Polarization

The continuum correlator mode provides full polarimetric information for both observing frequencies. The polarimetric spectral line modes (PA and PB) are also available for observations of linearly polarized spectral lines, or for observations of continuum objects where large field-of-view or high dynamic range is necessary. Spectral line modes `2AC', `2BD', and `4' do not provide linear polarization information.

For each observation requiring polarization information, 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 will 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. The instrumental polarization averaged among all baselines can vary by 0.3% on timescales of minutes to hours, limiting the believable fractional polarization to about 0.1%.

Wide field linear polarization imaging will be limited by the instrumental polarization beam. For a snapshot observation, the spurious linear polarization (after the standard polarization correction for the on-axis polarization response is applied) is $<$ 1% at angles less than $\lambda / 4D$ radians ($D$ is the antenna diameter), is 1 - 3% at $\lambda/2D$, and increases sharply beyond this, reaching 10% at $3\lambda/4D$. Since the instrumental polarization response is directed radially and rotates with parallactic angle, the spurious polarization will tend to average down for long integrations. However, if the object being observed is very bright, and has a low degree of linear polarization, errors in the polarization calibration will cause Stokes I flux to be scattered into the Q and U images, thus limiting the polarization dynamic range.

Ionospheric Faraday rotation is always present at 20 cm and 90 cm. The typical daily maximum rotation measure under quiet solar conditions is 1 or 2 radians/m$^2$, so the ionospherically-induced rotation of the plane of polarization at these bands is not excessive - 5 degrees at 20 cm, and perhaps 90 degrees at 90 cm. However, under active conditions, this rotation can be many times larger, such that accurate polarimetry is impossible at 20 cm.

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.

Circular polarization measurements are limited by the beam squint - the RCP and LCP primary beams are separated by 6 percent of the beamwidth along the axis perpendicular to the azimuth of the secondary focus feed position. Since circular polarization is determined from the difference between RCP and LCP signals, there results an appreciable error in all measurements of circular polarization off the pointing axis. The effect is large - the apparent circular polarization is $\sim$10% at $\lambda / 4D$, and $\sim$20% at $\lambda/2D$. This false circular polarization is antisymmetric with respect to the center of the antenna beam, so 12-hour observations should partially cancel out the effect - however, even so, the residual apparent circular polarization is probably only accurate to a few percent.

Observers should be aware that detailed polarization tests have not yet been carried out for the retrofitted EVLA antennas. In particular, we expect that the VLA-EVLA baselines may have non-closing calibration problems similar to the total-intensity calibration (see Section 3.11). This is complicated by the fact that some receivers on EVLA antennas will be completely new (e.g., wide band 5 GHz receivers), while others will be older VLA systems (e.g., some 8 GHz receivers).