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:

• MeerKAT L band polarimetric calibration

• MeerKAT64 widefield polarimetry of DEEP2

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

Using celestial calibrators

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. 3C 286 is seen to rapidly depolarise below 1 GHz, with an intrinsic rotation measure that gradually increases to about 0.12 rad/m² (Hugo & Perley, 2024). An updated 3C286 model is presented in Hugo & Perley, 2024. 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 at the MeerKAT L-band (Taylor & Legodi 2024).

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.

Using the noise diode and antenna calibration tables

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.

The reference antenna calibration tables needed for this method are attached below.

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 Observation Planning Tool (OPT) calculates and plots source H-V power values (in Jy) as part of the OPT simulate functionality, along with a correlator mode specific H-V limit (in Jy, horizontal line) based on a 5 minute polarisation scan. Users can ensure that sufficient H-V power is obtained by confirming that the chosen calibrator (3C 138 or 3C 286) has a H-V power above the provided limit across the LST range it is observed for. We recommend including a minimum of 10 minutes on a polarisation calibrator with sufficient H-V power for every 5 hours of observing. Where possible these 10 minutes should be split into two 5 minute scans to minimise the risk against scans being affected by either RFI or ionospheric effects. More than two scans can also be observed as these will provide better parallactic angle coverage of the polarisation calibrator.

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.

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.