Making a Fake Observation

The Miriad task uvgen can be used to simulate a variety of observing modes. Additionally it can simulate a number of systematic and random errors and non-ideal effects that a synthesis instrument will typically suffer.

Simulating a telescope approximately, let alone in some detail, requires a good many parameters. uvgen takes an appreciable number of parameters and input files to describe the telescope, correlator and source. Fortunately keyword and set-up files for some common situations exist (e.g. simulating the ATCA and VLA). Also the defaults are usually sensible.

Some inputs to uvgen, and typical values for the ATCA, are given below. Note that these are not the default values.

source= Source description file - see help file.
ant= Antenna location file - see help file.
baseunit=-51.02 Basic antenna increment (15m) in nanosec,
  for coorinates in a topocentric system.
corr=0,1,0,104 Correlator model - see help file.
spectra= Spectral line model - see help file.
ellim=12 Telescope elevation limit.
telescop=atca Telescope name - see help file.
stokes=xx,yy,xy,yx ATCA measures linear polarisations.
lat=-30 Latitude of Narrabri.
radec= Source RA and declination, in hours and degrees.
systemp=50 Typical receivers have a $T_{sys}$ of 50 K
pbfwhm= Primary beam size? Varies with frequency
leakage=2 Typical polarisation leakage
jyperk=12.7 System gain is about 12.7 Jy/K
pnoise= Rms antenna phase variation
gnoise= Rms antenna amplitude gain variation

These typical values for the ATCA, and also the VLA, can be found in in `keyword' files, found in the $MIRCAT directory.

Telescope Keyword Files
$MIRCAT/uvgen.def.atca Typical uvgen parameters for the ATCA
$MIRCAT/uvgen.def.vla Typical uvgen parameters for the VLA
The parameters in these files can be loaded from within the miriad shell using the source command. For example, to load typical parameters for the ATCA, use
      miriad% source $MIRCAT/uvgen.def.atca

By leaving some of uvgen's parameters unset (e.g. leakage, systemp, pbfwhm) you get an idealised telescope (no polarisation leakage, no receiver noise, no primary beam effects). Additionally there is no reason why you cannot request an ATCA-like telescope to produce Stokes I or circular polarisations!

The source and antenna description files are described in the help for uvgen, as are a number of other parameters needed in the simulation process. There are, however, a collection of description files for common situations. These are all kept in the $MIRCAT directory.

Source Description File
File Description
no.source Nothing! No source at all
point.source A 1 Jy unpolarised point source, at field centre
poff.source A point source offset from the field centre
qpoint.source A 1% linearly polarised point source
spiral.source A spiral

Antenna Description File
File Description
h75.ant ATCA configuration H75
h168.ant ATCA configuration H168
h214.ant ATCA configuration H214
0.122.ant ATCA configuration 0.122A
ew214.ant ATCA configuration EW214
ew352.ant ATCA configuration EW352
ew367.ant ATCA configuration EW367
0.375.ant ATCA configuration 0.375
0.75a.ant ATCA configuration 0.75A
0.75b.ant ATCA configuration 0.75B
0.75c.ant ATCA configuration 0.75C
0.75d.ant ATCA configuration 0.75D
1.5a.ant ATCA configuration 1.5A
1.5b.ant ATCA configuration 1.5B
1.5c.ant ATCA configuration 1.5C
1.5d.ant ATCA configuration 1.5D
3.0a.ant ATCA configuration 3.0A or 6.0A
3.0b.ant ATCA configuration 3.0B or 6.0B
3.0c.ant ATCA configuration 3.0C or 6.0C
3.0d.ant ATCA configuration 3.0D or 6.0D
vla_a.ant VLA configuration A
vla_b.ant VLA configuration B
vla_c.ant VLA configuration C
vla_d.ant VLA configuration D

Given the various standard keyword and description files kept in the $MIRCAT directory, it is not particularly difficult to set up the inputs to uvgen. For example, to simulate the `spiral' source using configuration 3.0A, in a continuum mode, you would give the following commands to the miriad front-end:

  miriad% source $MIRCAT/uvgen.def.atca
  miriad% source = $MIRCAT/spiral.source
  miriad% ant = $MIRCAT/3.0a.ant
  miriad% inp uvgen
and then set or unset other parameters as desired.

If you wish to model a particular telescope, the task telepar contains a database of telescope characteristics.

For models which are more complex than those that can be readily simulated with uvgen, one approach is to generate the visibilities are the appropriate u-v tracks, and then to use the task uvmodel to compute the visibilities, on those tracks, of a particular image model.

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