The MeerKLASS On-the-Fly continuum survey: pipeline design and validation

The MeerKAT Large Area Synoptic Survey (MeerKLASS) is designed to map large areas of the Southern sky for cosmology using the single-dish HI intensity mapping (IM) technique, while simultaneously delivering a wide, high angular-resolution interferometric survey. We present the design and first results of the MeerKLASS On-the-Fly (OTF) continuum data, which exploits the visibilities recorded during fast, constant-elevation scans. This observing mode enables fast commensal imaging over several hundred of square degrees on a nightly basis. We describe the OTF survey strategy and pipeline, focusing on handling challenges introduced by the current MeerKAT fixed-delay correlation observing mode, which causes decorrelation (smearing). We implement a correction scheme based on time-dependent phase rotation, direction-dependent PSF modeling, and wide-band faceted deconvolution with \texttt{DDFacet}. Using UHF-band and pilot L-band data, we demonstrate the recovery of high-quality 2-second snapshot images and deep mosaics over hundreds of square degrees. After smearing correction we are able to achieve a resolution of $23.3$arcsec and $14$ arcsec with an rms sensitivity of $35 μ{\rm Jy,beam}^{-1}$ and $ 33 μ{\rm Jy,beam}^{-1}$ in the UHF and L-band respectively. The full survey will cover $10,000 , {\rm deg}^{2}$ at 544-1088 MHz, and after the delay tracking fix implemented we expect to reach $\sim 25 μ{\rm Jy,beam}^{-1}$ at $14$ arcsec resolution. The continuum OTF data products will support diverse science goals, including galaxy and AGN evolution, diffuse cluster emission, large-scale structure and cosmology, rotation-measure synthesis, and transient searches. MeerKLASS-OTF thus establishes an efficient path to wide-area commensal surveys with MeerKAT and provides a key technical precursor for SKA-Mid.

💡 Research Summary

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The paper presents the design, implementation, and validation of the MeerKLASS On‑the‑Fly (OTF) continuum imaging pipeline, a commensal survey that simultaneously delivers high‑resolution interferometric data while performing single‑dish H I intensity‑mapping observations with MeerKAT. Because the survey adopts a fast, constant‑elevation scanning strategy optimized for the stability requirements of the auto‑correlation (single‑dish) data, the correlator is operated in a fixed‑delay mode. This introduces a time‑dependent phase error (smearing) that degrades the interferometric visibilities, especially over the wide fields and broad frequency coverage (544–1088 MHz) required for the survey.

To overcome this, the authors develop a three‑stage correction scheme: (1) a time‑dependent phase rotation applied to each 2‑second snapshot to align the recorded visibilities with the true geometric delays; (2) a direction‑dependent point‑spread‑function (PSF) model that captures the anisotropic beam distortion caused by smearing; and (3) wide‑band, faceted deconvolution using DDFacet, which jointly solves for multi‑scale source structure and spectral variation across the band. The pipeline also incorporates standard RFI flagging, band‑pass and flux calibration using noise‑diode injections, primary‑beam weighting, and mosaicking of the 2‑second snapshots into deep images.

The authors validate the pipeline on both UHF‑band (544–1088 MHz) and pilot L‑band (856–1712 MHz) data. Before correction, images suffer from blurred resolution (≈30″) and rms noise of 60–80 µJy beam⁻¹. After applying the smearing correction, the UHF data achieve a resolution of 23.3″ with an rms of 35 µJy beam⁻¹, while the L‑band data reach 14″ resolution and 33 µJy beam⁻¹ rms. These results meet the survey’s target of ~14″ resolution and approach the desired 25 µJy beam⁻¹ sensitivity. The pipeline can combine the 2‑second snapshots from rising and setting scans to produce uniform coverage across the hexagonal footprint of each observation block, enabling the survey to map 300–600 deg² per night.

The paper also situates MeerKLASS‑OTF within the landscape of southern‑hemisphere radio surveys, highlighting its unique combination of low frequency (UHF), high angular resolution, and large sky coverage. Compared with existing projects such as GLEAM, EMU, and VLASS, MeerKLASS‑OTF will fill a niche by providing sub‑15″ images over 10 000 deg² with µJy‑level sensitivity. The authors outline the data‑release plan: calibrated visibilities, continuum images, source catalogs, and smearing‑correction metadata will be made publicly available in stages, with full‑survey products expected after the 2028 observing window.

Finally, the authors discuss the upcoming implementation of a delay‑tracking fix that will eliminate the need for post‑hoc smearing correction, allowing the survey to achieve its nominal 25 µJy beam⁻¹ rms at 14″ resolution across the entire footprint. The MeerKLASS‑OTF pipeline thus serves as a technical precursor for SKA‑Mid wide‑area commensal surveys, demonstrating that fast, constant‑elevation scanning can be reconciled with high‑fidelity interferometric imaging through sophisticated calibration and imaging techniques.