The Allen Telescope Array (ATA) is a centimeter-wavelength radio telescope producing unique science in the areas of radio surveys and transient source detection. The ATA is the first radio telescope designed for commensal observing; it will simultaneously produce sensitive SETI (Search for Extraterrestrial Intelligence) surveys as well as large area continuum and spectroscopic surveys for conventional radio astronomy. The science of the ATA is enabled by a number of innovative technical developments. The ATA may serve as a prototype for the US design of the Square Kilometer Array (SKA), a next generation radio telescope currently in the planning phase. Located in Hat Creek, CA, formerly the site of the Berkeley Illinois Maryland Association (BIMA) millimeter-wavelength array, the ATA currently consists of 42 offset Gregorian antennas with continuous frequency response from 0.5 to 11.2 GHz, with the possibility of future upgrades to more antennas.
,'' provided financial support for a fraction of the cost of operating and producing science from the ATA. Other NSF awards and public and private sources of funding contributed to the overall support of operations and science. The ATA was shut down due to lack of funds in April 2011. The ATA has had a world-wide impact in astronomical discovery and radio telescope design. As the pioneer of the large-N-small-D (LNSD) concept, the ATA performed scientific investigations that have set the global standard for radio astronomy. Key achievements are: Establishment of the LNSD as the best system design for radio telescopes; A unique antenna design lowering costs and improving performance, now mirrored in new telescopes around the world; The ultra-wide band feed that makes the entire radio spectrum instantaneously available; Rapid design and deployment of new digital instrumentation in response to urgent scientific questions; The first demonstration of commensal observing, the ability to carry out multiple scientific programs simultaneously; Ground-breaking investigations of the discovery space for the most energetic and violent objects in the Universe including black holes and neutron stars through surveys of transient and variable radio sources;and, Detailed investigations of the structure and evolution of galaxies, demonstrating the important roles of magnetic fields on small and large scales. A diverse set of international facilities owes a significant conceptual and technological debt to the ATA. Fundamentally, the ATA showed the way in which the digital revolution transforms telescope design, leading to the LNSD. The technologies associated with the LNSD concept and the science that they enable have been adopted by a wide range of emerging telescopes. In some cases, these designs strongly resemble the ATA, such as the MeerKAT telescope in South Africa, and the Square Kilometer Array (SKA), a proposed multi-billion dollar international facility. For other telescopes, the emphasis on large numbers of antennas, wide field of view, rapid development of digital technology, and flexible digital signal processing all echo the ATA concept. Scientifically, the LNSD concept drives arrays towards surveys of very large areas of sky. The ATA has been the fore-runner of the new class of radio telescopes dedicated to large area surveys of the sky and complete use of the radio spectrum. Our science has been in three key areas of discovery: the time-domain Universe, the structure and evolution of galaxies, and wide-field imaging. The largest impact has been in time-domain surveys that probe the violent and variable Universe on timescales of milliseconds to decades and explore the properties of black holes, neutron stars, supernovae, and other energetic objects. These surveys caught massive eruptions from a black hole binary and a white dwarf binary in the Milky Way, searched the entire Andromeda Galaxy for giant pulses, and systematically explored variability of distant, faint radio sources. The importance of the ATA and its impact is demonstrated by the quality of its scientific and technical collaborators. They include researchers from leading institutions around the world: Caltech, Cornell University, Jet Propulsion Laboratory (JPL), Los Alamos National Laboratory (LANL), and Sydney University, among others. A total of 18 refereed papers based on ATA data have been published; additional papers based on data already obtained are forthcoming. Six US patents have been registered for ATA inventions. A total of 88 technical memos have been published and made available to the community. The legacy of the ATA at UC Berkeley is a deep reservoir of knowledge and skill in areas of telescope design and construction, digital signal processing, telescope operations, and radio astronomy surveys. Three UC Berkeley students will receive Ph.D.s based on their work with the ATA. Numerous undergraduates have participated in small and large roles, many going on to graduate careers in astronomy. ATA staff and students have been frequent participants in national and international conferences, promoting the achievements of the ATA. Current Berkeley radio telescopes, the Precision Array for Probing the Epoch of Reionization (PAPER) and the Combined Array for Millimeter Astronomy (CARMA), both owe part of their success to technical developments for and personnel trained by the ATA. The Radio Astronomy Laboratory at UC Berkeley stands poised to transform the ATA experience into new endeavors of discovery.