The implementation of new technologies has permitted astronomers to achieve enhanced spatial resolution from the ground at progressively shorter wavelengths, first in the radio region of the spectrum and, during the coming decade, at infrared and optical wavelengths. Recently, European astronomers have demonstrated impressively that adaptive optics can be successfully applied to astronomy. Adaptive optics permits diffraction-limited imaging over the full aperture of large telescopes. The pioneering U.S. effort in this area was cut back because of budget reductions at the NSF. The committee believes this program should be renewed so that the United States can take a leadership role in adaptive optics. Hence the committee has established adaptive optics as its highest-priority, moderate ground-based program (Chapter 1).
As discussed in more detail in Chapter 4, optical and infrared interferometry can achieve high spatial resolution over baselines longer than the aperture of a single mirror. The technology needed for the application of optical interferometry to faint sources was carried out on some bright stars in the 1980s. The Department of the Navy supported the building of two interferometers on Mt. Wilson. The NSF also established a number of modest programs. These demonstration projects deserve continued support.
The Very Large Array (VLA), recommended by the Greenstein Committee (NRC, 1972), operated with great power in the 1980s, annually providing observations for about 600 astronomers and data for over 200 research papers. During the 1980s, new receivers and computing techniques enhanced the power of the VLA to more than 10 times that of the original instrument, all without major changes in telescope hardware. However, the declining NSF budget has caused major problems at the VLA and its parent National Radio Astronomy Observatory (NRAO), as discussed by the Radio Astronomy Panel in the Working Papers (NRC, 1991) and summarized briefly here. A decade of diminishing funds has led to deferred maintenance that directly affects the reliability of the array. For example, the railroad track system over which the antennas are moved has deteriorated (Figure 3.1), making it difficult to configure the VLA for observations at different angular resolutions. Some of the vital instrumentation of the VLA is out of date: needed are low-noise receivers, fiber-optic transmission lines, a broadband digital correlator, and more powerful computers for data analysis. These items could improve the sensitivity of the instrument by up to a factor of 10, improve the frequency coverage and spectral resolution, and increase the maximum image size.