S-band capability and status
- Sharmila Goedhart
- Fernando Camilo
- Sean Passmoor
Overview
The Max-Planck-Institut für Radioastronomie (MPIfR) and the Max-Planck-Society, in partnership with SARAO, have equipped MeerKAT with S-band receivers to complement the UHF and L-band receiver systems, providing coverage in the 1.75 - 3.5 GHz frequency range. These receivers are now available for science in shared-risk mode.
Basic characteristics of the system are known, and limited list of calibrators have been characterised. Investigation is still under way regarding the following:
expanding the S-band calibrator list
off-axis polarisation performance and calibration
On the whole, the system is capable of high sensitivity imaging in Stokes I (see commissioning reports below). We note that polarisation leakage is significant in the upper half of the band and at this stage polarimetric observations in the S3 and S4 sub-bands are not advised - see discussion below.
High dynamic range imaging will require both self-calibration and direction-dependent corrections due to pointing irregularities which are enhanced at higher frequencies.
Sub-bands
While the receiver and digitizer bandwidths are 1.75 GHz, the MeerKAT correlator can only process 875 MHz at a time. Digital band selection allows the band to be centred at 2.2, 2.4, 2.6, 2.8 or 3.1 GHz.
Sub-band | Frequency range |
|---|---|
S0 | 1750 - 2625 MHz |
S1 | 1968 - 2843 MHz |
S2 | 2187 - 3062 MHz |
S3 | 2406 - 3281 MHz |
S4 | 2625 - 3500 MHz |
Switching between sub-bands requires 'rebuilding' of the sub-array. Including time for delay calibration and subsequent operator quality assurance checks, this process takes about 15 - 20 minutes. Note that if wider frequency coverage is required in the same time slot, but sensitivity is not as vital, multiple sub-arrays with a subset of antennas can be run simultaneously in different bands.
Sensitivity
The mean SEFDs per antenna are given in Figures 1 and 2 below. Note that these are preliminary results, yet to be refined by gain curves.
Due to logistical issues, availability of the S-band receivers is somewhat lower than that of UHF and L-bands. Users should assume a lower limit of 56 antennas for sensitivity estimates.
Commissioning results
Antenna pointing
Many of the earlier S-band commissioning reports mention antenna pointing as a possible cause of gain variations resulting in imaging artefacts and gain/flux scale transfer errors. This is an active area of work currently and was improved over the course of 2023.
The current mean blind pointing of the antennas is 40" at night and 50" during the day. Referenced pointing procedures are currently under development.
Users are advised to inspect the SDP calibration reports (or their own preliminary gain solutions) for high scatter in gain solutions before selecting a reference antenna.
Flux scale
The Reynolds (1994) and Partridge et al. (2016) flux scales were compared to the Perley-Butler (2017) flux scales in this report. The derived flux errors are generally less than 2% but we note large gain variations between observations in the S3 and S4 sub-bands, likely due to pointing errors. Work is currently under way to improve mechanical pointing and we expect that the situation has improved since these data were obtained.
Noise-limited imaging
An observation of the ‘DEEP2’ field was done in May 2021 in the S4 sub-band using 42 antennas. The on-source time was 7.5 hours. No significant issues were noted and the rms noise appears to be as expected (see Fig. 3).
Astrometry
A test observation of the COSMOS field at S4 compared to the VLA 3 GHz COSMOS survey shows astrometric offsets are ∼2% of a synthesised beam width at worst (I. Heywood priv. comm).
High dynamic range imaging
Two observations were conducted on the field containing the “Corkscrew” galaxy, an extremely bright object with complex structure (see Fig. 4). Strong artefacts were initially found near the bright core in the first observation.
Better results were found when centering the pointing position on the bright core. This reduces gain variations that would be introduced by pointing offsets. Direction dependent calibration produced significant improvement in image quality and dynamic range. More details can be found in this report.
Spectral line capabilities
Note that only the ‘wideband’ 32K channel mode is available at this time.
Test observations were made of three CH hyperfine-structure (hfs) transitions of CH at 3.3 GHz (in the S4 sub-band) towards Sgr B2, G09.622+0.19 and W51. Good agreement was found between the MeerKAT and previous VLA observations in both flux density and measured velocities.
Observations of transients
The ThunderKAT project conducted an observation of an X-ray binary and concluded that “Overall, the imaging capabilities at this high frequency with MeerKAT are impressive and although the targeted field is complex, the low RFI levels at this part of the band help to reach high sensitivity at ∼3 arcsec angular resolution.”
RFI
The dominant source of RFI appears to be from the Globalstar satellite constellation at 2483.5 - 2495.0 MHz (in the S3 sub-band), but this affects mainly the short baselines. Baseline dependent flagging should be used.
We are in the process of building up statistics on other less frequent transmissions, and will update the RFI page when we have more information.
Polarimetry
While plausible results have been obtained on a test source (see section V of the report on the Corkscrew Galaxy), we note that there is potentially high leakage in the S3 and S4 bands.
Users interested in doing sensitive polarimetric measurements, particularly across an extended area, are advised to avoid the higher frequency sub-bands. See De Villiers (2023) for more technical details.
Acknowledgments