The VLA has transformed many areas of radio astronomy with a powerful combination of angular resolution and brightness sensitivity. Even so, fifteen years of research with the VLA have also exposed the many ways in which its limitations direct, or bias, our observations. Depending on the field of study, we may be restricted to objects that are unusually luminous examples of their class, or unusually nearby, or not too extended, or in a particular redshift range, because of limits to the VLA design that were accepted in the 1970's. Many of these limits can be greatly relaxed today. A second transformation of the scientific capabilities of the VLA is possible if it could be returned to the state of the art in sensitivity, frequency coverage, angular and spectral resolution. An enhanced VLA could be a hundred times faster, several times more frequency-agile, and fifty times better at resolving details, for only a few tens of per cent of the replacement cost of the instrument.
For example, many observing programs are limited (or prohibited!) by sensitivity. The VLA's intermediate-frequency transmission system was designed with a maximum bandwidth of 100 MHz. This limits the sensitivity, particularly above 2 GHz, where wider bandwidths are permitted by the antennas, the feeds, and the interference conditions. These higher frequencies are very attractive for studies of thermal (stellar and circumstellar) sources in the Milky Way, of young supernovae in other galaxies, of polarimetry of the jets from active galactic nuclei, of the dense magnetized media in gas-rich clusters of galaxies, and for examining radio ``quiet'' source populations at high redshifts. Fiber optic signal transmission could increase the maximum bandwidth to 2 GHz, enabling a huge leap in the VLA's capability in all such astrophysical arenas.
Similarly, the VLA correlator is based on a custom EGL circuit that was the state of the art in the 1970's. Modern designs allow much greater spectral resolution, larger bandwidths, and increased flexibility. By reaching the state of the correlator art again in the 1990's, we could, for example, allow complete surveys of the neutral hydrogen gas in the Universe out to 0.1, avoiding any bias in present surveys of the local structure that are based on cataloging galaxies. We could also use an expanded correlator to exploit the sensitivity of broad-band systems to image wide fields of view in the continuum at lower frequencies, e.g., using bandwidth synthesis to image nearby galaxies and to inventory their stellar and interstellar emissions with less bias toward the brightest or most compact features.
The VLA antennas are useful to 45 GHz, and we now have experience with holography to optimize their performance. This increases interest in outfitting the VLA more completely at frequencies above 20 GHz, where we can explore thermal processes around galactic stars, image the structures of protoplanetary disks, detect young supernovae in nearby galaxies, and study CO in galaxies at high redshifts.
The u-v coverage of the VLA was optimized for the astrophysical purposes (and the computing context) of the 1970's. Computing limitations, both in software and in hardware, discouraged imaging and deconvolving large fields of view, especially at high angular resolution. These limitations have been reduced to the point where many wide-field and/or high-resolution experiments could now be supported if the VLA had the requisite inner and outer u-v coverage. Whether mosaicing large-scale galactic features or entire external galaxies, or making high-resolution images of the interior structures of star-forming regions, circumstellar winds, young supernovae, or jets from galactic X-ray sources and AGNs, we may now be limited by the available u-v coverage, not by our ability to compute the images.
Finally, some VLA subsystems were planned but never built for lack of development (infrastructure) funding. The 2.4 GHz system, which has always been of interest for planetary radar experiments and for Faraday depth studies of galactic and extragalactic sources, is now of increasing interest to all continuum projects as RFI from mobile communications systems proliferates at lower frequencies.
It has therefore been clear for some time that the VLA could be made a much more powerful tool for astrophysics at a moderate cost relative to the original investment (in today's dollars). Ideally, its construction would have been followed by funding a timely inflow of new technology into its infrastructure. This would have allowed for a steady expansion of its scientific capabilities. But in fact the operating budget has not kept pace with inflation, so only a few high-priority but incremental improvements have been possible, some with non-NSF funds. The imbalance between our technical ability to increase the VLA's scientific productivity and our capacity to fund technical improvements has grown steadily. This imbalance may only worsen in future, absent an explicit proposal to the NSF for a major re-capitalization of the VLA hardware.
This document reviews the technical possibilities for enhancing the VLA, and their scientific potential, as a first step toward such a proposal.