Subarcminute resolution imaging of radio sources at 74 MHz with the Very Large Array

Abstract

A new observing system operating at 74 MHz and providing an angular resolution of 20 sec is currently being tested at the Very Large Array (VLA). The system comprises a prime-focus dipole feed and amplifier installed on eight of the VLA's 25 m antennas. Although the system is of low efficiency, it works well for imaging strong sources (S approximately greater than or equal to 200 Jy) on long baselines (greater than 5 km) since self-calibration has sufficient signal to noise to remove phase errors on the short times scales (approximately 10 s) characteristic of ionospheric variations. We present maps of a few strong, well known radio sources imaged in most cases with unprecedented anguar resolution. For weaker sources, the ionospheric phase variations do not allow the increased integration times required for successful self-calibration. In order to overcome this, we have developed a procedure of dual-frequency ionospheric phase referencing utilizing simultaneous observations at 74 and 330 MHz to 'unwind' the effects of the ionosphere before self-calibration. We will discuss the procedure, which should be applicable to any future low frequency (less than 100 MHz), long baseline (greater than 5 km), ground-based interferometer which can obtain simultaneous, higher frequency measurements, and show examples of its applications.