Polarization Calibration

Current OSS Guidelines

For projects requiring imaging in Stokes Q and U, the instrumental polarization should be determined through observations of a bright calibrator source spread over a range in parallactic angle. In nearly all cases, the phase calibrator chosen can double as a polarization calibrator. The minimum condition that will enable accurate polarization calibration is four observations of a bright source spanning at least 90 degrees in parallactic angle. The accuracy of polarization calibration is generally better than 0.5% for objects small compared to the antenna beam size. At least one observation of 3C286 or 3C138 is required to fix the absolute position angle of polarized emission. 3C48 also can be used to fix the position angle at wavelengths of 6 cm or shorter. The results of a careful monitoring program of these and other polarization calibrators can be found at http://www.vla.nrao.edu/astro/calib/polar/.

High sensitivity linear polarization imaging may be limited by time dependent instrumental polarization, which can add low levels of spurious polarization near features seen in total intensity and can scatter flux throughout the polarization image, potentially limiting the dynamic range. Preliminary investigation of the EVLA’s new polarizers indicates that these are extremely stable over the duration of any single observation, strongly suggesting that high quality polarimetry over the full bandwidth will be possible.

The accuracy of wide field linear polarization imaging will be limited, likely at the level of a few percent at the antenna half-power width, by angular variations in the antenna polarization response. Algorithms to enable removable of this angle-dependent polarization are being tested, and observations to determine the antenna polarizations have begun. Circular polarization measurements will be limited by the beam squint, due to the offset secondary focus feeds, which separates the RCP and LCP beams by a few percent of the FWHM. The same algorithms noted above to correct for antenna-induced linear polarization can be applied to correct for the circular beam squint. Measurement of the beam squints, and testing of the algorithms, is ongoing.

Ionospheric Faraday rotation of the astronomical signal is always notable at 20 cm. The typical daily maximum rotation measure under quiet solar conditions is 1 or 2 radians/m2, so the ionospherically-induced rotation of the plane of polarization at these bands is not excessive – 5 degrees at 20 cm. However, under active conditions, this rotation can be many times larger, sufficiently large that polarimetry is impossible at 20 cm with corrrection for this effect. The AIPS program TECOR has been shown to be quite effective in removing large-scale ionospherically induced Faraday Rotation. It uses currently-available data in IONEX format. Please consult the TECOR help file for detailed information. In addition, the interim EVLA receivers generally have poor polarization performance outside the frequency range previously covered by the VLA (e.g., outside the 4.5–5.0 GHz frequency range for C band, and outside 1.3–1.7 GHz for L-band), and the wider frequency bands of these interim receivers may be useful only for total intensity measurements.

Revised Guidelines

Observing Recommendations

There are several strategies for deriving the instrumental polarization:

• Single scan observation of a zero polarization source (see catalog below)
• Several scans (minimum of 3 over 60 degrees of parallactic angle) of an unknown polarization source
• Two scans of a source of known polarization (see catalog below)

Low Frequency Considerations

• Ionosphere monitoring

Global Ionospheric TEC maps are available via: http://iono.jpl.nasa.gov/latest_rti_global.html

• Solar activity monitoring

Solar activity and general space weather can be reviewed at the NOAA site: http://www.swpc.noaa.gov/today.html

The site provides solar activity forecasts and geophysical (geomagnetic field) activity forecasts along with GOES X-ray flux values.

High Frequency Considerations

• Source list restrictions
• Need longer observations

Time stability

Current results (by band)

Frequency stability

Current results (by band)

Polarization Calibrator Catalog

CSO (Compact Symmetric Objects) are characteristically unpolarized and can be used over a range of frequencies (Gugliucci, N.E. et al. 2007, ApJ 661, 78); these make up the bulk of the zero/unpolarized sources.

High pol 3C148 3C286

Monitoring Observations

Description and scope.

List of observation dates and results.

Post-processing Guidelines

Perhaps just a pointer to the CASA guides page for the relevant section.