Polarisation calibration
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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:
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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
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). 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 at the MeerKAT L-band.
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.
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.