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Basic Information on gmakes
Task: gmakes
Purpose: Convert a visibility dataset into a gain visibility dataset
Categories: calibration
GMAKES creates a gain calibration dataset (which in actuality
is a regular visibility dataset) from a single visibility dataset
assuming a model for the observed source (point source or planet)
** In antenna based mode the visibilities are written as if
the data is an autocorrelation dataset
NOTE: this is the new GMAKES, with new keyword focus=
Key: vis
Name of input visibility dataset. Only one dataset can be given;
to use multiple datasets, use UVCAT with the proper LINE= to speed
up I/O in this program.
No default.
Key: select
Standard uv data selection criteria. Generally this should not include
a "dra" and "ddec" selection, as GMAKES automatically matches data
with the appropriate pointing center.
Key: model
Name of the input models. Several models can be given, which can
cover different channel ranges, different pointing centers, and
different polarizations of the input visibility data. 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. If
no models are given, a point source model is assumed.
(see also the ``apriori'' option below)
Key: clip
Clip level. Anything, in the input model, below the clip level is set
to zero. Default is 0.
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.
**bug** For multisource datasets and long intervals pathological
situations could occur where the computed timestamps are wrong.
Key: options
This gives several processing options. Possible values are:
amplitude Perform amplitude and phase self-cal.
phase Perform phase only self-cal.
baseline Baseline based gains!! ** NOT IMPLEMENTED YET **
polarized The source is polarised. By default the source is
assumed to be unpolarised. For a polarized source,
SELFCAL cannot perform polarization conversion. That
is, if the model is of a particular polarization, then
the visibility file should contain that sort of
polarization. For example, if the model is Stokes-Q,
then the visibility file should contain Stokes-Q.
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.
relax Relax the convergence criteria. This is useful when
GMAKES'ing with a very poor model.
apriori This is used if there is no input model, and the
source in the visibility data is either a planet,
or a standard calibrator. This causes the model data
to be scaled by the known flux of the source. For a
planet, this flux will be a function of baseline. If
the source is a point source, the ``apriori'' option
is only useful if the ``amplitude'' and ``noscale''
option are being used. For a planet, this option
should also be used for a phase GMAKES, to get the
correct weighting of the different baselines in the
solution.
noscale Do not scale the model. Normally the model is scaled
so that the flux in the model visibilities and the
observed visibilities are the same. Generally this
option should be used with at least the apriori option.
It must be used if GMAKES is being used to determine
Jy/K, and should also be used if the model is believed
to have the correct scale.
Note that "amplitude", "phase" and "baseline" are mutually exclusive.
The default is options=apriori,amp,noscale. Note that this is
different from the defaults for SELFCAL.
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, for a given solution interval, is the antennae with the
greatest weight.
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: 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.
Default: wide,2,1,1,1; i.e. the first two (digital) wide band channels.
Key: fluxes
List of sources and associated fluxes in Jy to be used for
calibration. The format is name of the source, followed by
its flux, and optionally more sources, e.g.
fluxes=3c273,10.3,3c84,12.3,bllac,2.2
Currently no frequency dependency can be specified.
Key: fluxtab
Name of the calibrators flux table file. The user
can also supply fluxes by hand (see keyword FLUXES=) in
which case the flux table derived values are ignored.
* ? Note that reported frequencies of 0.0 means the flux was
? determined from interpolation accross different frequencies.
Default: $MIRCAT/cals.fluxes
Key: error
The percentage error in amplitude, and the phase error above which
antennae are reported when the gain solution deviates from the
observed values.
Default: 20,20 (20% in amplitude, 20 degrees phase error).
Note: not implemented yet.
Key: focus
Maximum (sum of) focus change allowed (in volts) to force a break point
and hence start of a new averaging interval. Note: a new uv variable,
sfocus, is copied to the output gain visibility dataset.
NOTE: CARMA has no focus values (yet)
Default: 0.1 (volts)
Key: out
Output gain visibility dataset. No default.