The Allen Telescope Array is a response to one of the most enticing sirens to beckon the SETI community: a major telescope that can be dedicated to the search. Despite the seductiveness of this idea, construction of an instrument designed to meet the requirements of full-time SETI has always foundered on the large costs.
That situation has changed. Thanks to the far-sighted benevolence of many donors, including technologists Paul Allen (co-founder of Microsoft) and Nathan Myhrvold (former Chief Technology Officer for Microsoft), the first 42 elements of the ATA are conducting SETI searches every day of the week.
The instrument, called the Allen Telescope Array, is situated at the Hat Creek Observatory, located in the Cascade Mountains just north of Lassen Peak, in California.
Radio SETI experiments have historically relied on existing radio astronomy telescopes. While this allows such searches to be conducted on quite large instruments (for example, the 305 m Arecibo dish, in Puerto Rico), the amount of telescope time available for the search is necessarily restricted. Project Phoenix, for example, took control of the Arecibo telescope for approximately three weeks in the spring and a similar block of time in the fall. Since observations were only made at night, this amounted to a total of only three weeks of full-time observing annually.
During the period from September 1998 through March 2004, Project Phoenix observed for a total of 100 days at Arecibo. That’s only 5% of the available time. The Allen Telescope Array offers SETI scientists access to an instrument seven days a week, and permits the search of several different targets (primarily exoplanet systems) simultaneously. As a result, the Allen Telescope Array is speeding up SETI targeted searching by a factor of at least 100.
Because of its ability to study many areas on the sky at once, and with greatly improved access to a telescope, the ATA will allow an expansion from Project Phoenix’s stellar reconnaissance of 1,000 stars to a million or more nearby stars over the course of the next two decades.
The fundamental idea behind the Allen Telescope Array was generated during a series of workshops held in 1997 - 1999 in which a group of scientists, engineers, and technologists considered how best to pursue SETI in the coming two decades (the SETI Science and Technology Workshops). The favored scheme was an array of relatively small dishes (antennas), with a pseudo-random arrangement on the ground extending over about 1 km. This provides a very high quality beam shape (the spot in the sky to which the telescope is most sensitive), and thanks to the relatively large number of antennas, also minimizes (unwanted) sensitivity outside the primary beam.
The Allen Telescope Array is optimized to cover frequencies between 500 and 10,000 MHz, which is more than five times the range searched in Project Phoenix. Thanks to a generous donation from Franklin Antonio (co-founder and Chief Scientist at Qualcomm) the ATA receivers are being replaced with new systems that will cover 1,000 to 15,000 MHz, and offer improved sensitivity and greater reliability.
Software for the search, dubbed SonATA (SETI on the ATA) is a software-only search system implemented with enterprise servers donated by Dell and Intel. Detailed descriptions of the SonATA system that began operation in 2010 can be found here.
By building the new telescope as an array, several major advantages can be realized. To begin with, many “pixels” can be generated on the sky at once. Rather than looking at only one star at a time, as the Arecibo telescope and its kin are constrained to do, several stars can be examined simultaneously. This again speeds up the process of stellar reconnaissance. In some cases, it may be desirable to sacrifice spectral resolution (typically 1 Hz) in order to gain additional pixels. In other words, one can trade amount of sky covered for sensitivity to very narrow-band signals. Depending on the type of signal we expect, this might be a judicious trade-off.
In addition, it is easy to expand an array by merely buying additional antennas and connecting them into the system. Single, large dish antennas are not amenable to such simple improvement. The bottom line is compelling. Because of its ability to study many areas on the sky at once, with more channels and every day of the week, the Allen Telescope Array will be able to check out a truly significant sample of the cosmic haystack. This is not an incremental step forward: the Allen Telescope Array will increase the stellar reconnaissance by orders of magnitude. It is a very large step for SETI research.
The design for the Allen Telescope Array’s antennas – so clearly different from the type of “tripod” arrangement found in many backyard satellite dishes – incorporates so-called offset optics because sometimes, as in football, going to the side can reduce interference. The antennas use what’s known as an offset Gregorian system. A secondary mirror bounces incoming radio signals collected by the large (6.1 meter diameter) primary reflector back to the feed antenna (hidden from view by a fabric shroud) where they are amplified and sent on their way to the control buildings.
By introducing a secondary mirror and a surrounding shroud, the antenna is less likely to pick up noisy radiation from the (relatively hot) ground surrounding the telescope. Moving the reflector assembly off-center minimizes the chance of terrestrial signals bouncing off the antenna structures and interfering with our study of cosmic emissions. This offset design has also been used for the 100 m Robert C. Byrd Telescope, which is in operation in West Virginia.