Instrumental Polarization Calibration (D,X)

NRAO Home > CASA > CASA Cookbook and User Reference Manual

4.4.5 Instrumental Polarization Calibration (D,X)

Full support for instrumental polarization calibration for the circular feed basis (e.g., JVLA) is provided in CASA. Support for the linear feed basis (e.g., ALMA) will be deployed and documented for the general user in the 3.4 release. The following examples assume the circular basis.

The inputs to polcal are:

#  polcal :: Determine instrumental polarization calibrations
vis                 =         ’’        #  Name of input visibility file
caltable            =         ’’        #  Name of output gain calibration table
field               =         ’’        #  Select field using field id(s) or field name(s)
spw                 =         ’’        #  Select spectral window/channels
intent              =         ’’        #  Select observing intent
selectdata          =      False        #  Other data selection parameters
solint              =      ’inf’        #  Solution interval
combine             =     ’scan’        #  Data axes which to combine for solve (scan, spw,
                                        #   and/or field)
preavg              =       -1.0        #  Pre-averaging interval (sec)
refant              =         ’’        #  Reference antenna name(s)
minblperant         =          4        #  Minimum baselines _per antenna_ required for solve
minsnr              =        3.0        #  Reject solutions below this SNR
poltype             =     ’D+QU’        #  Type of instrumental polarization solution (see help)
smodel              =         []        #  Point source Stokes parameters for source model.
append              =      False        #  Append solutions to the (existing) table
gaintable           =       [’’]        #  Gain calibration table(s) to apply
gainfield           =       [’’]        #  Select a subset of calibrators from gaintable(s)
interp              =       [’’]        #  Interpolation mode (in time) to use for each gaintable
spwmap              =         []        #  Spectral windows combinations to form for
                                        #   gaintables(s)
gaincurve           =      False        #  Apply internal VLA antenna gain curve correction
opacity             =         []        #  Opacity correction to apply (nepers), per spw
async               =      False        #  If true the taskname must be started using polcal(...)

The polcal task uses many of the standard calibration parameters as described above in § 4.4.1.

The key parameter controlling polcal is poltype. The choices are:

: ’D’ — Solve for instrumental polarization (leakage D-terms), using the transform of an IQU model; requires no parallactic angle coverage, but if the source polarization is non-zero, the gain calibration must have the correct R-L phase registration. (Note: this is unlikely, so just use ’D+X’ to let the position angle registration float.) This will produce a calibration table of type D.
: ’D+X’ — Solve for instrumental polarization D-terms and the polarization position angle correction, using the transform of an IQU model; this mode requires at least 2 distinct parallactic angles to separate the net instrumental polarization and the PA. This will produce a calibration table of type ’D’. ALERT: no table of type ’X’ will be produced, so you must follow this by a run of polcal with polmode=’X’ (see below).
: ’D+QU’ — Solve for instrumental polarization and source Q + iU; requires at least 3 distinct parallactic angles to separate the net instrumental polarization from the source Q and U. Effectively sets the polarization PA to the value if the R-L phase difference were 0^∘. This will produce a calibration table of type ’D’.
: ’X’ — Solve only for the position angle correction; best to use this after getting the D-terms from one of the above modes. Requires the observation of a calibrator with known Q + iU (or at least known U∕Q). This will produce a calibration table of type ’X’.

There are channelized solution modes for the above options. For example, substitute ’Df’ for ’D’ in the ’D*’ modes described above to get a channelized D-term solution.

ALERT: polcal will obtain a separate D-term solution for each field supplied to it. This limitation will be relaxed in the future, enabling more sensitive solutions.

   4.4.5.1 Heuristics and Strategies for Polarization Calibration
   4.4.5.2 A Note on channelized polarization calibration
   4.4.5.3 A Polarization Calibration Example

[next] [prev] [prev-tail] [front] [up]

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

This code is available under the terms of the GNU General Public Lincense