Basic Information on gpcal

Task: gpcal
Purpose: Gain/phase/polarization calibration of dual feed data.
Categories: calibration

        Gpcal is a MIRIAD task that 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').
 
        With nfbin>1, GPCAL writes additional tables 'gainsf' and
        'leakagef' with solutions for frequency binned data.
        Not many other tasks currently read these tables, but
        they will be applied in preference to the single frequency
        versions if present.
 
        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 their 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.
 

Key: vis
        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.

Key: select
        Standard uv selection. Default is all data.

Key: line
        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).

Key: flux
        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. You may specify an I,Q,U,V flux density for each of the
        nfbins that you request, but all values must be present. If you
        set nfbins higher than 1, then any bins without information
        provided here will use the values from the first bin. If you
        want to use spec to describe the Stokes I flux density, then
        you only need to put the flux density at the reference frequency
        as the first parameter here, while all other I values are
        ignored.

Key: spec
        The reference frequency (GHz), spectral index and up to two
        higher order terms. Only used if nfbin>1. The spectral index
        terms default to zero.

Key: refant
        The reference antenna.  Default is 3.  The reference antenna
        must be present throughout the observation.  Any solution
        intervals where the reference antenna is missing are discarded.

Key: minants
        The minimum number of antenna that must be present before a
        solution is attempted. Default is 2.

Key: interval
        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.

Key: nfbin
        The number of frequency bins. The default is 1. Use nfbin>1 to
        solve for variation across the band in the gain and leakage
        parameters.
        Works best for uv files with a single spectral window, i.e.,
        after uvsplit.

Key: tol
        Error tolerance. The default is 0.001 which should be adequate.

Key: xyphase
        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.

Key: options
        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 the pre-August 1994 ATCA
                     flux density scale below 11 GHz and the
                     pre-May 2016 mm fluxscale above 11 GHz.
                     See the help on oldflux for more information.
          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. Use with caution.
          noamphase  Do not solve for the amplitude and phase. The
                     default is to solve for amplitude and phase. This
                     option is rarely useful.
          linear     Expect/handle data from feeds that are linearly
                     polarised. This is the default.
          circular   Expect/handle data from feeds that are circularly
                     polarised.
 
        The following are options for advanced users, and should be used
        with forethought and caution.
          reset      If calibration tables (leakage and gains) are
                     present within the input dataset, GPCAL usually
                     uses these in determining default XY phases,
                     leakages and flux scale.  This is usually the
                     desired behaviour.  The "reset" option suppresses
                     this behaviour, and starts GPCAL with a clean
                     slate.
          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.
 
Revision: 1.23, 2020/11/23 04:42:34 UTC