Background on Long Wavelength Astronomy

Even though radio astronomy began at 20 MHz with Karl Jansky, until recently ionospheric effects severely limited the angular resolution and sensitivity of all existing low-frequency telescopes. Other barriers included radio frequency interference (RFI), the need for non-coplanar-array imaging, and other problems exacerbated by past limitations in computational power. As a result, low-frequency (ν < 100 MHz) imaging was largely abandoned in the race for higher angular resolution and sensitivity.

Today this region remains one of the most poorly explored regions of the electromagnetic spectrum despite its great scientific potential. It is a region that favors studies of non-thermal and coherent emission processes, and provides an intrinsic link to shock physics, high-energy phenomena, and the high-red-shift Universe. It can provide unique insights into the interaction of thermal and non-thermal sources through absorption and scattering processes, and the intrinsically large field of view and high surface-brightness sensitivity are often of great advantage.

During the 1990s NRL and NRAO worked together to develop a "Phase 0" in the goal of creating a high resolution, high sensitivity long wavelength telescope. Phase 0 consisted of adding a 74 MHz capability at the VLA. The system was a narrow-band, modest implementation of a much more sensitive, broad-band system originally envisaged in VLA Technical Memorandum #146 (Perley and Erickson 1984). That earlier vision had been motivated by the recognition that the then new technique of self-calibration might be capable of lifting the ionospheric limitation on baseline length. That barrier had restricted the aperture size of previous low frequency telescopes to < 5 km, thereby greatly restricting their angular resolution, and because of confusion, their sensitivity as well.

Figure 1: (a) physics of shock- and (b) pulsar-powered supernova remnants, emission from (c) merger-driven cluster relics & halos and from (d) AGN-powered radio galaxies.

An initial 8-antenna system developed at the VLA was very successful, being the first low-frequency interferometer to "break the ionospheric barrier" (Kassim et al. 1993). It successfully demonstrated that self-calibration could, at least to first order, remove ionospheric effects and permit imaging on long baselines (>5 km). Its reliance on an over-determined problem in which antenna-based corrections to ionospheric phase distortions could be readily extracted from the self-calibration process worked well at the VLA. The required antenna-based phase corrections were derived from simultaneously obtained 330 MHz data that utilized all 27 VLA antennas and possessed intrinsically much greater signal to noise. Using this trial system, several of the best known sources in the sky were resolved and imaged for the first time (Fig. 1), and a number of unique scientific results were extracted. At the same time as this system was being developed, solutions to key challenges common to all low frequency interferometer observations, such as RFI-excision and wide-field imaging, were being developed on the VLA 330 MHz system that NRL had also worked with NRAO to develop during the 1980s.

Figure 2: The sensitivity and resolution of the 74 MHz VLA compared to other long wavelength instruments.

Based on this success, NRL obtained additional funding to build the receivers and to work with NRAO to extend the 74 MHz system to all 27 antennas of the VLA. When the techniques developed at 330 MHz, aided by the ongoing revolution in computational power, were applied to the completed 27-antenna 74 MHz data stream, it quickly demonstrated the ability to map thousands of sources, demonstrating a significant leap forward compared to past capbilities. In fact the full system made self-calibration so much more robust that the previous prerequisite for "phase-transfer" from simultaneous 330 MHz observations was no longer required. As a result, the 74 MHz VLA system is now by far the most powerful interferometer in the world working below 150 MHz (Fig. 2), and has attracted a wide variety of scientific projects in the areas of solar system (planetary emission, solar bursts), Galactic (supernova remnants, ISM), and extragalactic (clusters, radio galaxies) astrophysics. For more information on our scientific results, please see our low frequency publication list. An ongoing major project is the VLA Low Frequency Sky survey ( VLSS ) a 74 MHz compliment to the successful NVSS 20 cm VLA sky survey.

The full 74 MHz VLA system has been available to the general scientific community since 1998, and has a growing international user community conducting unique observations in many different areas of astrophysics. Many of the technical innovations developed during the course of its development have also had tangible benefits for 330 MHz, and higher frequency observations, as well. In 2002 a 74 MHz receiver was added to the Pie Town VLBA antenna, and successful images synthesized from baselines up to 73 km represented another major milestone in long wavelength radio astronomy. While the improvement in resolution afforded by the expansion to Pie Town is impressive, the restriction to a narrow band and the continued poor sensitivity relative to the higher frequency VLA systems, reveal that future significant steps forward require development of a new instrument, as discussed further in the next section.