Very Large Array (VLA)/
Expanded Very Large Array (EVLA)

The Very Large Array (VLA) is a 27-element interferometer whose 25 m parabolic antennas can be deployed in four principal configurations to synthesize diffraction-limited apertures having diameters d ≈ 1, 3.4, 11, and 36 km (Figure 4.2.1). It operates full-time with seven discrete frequency bands ranging from l = 1 m to 6 mm (0.3 GHz to 50 GHz) and part-time at l = 4 m (74 MHz). It’s imaging resolution of 50 milliarcseconds at l = 7 mm (43 GHz) is comparable with the highest resolution of the Hubble Space Telescope. The VLA was dedicated in 1980. Since the mid-1980s it has produced an average of 170 refereed papers per year (see Appendix A for detailed statistics since 1998), a rate of productivity that continues even as the VLA celebrates the 25th anniversary of its dedication.

Over the years, the VLA has added significant capabilities including:

• The λ = 3.6 cm (8.4 GHz) receiving system funded by NASA in the late 1980s.
• The recently completed λ = 7 mm (43 GHz) receiving system jointly funded by Mexico, Germany, and the NSF.
• Beam-forming capability for observing as an element of a VLBI array.
• A fiber-optic link to the Pie Town VLBA antenna, providing an improvement by a factor of two in resolution

However, the fundamental data-processing electronics of the VLA remained essentially unchanged for 25 years, preventing it from reaching its current potential as the world’s most versatile radio telescope. The Expanded VLA (EVLA) will revolutionize its ability to make high-sensitivity and high-resolution images, ensuring that NSF centimeter-wave facilities will stay at the forefront of discovery in modern astronomy for the next two decades. With this potential in mind, the 2001 National Academy of Sciences decadal survey recommended the EVLA as the second-ranked major ground-based project for the current decade:

The EVLA — the revitalization of the VLA, the world’s foremost centimeter-wave radio telescope—will take advantage of modern technology to attain unprecedented image quality with 10 times the sensitivity and 1000 times the spectroscopic capability of the existing VLA. The addition of eight new antennas will provide an order-of-magnitude increase in angular resolution. With resolution comparable to that of ALMA and [JWST], but operating at much longer wavelengths, the EVLA will be a powerful complement to these instruments for studying the formation of proto-planetary disks and the earliest stages of galaxy formation.—Astronomy and Astrophysics in the New Millennium, National Academy Press (AANM 2001)

The EVLA preserves the large existing investment in VLA antennas and civil infrastructure, and it adds new wideband receiver systems, a state-of-the-art correlator, a fiber-optic data-transmission system, digital electronics, and a new on-line control system to yield a new instrument with the following improvements over the present VLA:

• Continuum sensitivity improvements from up to a factor of 5 for λ > 3 cm (< 10 GHz) to more than 20 between λ = 3 and 0.6 cm (10 and 50 GHz).
• Operation at any wavelength from λ = 0.6 to 30 cm (1.0 to 50 GHz) yielding two pairs of signals, each pair with opposite polarizations and up to 4 GHz bandwidth, independently tunable to any frequency within any given band.
• A flexible new correlator which will provide over 16,384 frequency channels, process the full EVLA bandwidths, and give frequency resolution better than 1 Hz if necessary.