Low-frequency lessons from the 74-MHz Very Large Array

Abstract

The 74 MHz system on the National Radio Astronomy Observatory's Very Large Array (VLA) has opened a high-resolution, high-sensitivity window on the electromagnetic spectrum at low frequencies. It provides us with a unique glimpse into both the possibilities and challenges of planned low-frequency radio interferometers such as LOFAR, the LWA, and the SKA. Observations of bright, resolved radio sources at 74 MHz provide new scientific insights into the structure, history, and energy balances of these systems. However many of these scientifically motivated observations will also be critical to testing the scientific fidelity of new instruments, by providing a set of well-known standard sources. We are also using the 74 MHz system to conduct a sky survey, called the VLA Low-frequency Sky Survey (VLSS). When complete it will cover the entire sky above -30 degrees declination, at a 5σ sensitivity of 0.5 Jy/bm^-1, and a resolution of 80" (B-configuration). Among its various uses, this survey will provide an initial grid of calibrator sources at low frequency. Finally, practical experience with calibration and data reduction at 74 MHz has helped to direct and shape our understanding of the design needs of future instruments. In particular, we have begun experimenting with angle-variant calibration techniques which are essential to properly calibrate the wide field of view at low frequencies.