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4.3.4.1 Using Calibration Models for Resolved Sources

If the flux density calibrator is resolved at the observing frequency, the point source model generated by setjy will not be appropriate. If available, a model image of the resolved source at the observing frequency may be used to generate the appropriate visibilities using the modimage parameter (or in older versions explicitly with the ft task). To use this, provide modimage with the path to the model image. Remember, if you just give the file name, it will assume that it is in the current working directory. Note also that setjy using a model image will only operate on that single source, thus you would run it multiple times (with different field settings) for different sources.

Otherwise, you may need to use the uvrange selection (§ 4.4.1.2) in the calibration solving tasks to exclude the baselines where the resolution effect is significant. There is not hard and fast rule for this, though you should consider this if your calibrator is shows a drop of more than 10% on the longest baselines (use plotxy, § 3.3.2, to look at this). You may need to do antenna selection also, if it is heavily resolved and there are few good baselines to the outer antennas. Note that uvrange may also be needed to exclude the short baselines on some calibrators that have extended flux not accounted for in the model. Note: the calibrator guides for the specific telescopes usually indicate appropriate min and max for uvrange. For example, see the VLA Calibration Manual at:

http://www.vla.nrao.edu/astro/calib/manual/

for details on the use of standard calibrators for the E/VLA.

Model images for some flux density calibrators are provided with CASA:

e.g., these are found in the data/nrao/VLA/CalModels sub-directory of the CASA installation. For example, just point to the repository copy, e.g.

   modimage = ’/usr/lib/casapy/data/nrao/VLA/CalModels/3C48_C.im’

or if you like, you can copy the ones you wish to use to your working directory.

The models available are:

 
3C138_L.im  3C147_L.im  3C286_L.im  3C48_L.im  
3C138_C.im  3C147_C.im  3C286_C.im  3C48_C.im  
3C138_X.im  3C147_X.im  3C286_X.im  3C48_X.im  
3C138_U.im  3C147_U.im  3C286_U.im  3C48_U.im  
3C138_K.im  3C147_K.im  3C286_K.im  3C48_K.im  
3C138_Q.im  3C147_Q.im  3C286_Q.im  3C48_Q.im

(more calibrator models for the JVLA are available at
https://science.nrao.edu/facilities/evla/data-processing/flux-calibrator-models-for-new-evla-bands These are all un-reconvolved images of AIPS CC lists. It is important that the model image not be one convolved with a finite beam; it must have units of Jy/pixel (not Jy/beam).

Note that setjy will rescale the flux in the models for known sources (e.g. those in Table 4.1) to match those it would have calculated. It will thus extrapolated the flux out of the frequency band of the model image to whatever spectral windows in the MS are specified (but will use the structure of the source in the model image).

ALERT: The reference position in the modimage is currently used by setjy when it does the Fourier transform, thus differences from the positions for the calibrator in the MS will show up as phase gradients in the uv-plane. If your model image position is significantly different but you don’t want this to affect your calibration, then you can doctor either the image header using imhead (§ 6.2) or in the MS (using the ms tool) as appropriate. In an upcoming release we will put in a toggle to use or ignore the position of the modimage. Note that this will not affect the flux scaling (only put in erroneous model phases); in any event small position differences, such as those arising by changing epoch from B1950 to J2000 using regridimage (§ 6.11), will be inconseqential to the calibration.

This illustrates the use of uvrange for a slightly resolved calibrator:

  # Import the data  
  importvla(archivefiles=’AS776_A031015.xp2’, vis=’ngc7538_XBAND.ms’,  
            freqtol=10000000.0, bandname=’X’)  
 
  # Flag the ACs  
  flagautocorr(’ngc7538_XBAND.ms’)  
 
  # METHOD 1:  Use point source model for 3C48, plus uvrange in solve  
 
  # Use point source model for 3C48  
  setjy(vis=’ngc7538_XBAND.ms’,field=’0’);  
 
  # Limit 3C48 (fieldid=0) solutions to uvrange = 0-40 klambda  
  gaincal(vis=’ngc7538_XBAND.ms’, caltable=’cal.G’, field=’0’,  
          solint=60.0, refant=’10’, selectdata=True, uvrange=’0~40klambda’,  
          append=False, gaincurve=False, opacity=0.0)  
 
  # Append phase-calibrator’s solutions (no uvrange) to the same table  
  gaincal(vis=’ngc7538_XBAND.ms’, caltable=’cal.G’, field=’2’,  
          solint=60.0, refant=’10’, selectdata=True, uvrange=’’,  
          append=True, gaincurve=False, opacity=0.0)  
 
  # Fluxscale  
  fluxscale(vis=’ngc7538_XBAND.ms’, caltable=’cal.G’, reference=[’0137+331’],  
          transfer=[’2230+697’], fluxtable=’cal.Gflx’, append=False)

while the following illustrates the use of of a model:

  # METHOD 2: use a resolved model copied from the data respository  
  #   for 3C48, and no uvrange  
  # (NB: detailed freq-dep flux scaling TBD)  
 
  # Copy the model image 3C48_X.im to the working directory first!  
 
  setjy(vis=’ngc7538_XBAND.ms’, field=’0’, modimage=’3C48_X.im’)  
 
  # Solutions on both calibrators with no uvrange  
  gaincal(vis=’ngc7538_XBAND.ms’, caltable=’cal.G2’, field=’0,2’,  
          solint=60.0, refant=’10’,  
          append=False, gaincurve=False, opacity=0.0)  
 
  # Fluxscale  
  fluxscale(vis=’ngc7538_XBAND.ms’, caltable=’cal.G2’, reference=[’0137+331’],  
          transfer=[’2230+697’], fluxtable=’cal.G2flx’, append=False)  
 
  # Both methods give 2230 flux densities ~0.7 Jy, in good agreement with  
  #   AIPS

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