National Radio
Astronomy Observatory

4.3.4 Setting the Flux Density Scale using (setjy)

When solving for visibility-plane calibration, CASA calibration applications compare the observed DATA column with the Fourier transform of calibrator model when it is provided (if no model is specified, a point source at the phase center is assumed).

The setjy task is then used to set the proper flux density and attaches a model image of the calibrator to the MS if specified. For sources that are recognized flux calibrators (listed in Tables 4.1 and 4.2), setjy can calculate the flux densities as a function of frequency (and time, for Solar System objects). Otherwise, the flux densities should be manually specified.

For the JVLA, the default source models are customarily point sources defined by the Baars, Perley-Taylor 99, or Perley-Butler 2010 flux density scales (’Perley-Butler 2010’ is standard by default), or point sources of unit flux density if the flux density is unknown. In fact, the model can be any image in Jy/pixel units (models typically generated by the clean task).

Optionally, the MODEL column can be filled with the Fourier transform of (option usescratch=T is setjy, ft, and clean). But for most measurement sets, the performance and data storage requirements are less demanding without the MODEL_DATA column.

The inputs for setjy are:

By default the setjy task will cycle through all fields spectral windows and channels, (one solution per spw with scalebychan = False) , setting the flux density either to 1 Jy (unpolarized), or if the source is recognized as one of the calibrators in the above table, to the flux density (assumed unpolarized) appropriate to the observing frequency. For example, to run setjy on a measurement set called data.ms:

Models of available calibrator sources can be listed by setting listmodels=True. setjy will then come up with all images that are in the paths where calibrator models for known telescopes are stored. It will also show all images in the working directory - any image there could potentially be a calibrator model. If the calibrator model is found by listmodimages it can be used in the modimage parameter without a path.

The fluxdensity parameter can be used to specify the flux of the calibrator in all Stokes parameters. Is is thus a list of values [I,Q,U,V], e.g. [’12Jy’,’13mJy’,’0Jy’,’0Jy’]. In addition, a spectral index can be specified via spix and a reference frequency reffreq (using the definition: S = fluxdensity ×^spix).

Most calibrator sources are based on radio emission from quasars and jets. The spectral indices of these sources are such that at (sub)mm wavelengths the majority of these sources become too weak and variable to be reliable flux estimators. Alternatives are thermal objects such as planets, moons, and asteroids. Those sources, however, are all Solar System objects, which implies that they move and may be (strongly) resolved. The standard=’Butler-JPL-Horizons 2010’ option of setjy includes flux density calibration using Solar System objects. CASA currently supports the objects listed in Table 4.2. These names are recognized when they are used in the ’field’ parameter in setjy. In that case, setjy will obtain the geocentric distance and angular diameter at the time of the observation from a (JPL–Horizons) ephemeris and calculate model visibilities. Currently the objects are modeled as uniform temperature disks, but effects like primary beam attenuation and limb darkening will be accounted for soon. Note that this model may oversimplify the real structure, in particular asteroids.

An example, using data from the M99 tutorial in http://casaguides.nrao.edu/index.php?title=CARMA_spectral_line_mosaic_M99:

setjy(vis=’c0104I’, field=’MARS’, spw=’0 2’, standard=’Butler-JPL-Horizons 2010’)

Tip: Running casalog.filter(’INFO1’) before running setjy with a Solar System object may send the logger a reference to the temperature measurement. Use casalog.filter(’INFO’) to restore the normal logging level.

The source model will be attached to the MS and applied to all calibration steps when usescratch=False. usesratch=True fills the MODEL_DATA column with the Fourier transform of the model. As of CASA 3.4. we found that under some circumstances, creation of the MODEL column may prevent memory issues and if tasks fail, we recoomend to set usesratch=True. Note that currently setjy will not transform a full-Stokes model image such that all polarizations are applied correctly. You need to use ft for this.

To limit this operation to certain fields and spectral windows, use the field and/or spw parameters, which take the usual data selection strings (§ 2.3). For example, to set the flux density of the first field (all spectral windows)

or to set the flux density of the second field in spectral window 17

The full-polarization flux density (I,Q,U,V) may also be explicitly provided:

ALERT: The apparent brightness of objects in the Solar System will vary with time (Fig. 4.3.4) because of the Earth’s motion relative to them, if nothing else. If the field index of a flux calibrator spans several days, you should run setjy more than once, limiting each run to a suitable timerange by using the timerange, scan, and/or observation selection parameters. Note that it is the field index that matters, not the name. Typically concat assigns moving objects a new field index for each observation, so usually it is not necessary to select a time range in setjy. However, it is worth checking with listobs, especially for planets.

More information about CASA may be found at the CASA web page

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