Very Long Baseline Array (VLBA)

The Very Long Baseline Array (VLBA) is a transcontinental interferometer consisting of 10 identical 25 m antennas on baselines up to 8,000 km (see Figure 4.3.1). The VLBA can resolve high-brightness sources as small as 1 pc and measure their positions with 0.1 pc (or better) accuracy anywhere in the Universe. The VLBA was the leading ground-based project recommended by the 1980 decadal survey of the National Academy of Sciences, and it was dedicated in 1993. It relies on the techniques of Very Long Baseline Interferometry (VLBI) first demonstrated nearly 40 years ago. During that 40-year period, the VLBA has been the only facility designed specifically for and dedicated to full-time astronomical VLBI. The design includes:

• Ten identical antennas.
• A distribution of antenna locations specifically chosen to optimize aperture-plane coverage and to take advantage of the large additional collecting area of the VLA.
• Routine operation of the array on an unattended basis, with minimal staff (two site technicians) required to perform routine maintenance at each antenna location.
• A dedicated data processor (correlator) matched to the characteristics of the antennas and their recording systems.
• An overall maintenance/operations plan that exploits economies of scale using VLA staff located in Socorro, New Mexico.

The VLBA covers λ = 28 cm to 3 mm (1.2 GHz to 96 GHz) in eight discrete bands plus two narrow sub-gigahertz bands, including the primary spectral lines that produce high-brightness maser emission. Its unique high-frequency capabilities enable the VLBA to image features within tens of Schwarzschild radii of the massive black holes in the Galactic Center and in the radio galaxy M87. The VLBA can be scheduled dynamically on a full-time basis, although about 30% of its observing time is allocated on fixed schedules to accommodate the requirements of other co-observing telescopes. Since the data from the VLBA are transported via magnetic media, its observing time has been restricted to about 50% to 55% of the hours in a year by limited tape-recorder capacity. Observing time will increase to 65% or more by the end of 2006, once the full transition is completed to direct recording on transportable high-capacity computer disks.

The continuum sensitivity can be improved by a factor of more than 5 by adding the GBT and the phased VLA to the VLBA.