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Basic Information on gpcal
Task: gpcal
Purpose: Gain/phase/polarization calibration of dual feed data.
Categories: calibration
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.
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.
Key: refant
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.
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: 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 a pre-August 1994 ATCA flux
density scale. See the help on oldflux for more
information.
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.