Polarisation calibration

 

Introduction

The MeerKAT receivers have two orthogonal linear feeds. The correlator always produces all four polarisation products, though users who only need Stokes I images can choose to only download the parallel-hand (HH and VV) products from the archive. Although the polarisation characteristics of the receivers appear to be stable on a timescale of months (Plavin et al. 2020), it is recommended to include absolute polarisation calibrators in each observation, if possible.

There are somewhat different approaches to polarisation calibration, both in observational set-up and reduction methods, which depend on calibration source LST-coverage, reduction package used, and the user’s dynamic range and fidelity requirements. Moreover, much work is still being done on spectropolarimetric calibration across the full field of view. Below we attach two reports demonstrating calibration using Obit and CASA, respectively:

 

For polarimetry above 1380 MHz, or past the half-power beam width of the primary beam, please have a look at our page on the MeerKAT primary beam measurements.

Leakage calibration

A strong unpolarised calibrator such as J1939-6342 can be used to calibrate the leakage terms. J0408-6545 could be used but is weakly polarised at the 0.1% level. Alternatively, a strong calibrator can be observed over a range (at least 60°) of parallactic angles.

Absolute polarisation angle

MeerKAT currently uses J1331+3030 (3C 286) as the preferred fundamental polarisation reference calibrator. However, due to its high northern declination, it is only visible to the telescope for ~5 hours above 20° elevation. The other commonly used calibrator is 3C 138. The properties of these calibrators are summarised in Table 1 below. However, we have been finding anomalous results for 3C 138, and there are some indications that the model for 3C 286 may not be accurate in the UHF band. Work is underway to make new measurements. In the meantime, users are requested to contact the helpdesk if they are concerned about their results. Note that despite uncertainties in calibrating the polarisation angle, it is still possible to obtain significant scientific results, e.g. Cotton et al. (2020).

Table 1: Properties of suggested polarisation calibrators.

Calibrator

RM (rad / m2)

Fractional linear power (%)

Average linear angle (deg)

S1.28GHz Total intensity (Jy)

Calibrator

RM (rad / m2)

Fractional linear power (%)

Average linear angle (deg)

S1.28GHz Total intensity (Jy)

3C 286

0.00 +/- 0.2

8.6 to 9.9

33

15.77

3C 138

-0.80 +/- 0.3

5.6 to 8.4

-14 to -10

8.99

While we have identified 10 new polarised sources with low (< 10 rad/m2) rotation measure, we have only recently started a monitoring programme to determine their long-term stability. Users interested in using one of these potential calibrators should contact the helpdesk when setting up their observation for scheduling.

In the case where no absolute polarisation calibrator is available in the required LST range, the only option is to follow the polarisation calibration procedure outlined in the Obit report mentioned above. Note that this can only be done using either the SDP pipeline or Obit.

Parallactic angle of the polarisation calibrator

If a polarisation calibrator is included to calibrate the H-V phase of the system the observer should
consider an optimal LST time for the calibrator visit to ensure that power is available on the crosshand visibilities for their choice of parallactic angle (i.e. the parallactic angle rotated Stokes U component should be maximized). The measured fractional polarisation power for each feed should be at least 0.45 Jy (~5.3% on 3C 138 and 3% for 3C 286).

At present the OPT system will not warn the observer if this is not the case, but this functionality is being worked on. An ipython notebook is available to calculate the HV power for a given scan start time. Please contact the helpdesk should you require assistance.

Cross-hand delays and V/H phase difference

The delay calibration script executed at the subarray build calculates the cross-hand polarisation phase offsets using the noise diodes. The polarisation characteristics of the receiver chain appear to remain stable to less than 10 ps in delay over timescales of hours, certainly within the timescale of a typical horizon-to-horizon track.

Note that when digitizers are rebooted or power cycled, a random delay of -4 to 4 samples may be introduced relative to the other polarisation on the antenna. This can lead to jumps of multiples of 90° in leakage phase solutions for the affected antenna from one epoch to another.

These delays can also be calibrated in the standard manner in CASA using the polarisation calibrator or by using the pipeline solutions derived from the delay calibration prior to the start of the observation.

Ionospheric Faraday rotation

It is becoming clear that the ionosphere can introduce significant Faraday rotation at lower frequencies, especially in the UHF band. Work is pending on implementing total electron content (TEC) measurements derived from on-site dual-frequency GPS receivers. Anyone interested in becoming involved in this project, or requiring updates, is welcome to contact the helpdesk.