MeerKAT follows standard interferometric calibration procedures. Here we list a high-level overview of the recommended observational set-up for calibration, with more details given in the sections below. These strategies are applicable to both UHF and L-band.
In general, imaging observations are preceded by a delay calibration on the subarray build, done by the telescope operators. This is applied at the correlator and does not need to be solved for again, though it can be a useful sanity check to solve for delays on the bandpass calibrator during reductions.
A typical duration and cadence for bandpass calibrator visits is for 10 minutes every 3 hours.
It is recommended to visit a phase calibrator every 30 minutes for a duration of 2 minutes.
An absolute polarisation calibrator such 3C286 or 3C138 should be visited at least once during the observation if the user wishes to have polarisation angle information. J1939-6342 is effectively unpolarised and can be used to solve for leakage. The gain calibrator can generally be used to obtain more extensive parallactic angle coverage.
Some a priori corrections are generally applied to the correlator before imaging observations are done: key among them is a delay calibration. While the cable delays of the array are stable, small timing offsets can be introduced during synchronisation of the digitizers. After initialising the array the standard operating procedure is to run a short delay calibration observation. During the observation, the noise diode, as well as a bright, well-known source, are used to calculate and apply time-variable solutions for the antenna-based delays.
The delay calibration observation consists of multiple stages: initially, predefined complex gains are applied across the band in the correlator for each antenna; subsequently, a suitable calibrator is observed and simple antenna-based delays are calculated; next, the noise diodes are activated and cross-polarisation delay, as well as phase, is measured for the entire array. The delays are derived and combined by the real-time calibration pipeline. The delays are applied to the data with the exception of the cross-polarisation phases which are stored in the observation metadata and can be applied at a later stage.
Flux and bandpass calibration
Flux calibration requires that the gains for a given gain calibrator which were scaled to a flux density of unity be re-scaled to the true flux density of said gain calibrator. This scaling requires measuring the flux density of the gain calibrator by computing the ratio of the (flux calibrated) gains of the flux calibrators to the uncalibrated gains for a whole observation on the gain calibrator. The scaling factor will be different for each gain calibrator used in the observation.
J1939-6342 is the recommended and preferred MeerKAT flux calibrator at L-band and can be used without a multi-component model. Please see this page for more information on source structure and models to be used, particularly at UHF. J0408-6545 is also used but has more contamination from secondary sources. Due to MeerKAT's wide field of view and sensitivity, there is some structure introduced into the bandpass response by secondary sources in the field. The scale of these effects on the system bandpass ranges from 0.3% to 1.0% at L band (and are far more severe at UHF). The effects can be mitigated to a minor degree through fringe washing with long solution intervals. However, if an accurate bandpass correction is required, it is recommended that a source model be used for the calibrators.
The structure of the MeerKAT bandpass response is stable to within 3% over 3 hours. A typical duration and cadence for bandpass calibrator visit is 10 minutes every 3 hours. However, for spectral line observations, the required time on the bandpass calibrator will depend on the continuum flux density of your target; if the target has a similar flux density to that of the calibrator, the per-channel bandpass solutions will transfer noise to the target spectrum.
Gain and phase stability
The MeerKAT receivers are temperature stabilised, ensuring that the typical amplitude response remains stable to levels well below 2-3% over the span of a horizon to horizon observation. Under typical ionospheric conditions, the L-band system phase is stable to within 1% level over time-spans exceeding 1 hour (Figure 1). It is recommended to visit a phase calibrator every 30 minutes for a duration of 2 minutes. Two minutes is sufficient for any of the calibrators listed in the Observation Planning Tool (OPT).
Figure 1: The accumulative divergence in fractional gain phase as a function of time and antenna distance from the array centre. Most of the gain variation can be attributed to antennae dominated by longer spacings.
Given MeerKAT’s large field of view and sensitivity, contributions from secondary sources in the calibrator field can be significant (see discussion above under Flux and bandpass calibration). Work is currently underway to develop models of our standard calibrator fields. Further information will be added to our pages on MeerKAT L-band and UHF calibrators when available. Note that fits images for L-band calibrators are available, UHF still to follow.
Note that MeerKAT currently does not make use of noise diodes to calibrate system gain or Tsys during standard interferometric imaging observations, though the capability exists to fire the noise diodes in a wide range of timing patterns.