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Basic Information on selfcal
Purpose: Determine self-calibration of calibration gains.
Categories: calibration, map making
SELFCAL is a MIRIAD task to perform self-calibration of
visibility data. Either phase only or amplitude and phase
calibration can be performed. The input to SELCAL are a
visibility data file, and model images. This program then
calculates the visibilities corresponding to the model,
accumulates the statistics needed to determine the antenna
solutions, and then calculates the self-cal solutions.
Name of input visibility data file. No default.
Standard uv data selection criteria. See the help on select
for more information.
Name of the input models. Several models can be given, which
can cover different channel ranges and different pointing and
sources 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. The units of the model MUST be
Jy/pixel, rather than Jy/beam, and 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.
Instead of a model image, you can also use a visibility model.
Visibility models should have the same layout as the data and
the same selections will be applied.
NOTE: When you give SELFCAL a model, it will, by default, select
the data associated with this model from the visibility data-
set. This includes selecting the appropriate range of channels
and the appropriate pointing/source (if options=mosaic is used).
If you use a point source model, if it YOUR responsibility to
select the appropriate data.
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.
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.
The number of frequency bins. The default is 1. Use nfbin>1 to
solve for variation across the band in the gains.
Works best for uv files with a single spectral window, i.e.,
This gives several processing options. Possible values are:
amplitude Perform amplitude and phase self-cal.
phase Perform phase only self-cal.
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.
mmfs Same as mfs, but with multiple input models,
each applying to a subset of the bandwidth.
For use with nfbin>1: models will be allocated to
bins based on their centre frequency.
relax Relax the convergence criteria. This is useful when
selfcal'ing with a very poor model.
noscale Do not scale the gains. By default the gains are
scaled so that the rms gain amplitude is 1. In
this way, the total flux is not constrained to
agree with the model. If you are using nfbin>1
you may need this to make the data follow the
spectral shape of the model.
mosaic This causes SELFCAL to select only those
visibilities whose observing center is within plus
or minus three pixels of the model reference pixel.
This is needed if there are multiple pointings or
multiple sources in the input uv file. By default
no observing center selection is performed.
Note that "amplitude" and "phase" are mutually exclusive.
The default is options=phase.
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.
This sets the reference antenna, which is given a phase angle of
zero. The default, for a given solution interval, is the
antenna with the greatest weight.
If MODEL is blank, then the flux density (Jy) of a point source
model can be specified here. The default is 1.
Additional spectral parameters can be specified as follows:
Reference frequency in GHz, spectral index, up to two higher
order alpha terms (as produced by uvsfit or uvfmeas with
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.
The visibility linetype to use, in the standard form, viz:
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.
Revision: 1.23, 2021/07/02 03:23:16 UTC
Generated by firstname.lastname@example.org on 02 Jul 2021