user specifies which parameters are to be solved for. In the case of leakages and xyphases, GPCAL will check for the existence of items ``leakage'' and ``gains'' in the input data-set. If present, these will be used as the initial estimates of these parameters. If you are not solving for these parameters, they will be held at there initial value through the solution process. After converging on a solution, and if the xyphase offsets or leakage parameters have been modified, GPCAL will write out their current values to the appropriate items.

Conventions: Unfortunately there has been a number of changes in the `sign conventions' used within Miriad. For a discussion of the conventions, past sign errors and how they affect you, see the memo ``The Sign of Stokes-V, etc'' by Bob Sault.  


Input visibility data file. The data should be either raw linear or raw circular polarisations. No default. The visibility data must be in time order.
Standard uv selection. Default is all data.
Standard line-type specification. Multiple channels can be given. Generally it is better to give multiple channels, rather than averaging them into a `channel-0'. The default is all the channel data (or all the wide data, if there is no channel data).
The values of the I,Q,U,V Stokes parameters. If no values are given, and it is a source known to GPCAL, GPCAL uses its known flux as the default. If GPCAL does not know the source, the flux is determined by assuming that the rms gain amplitude is 1. If the option `qusolve' is used, the given fluxes for Q and U are used as the initial estimates. Also see the "oldflux" option.
The reference antenna. Default is 3. The reference antenna needs to be present throughout the observation. Any solution intervals where the reference antenna is missing are discarded.
The minimum number of antenna that must be present before a solution is attempted. Default is 2.
This gives one or two numbers, both given in minutes, both being used to determine the extents of an amplitude calibration solution interval. The first gives the max length of a solution interval. The second gives the max gap size in a solution interval. A new solution interval is started when either the max time length is exceeded, or a gap larger than the max gap is encountered. The default is max length is 5 minutes, and the max gap size is the same as the max length. The polarisation characteristics are assumed to be constant over the observation.
Error tolerance. The default is 0.001, which should be adequate.
Generally the use of this parameter has been superceded.

Initial estimate of the XY phase of each antenna. The default is 0 for all antennas. If the XY phase has not been applied to the data, then it is important that this parameter is set correctly, particularly for the reference antenna.

These options determine what GPCAL solves for. There are many permutations, the more obscure or useless of which are not supported. The option values are used to turn on or off some of the solvers. Several options can be given, separated by commas. Minimum match is used. Some combinations of these options are not supported.
  xyvary     The XY phase varies with time. By default the XY phase
             is assumed to remain constant.
  qusolve    Solve for Q and U fluxes. Good parallactic
             angle coverage is required for this.
  oldflux    This causes GPCAL to use a pre-August 1994 ATCA flux
             density scale. See the help on "oldflux" for more
  circular   Expect/handle circularly polarised data.
  nopol      Do not solve for the instrumental polarisation
             leakage characteristics. The default is to solve
             for the polarisation leakages on all feeds except
             the X feed of the reference antenna.
  noxy       Do not solve for any XY phase offset. The default is to
             solve for the XY phase offset on all antennas
             except for the reference antenna.
  nopass     Do not apply bandpass correction. The default is
             to apply bandpass correction if possible. This is
             rarely useful.
  noamphase  Do not solve for the amplitude and phase. The 
             default is to solve for amplitude and phase. This is
             rarely useful.

The following are options for advanced users, and should be used with caution. Contact Bob Sault for guidance on their applicability.

  xyref      Solve for the XY phase of the reference antenna. To
             do this, the source must be linearly polarized and you
             must have reasonable parallactic angle coverage. This
             option can be used with ``noxy'', in which case GPCAL
             solves for the offset of the reference antenna.
  polref     Solve for the instrumental polarization leakage
             of the X feed on the reference antenna. This can
             be combined with ``nopol'', in which case GPCAL
             solves for X feed of the reference antenna only.
  vsolve     Solve for the Stokes-V of the source. This is only
             possible for linear feeds and a preliminary leakage
             solution for the observation already exists. This
             preliminary solution must be formed from a
             calibrator with known Stokes-V.




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