Begun in 2001 with a total budget of around $100M, the Expanded Very Large Array (EVLA) project is the only major
upgrade to the VLA undertaken since the interferometer was dedicated in 1980. The goal of this 11-year long project is
to improve all the observational capabilities of the original VLA - except for collecting area and spatial resolution - by
at least an order of magnitude. To achieve this, the 28 VLA antennas have been modernized with new digital data
transmission systems that link to a new, wideband, fiber optic digital LO/IF system, and eight new sets of cooled
receivers are under construction that will offer full frequency coverage from 1 to 50 GHz, with instantaneous bandwidths
up to 8 GHz provided by two independent dual polarization frequency pairs. The new WIDAR correlator provided by
NRAO's Canadian EVLA partner replaced the old VLA correlator in early 2010 and is currently undergoing
commissioning.
The long duration of the EVLA construction project coupled with the need to maintain the scientific productivity and
user base of the telescope obviously precluded shutting down the old array while new infrastructure was built and
commissioned. Consequently, the construction plan was based on the fundamental assumption that the old VLA would
continue to operate as new EVLA capabilities gradually came online; in some cases, additional complexity had to be
designed into new hardware in order to maintain transitional interoperability between the old analog and new digital
systems as the latter were installed and commissioned. As construction has advanced, operations has increasingly had to
coexist side by side with EVLA commissioning and verification. Current commissioning plans attempt to balance
making new EVLA capabilities available to the user community as soon as they have been installed and verified, and
maintaining a stable and robust end-to-end data acquisition and delivery process for the user community.
The Expanded Very Large Array (EVLA) is an international project to improve the scientific capabilities of the Very
Large Array (VLA), an aperture synthesis radio telescope consisting of 27, 25-meter diameter antennas distributed in a
Y-shaped configuration on the Plains of San Augustin in west-central New Mexico. The EVLA's major science themes
include measuring the strength and topology of magnetic fields, enabling unbiased surveys and imaging of dust-shrouded
objects that are obscured at other wavelengths, enabling rapid response to and imaging of rapidly evolving transient
sources, and tracking the formation and evolution of objects in the universe. The EVLA's primary technical elements
include new or upgraded receivers for continuous frequency coverage from 1 to 50 GHz, new local oscillator,
intermediate frequency, and wide bandwidth data transmission systems to carry signals with 16 GHz total bandwidth
from each antenna, and a new digital correlator with the capability to process this bandwidth with an unprecedented
number of frequency channels for an imaging array. The project also includes a new monitor and control system and new
software that will provide telescope ease of use. The project was started in 2001 and is on schedule and within budget.
Scientific observations with the new correlator started in March 2010. The structural modifications that convert the VLA
antennas to the EVLA design were completed in May 2010. The project will be complete in December 2012 when the
last receiver will be installed on an antenna.
The 74 MHz system on the National Radio Astronomy Observatory's Very
Large Array (VLA) has opened a high-resolution, high-sensitivity
window on the electromagnetic spectrum at low frequencies. It
provides us with a unique glimpse into both the possibilities and
challenges of planned low-frequency radio interferometers such as
LOFAR, the LWA, and the SKA. Observations of bright, resolved radio
sources at 74 MHz provide new scientific insights into the structure,
history, and energy balances of these systems. However many of these
scientifically motivated observations will also be critical to testing
the scientific fidelity of new instruments, by providing a set of
well-known standard sources. We are also using the 74 MHz system to
conduct a sky survey, called the VLA Low-frequency Sky Survey (VLSS).
When complete it will cover the entire sky above -30 degrees
declination, at a 5σ sensitivity of 0.5 Jy/bm-1, and a resolution of 80" (B-configuration). Among its various uses, this
survey will provide an initial grid of calibrator sources at low
frequency. Finally, practical experience with calibration and data
reduction at 74 MHz has helped to direct and shape our understanding
of the design needs of future instruments. In particular, we have
begun experimenting with angle-variant calibration techniques which
are essential to properly calibrate the wide field of view at low
frequencies.
