Polarization Calibration

Revised 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. The phase calibrator chosen for the observations can also double as a polarization calibrator provided it is at a declination where it moves through enough parallactic angle during the observation (roughly Dec 15deg to 50deg for a few hour track). The minimum condition that will enable accurate polarization calibration from a polarized source (in particular with unknown polarization) is three observations of a bright source spanning at least 60 degrees in parallactic angle (if possible schedule four scans in case one is lost). This can usually be obtained for your gain calibrator if it is at Dec>20 and your track is 4 hours or longer - check the reporting from the OPT when setting up your observations. If a bright unpolarized unresolved source is available (and known to have very low polarization) then a single scan will suffice to determine the leakage terms. 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 7.5 cm or shorter (4GHz and higher). The results of a careful monitoring program of these and other polarization calibrators can be found at http://www.aoc.nrao.edu/~smyers/evlapolcal/polcal_master.html

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.

Detailed Guidelines

Observing Recommendations

There are several strategies for deriving the Q/U angle calibration:

• Observation of a primary polarization standard (Category A)
• Observation of a secondary polarization calibrator (Category B with Note 3) with auxilary monitoring observations to transfer from primary.

This calibration is needed to set the polarization vector angle 0.5*arctan(U/Q) and should be done in all cases.

There are several strategies for deriving the instrumental polarization:

• Single scan observation of a zero polarization source (Category C)
• Several scans (minimum of 3 over 60 degrees of parallactic angle) of an unknown polarization source (Category B or your gain calibrator, if bright enough)
• Two scans of a source of known polarization (Category A or B with transfer)

See Tables 1-4 below for Category A-D source catalogs.

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

Wide Band Considerations

• Performance over 1, 2 and 8 GHz band widths

Time stability

Current results (by band)

Frequency stability

Current results (by band)

Polarization Calibrator Catalog and Selection

The following sources are known to be useable for polarization calibration. These consist of a few "pol standard" sources with known stable polarization (for Q/U angle calibration), plus a number of "bright" sources with "monitored" variable flux densities and polarization. Some of these are seen to have only "moderate variablity" and could be used as secondary angle calibrators if you can transfer the angle from the monitoring observations. Assume others (particularly "flat spectrum") are highly variable. There are also a few "bright, low pol" sources available as leakage calibrators (but the can have measureable polarization at high frequencies).

NOTE: Be sure to use the EVLA OPT Source catalog to get the standard J2000 positions and see the fluxes.

Calibration Selection Procedure:

• Select Polarization Standard (to calibrate polarization angle Q/U) - optimally select one Category A source and observe at least one scan. Alternative: use a "moderately variable" Category B calibrator and use monitoring information (may have to submit your own SB for this) to transfer from a primary.
• Select Leakage Calibrator (to determine intrumental polarization) - optimally select one Category C low-polarization source or Category B secondary source (or your gain calibrator) in optimal Dec range (see Table 2 note 3) for PA coverage during run (if long enough). Single scans ok for Category C. Alternative: try a Category D CSO if no other options available.

Notes:

• A1. 3C48 is weak at high frequency and somewhat resolved in larger configurations. Depolarized below 4GHz.
• A2. 3C286 is our foremost primary calibrator and should be used if available.

Notes:

• B1. In optimal Declination range to be used as leakage calibrator with PA coverage. Recommended as calibrators and if necessary can be used as secondary standards with monitoring.
• B2. Low polarization at low frequencies (L, sometimes S,C), do not use as angle calibrator.
• B3. Highly variable and interesting in its own right.

Notes:

• C1. Very bright and low polarization (<1%), but variable flux density. Approaches 1% polarized at 43GHz.
• C2. Weak at high frequency, but stable flux and very low polarization.
• C3. Weak at high frequency, bright and low polarization below 9GHz.

The following northern sources are known to be CSO (Compact Symmetric Objects) and are characteristically unpolarized. They can be used over a range of frequencies (Gugliucci, N.E. et al. 2007, ApJ 661, 78) as "low pol" leakage calibrators. CSOs tend to be on the weak side and should be used with care at higher frequencies. We have not used these with the EVLA and thus rate them as "secondary" unpolarized calibrators. Let us know if you use these so we can evaluate their performance.

WARNING: the positions given below are B1950, use the Source names in the EVLA OPT to get the J2000 positions.

Comments:

• at least one "pol standard" (ideally from Category A) should be included for angle calibration
• "bright" sources are easily useable as leakage calibrators with PA coverage (and probably good for bandpasses to boot!)
• "monitored" sources can be found at http://www.vla.nrao.edu/astro/calib/polar/
• "steep spectrum" sources are likely weak at high frequencies
• "flat spectrum" sources are likely bright at high frequencies but variable
• "moderately variable" sources may be useable in a pinch if you can get a nearby (in time) monitoring observation (see below)

Monitoring Observations

For the VLA, a decade-long monitoring program was carried out with the goal of allowing transfer from our standard sources to bright sources useable as VLBA calibrators. The results of this can be found at http://www.vla.nrao.edu/astro/calib/polar/

We are in the process of beginning such a program for the EVLA. There is no pipeline produced monitoring results as of this time, but intrepid users can find the data in the public archive https://archive.nrao.edu/archive/archiveproject.jsp under project code TPOL0003.

The VLA database (particularly before the transition in 2008) can be used to see the level of variability in these sources, and to get an idea of the flux density ranges to expect.

Post-processing Guidelines

For CASA reduction and analysis of polarization data, please see the following links:

• [Instrumental Polarization Calibration]
  • [Heuristics and Strategies for Polarization Calibration]
  • [A Note on Channelized Polarization Calibration]
  • [A Polarization Calibration Example]