The Caltech-NRAO Stripe 82 Survey (CNSS). II. On-the-fly Mosaicking Methodology

Telescope slew and settle time markedly reduce the efficiency of wide-field multi-epoch surveys for sensitive interferometers with small fields of view. The overheads can be mitigated through the use of on-the-fly mosaicking (OTFM), where the the antennas are driven at a non-sidereal rate and visibilities are recorded continuously. Here we introduce the OTFM technique for the Very Large Array (VLA), and describe its implementation for the Caltech-NRAO Stripe 82 Survey (CNSS), a dedicated five-epoch survey for slow transients at the S band (2–4 GHz). We also describe the OTFSim tool for planning dynamically scheduled OTFM observations on the VLA, the latest imaging capabilities for OTFM in CASA, and present a comparison of OTFM observations with pointed observations. Using the subset of our observations from the CNSS pilot and final surveys, we demonstrate that the wide-band and wide-field OTFM observations with the VLA can be imaged accurately, and that this technique offers a more efficient alternative to standard mosaicking for multi-epoch shallow surveys such as the CNSS and the VLA Sky Survey. We envisage that the new OTFM mode will facilitate new synoptic surveys and high-frequency mapping experiments on the VLA.