Ionospheric phase errors degrade high-resolution radio images below
100 MHz, and they differ significantly from the tropospheric errors
which dominate at high frequencies. The ionosphere is so high
(~400 km) and the VLA primary beam is so wide (~0.2 rad) that
the intersection of the beam with the ionospheric screen is larger
than the "isoplanatic patch" size, a phase coherent region on the
sky. Antenna-based calibration techniques developed at higher
frequencies cannot be used because ionospheric phase errors vary
significantly across the field-of-view of each antenna. This paper
describes the "field-based calibration" technique adopted for the
74 MHz VLA Low--frequency Sky Survey (VLSS) being made with the 10 km
"B" configuration. This technique is useful for a range of array
sizes but fails on baselines longer than the linear size of the
isoplanatic patch, a few 10s of km at 74 MHz. Implications for
designing larger low-frequency arrays are discussed.
The Expanded Very Large Array project has the top-level goal of
enhancing the performance of the Very Large Array by an order of
magnitude or more in all areas: sensitivity, frequency coverge,
spectral resolution, and spatial resolution. The project is being
implemented in two, overlapping phases: Phase I, which began in 2000
and will finish by 2012 addresses all new capabilities except spatial
resolution, and Phase II, which will improve tenfold the spatial
resolution, and which is planned to begin in 2006, and finish by 2013.
Progress in Phase I is very good, with first light and first fringes
having been achieved, and tests of the new hardware and software now
underway. A proposal for funding Phase II has now been delivered to
the National Science Foundation. A critical component of the project
is the new correlator, being designed and built by the Canadian
Herzberg Institute of Astrophysics at the DRAO in Penticton, BC
Canada. This new advanced correlator will be delivered beginning in
late 2008. First shared-risk science with the early portions of the
correlator will be done in late 2007.
The first, serendipitous, radio-astronomical observations by K. Jansky were at decametric wavelengths. However, after the initial pioneering work, long-wavelength radio astronomy was largely abandoned in the quest for higher angular resolution because ionospheric structure was thought to limit interferometric imaging to short (< 5 km) baselines. The long-wavelength (LW, 2 - 20 m or 15 - 150 MHz) portion of the electromagnetic spectrum thus remains poorly explored. The NRL-NRAO 74 MHz observing system on the Very Large Array has demonstrated that self-calibration techniques can remove ionospheric distortions over arbitrarily long baselines. We describe the scientific justification and initial technical design of the Low Frequency Array (LOFAR) -- a fully electronic, broad-band antenna array operating in the 15 - 150 MHz range with a collecting area of 1 km2 at 15 MHz. The longest baselines may be 500 km, providing an angular resolution of 10' at 15 MHz and 1' at 150 MHz. The combination of large collecting area and high angular resolution will enable LOFAR to produce images with sensitivities of order 1 mJy at 15 MHz and 300 (mu) Jy at 150 MHz. As such LOFAR will represent an improvement of 2 - 3 orders of magnitude in resolution and sensitivity over the state of the art. A key operational goal of LOFAR will be solar observations -- both passive imaging and radar imaging. In the latter mode LOFAR will serve as the receiver for bi-static observations of the Sun, with particular emphasis on the imaging of coronal mass ejections. LOFAR will serve as an astrophysical laboratory to study the origin, spectrum, and distribution of the Galactic cosmic ray electron gas and as an instrument to probe the high-redshift Universe.
The National Radio Astronomy Observatory is proposing to upgrade and expand the Very Large Array (VLA). This project, known as the VLA Expansion Project, will transform the VLA into the Expanded Very Large Array (EVLA) -- a new telescope whose observational capabilities will be at least an order of magnitude better than those of the VLA in every key instrumental parameter: sensitivity, frequency flexibility, spectral resolution, and spatial resolution. These improvements will be obtained at less than one third the investment cost of the VLA because the antennas, array design and infrastructure will be retained, while most of the electronics signal transmission system, and signal processing will be replaced with modern systems. The EVLA will be utilized by scientists from around the world for cutting-edge research throughout astronomy, providing unique information in such diverse areas as magnetic fields, cosmic sources in dusty regions, transient phenomena, and the formation of stars and galaxies.
Non-coplanar sampling of the visibility function measured by interferometric arrays leads to difficulties in imaging wide-fields. Unlike the case for co-planar sampling or small fields of view, the relationship between sky brightness and the visibility is not a simple two-dimensional Fourier transform, and so the usual methods of image reconstruction cannot be applied. We describe and analyze some of the many schemes which have been advocated to overcome this problem. The most promising is based upon an observation by Clark that if the sky brightness is thought of as lying on a surface embedded in a three dimensional space, a Fourier relationship does hold.
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