Radio astronomy is uniquely coupled to exponential trends in computation because the optics (cross-correlation, beamforming, and imaging) and spectrometry (i.e. channelization) can now be done digitally. Inexpensive analog-to-digital converters (ADCs) can sample signals from large numbers of antennas and graphics processing units (GPUs) allow us to coherently process wide-field radio data in real time, motivating large-N aperture arrays at moderate cost. We describe the 256-antenna Coherent All-Sky Monitor (CASM-256), a dense aperture array operating at 375-500\,MHz, currently being deployed at the Owens Valley Radio Observatory (OVRO) in Big Pine, California. The large field-of-view (FoV∼104\,deg2) and point-source sensitivity of CASM-256 will allow it to detect local Universe fast radio bursts (FRBs). The nearby sample is ideal for unveiling the physical origin of FRBs, measuring the baryonic content of nearby galaxy halos, and discovering prompt multi-wavelength and multi-messenger counterparts to FRBs. CASM will search for fast transients in the Milky Way such as FRB analogs, pulsar giant pulses, and the new source class known as long-period radio transients. We describe the instrument and present on-sky data from the first two dozen antennas, including an operational real-time GPU based FRB search pipeline. We emphasize the scalability of the concept and describe paths to a future CASM array with tens of thousands of antennas that could detect one million FRBs.
@article{arxiv.2604.13903,
title = {The 256-antenna Coherent All-Sky Monitor},
author = {Liam Connor and Vikram Ravi and Pranav Sanghavi and Vishnu Balakrishan and Luke Chung and Saren Daghlian and Liam Dunn and Anthony Griffin and Charlie Harnach and Mark Hodges and Andrew Jameson and Michael Gutierrez and Calvin Leung and Mei Lin and Advait Mehla and Obinna Modilim and Nimesh Patel and Kendrick Smith and Lingzhen Zeng},
journal= {arXiv preprint arXiv:2604.13903},
year = {2026}
}