# Allen Telescope Array

### Allen Telescope Array

 Organization The Allen Telescope Array (ATA-42), October 11, 2007 SETI Institute Radio Astronomy Laboratory Hat Creek Radio Observatory 60 cm–2.679 cm (0.5 to 11.2 Gigahertz (GHz)) 6.1 × 7.0 metres (20.0 ft × 23.0 ft) 2.4 metres (7 ft 10 in) seti.org

The Allen Telescope Array (ATA), formerly known as the One Hectare Telescope (1hT) is a radio telescope array dedicated to astronomical observations and a simultaneous Search for Extraterrestrial Intelligence (SETI).[1][2] The array is situated at the Hat Creek Radio Observatory, 290 miles (470 km) northeast of San Francisco, California.

Originally developed as a joint effort between the SETI Institute and the Radio Astronomy Laboratory (RAL) at the University of California, Berkeley (UC Berkley) with funds obtained from an initial US$11.5 million donation by the Paul G. Allen Family Foundation, the project completed the first phase of construction and become operational on 11 October 2007 with 42 antennas (ATA-42), after Paul Allen (co-founder of Microsoft) pledged an additional$13.5 million to support the construction of the first and second phases.[3][4]

## Overview

The ATA is a centimeter-wave array that pioneers the Large-Number Small-Diameter concept of building radio telescopes. Compared to a large dish antenna, large numbers of smaller dishes are cheaper for the same collecting area. However, to get similar sensitivity, the signals from all telescopes must be combined. This requires high performance electronics, and had been prohibitively expensive. However, due to the declining cost of the electronic components, the required electronics became practical, resulting in a large cost saving over telescopes of more conventional design. This is informally referred to as "replacing steel with silicon".

The ATA has four primary advantages for scientific studies over all major radio telescopes built to date: a very wide field of view (2.45° at λ = 21 cm), complete instantaneous frequency coverage from 0.5 to 11.2 Gigahertz (GHz), multiple simultaneous backends, and active interference mitigation. The instantaneous area of sky imaged is 17 times that of the Very Large Array. The instantaneous frequency coverage of more than four octaves is unprecedented in radio astronomy and is the result of a unique feed, input amplifier, and signal path design. Active interference mitigation will make it possible to observe even at the frequencies of many terrestrial radio emitters.

Because all-sky surveys are an important part of the science program, the efficiency of the ATA will be increased by doing radio astronomy and search for extraterrestrial intelligence (SETI) searches simultaneously. The telescope will do this by splitting the signals in the control room prior to final processing. Simultaneous observations are possible because for SETI, several target stars will lie within the large field of view afforded by the 6 m dishes wherever the telescope is pointed. Thus, by agreement between the RAL and the SETI Institute, the needs of conventional radio astronomy will determine the pointing of the array.

The ATA is planned to comprise 350 six-meter dishes and will make possible large, deep radio surveys that were not previously feasible. The telescope design incorporates many new features, including hydroformed antenna surfaces, a log-periodic feed covering the entire range of frequencies from 500 Megahertz (MHz) to 11.2 GHz, and low-noise, wide-band amplifiers with a flat response over the entire band making it possible to amplify the sky signal directly. This amplified signal, containing the entire received bandwidth, is brought from each antenna to the processing room on optical fiber cables. This means that as electronics improve and wider bandwidths are feasible, only the central processor needs to change, and not the antennas or feeds.

The instrument was operated and maintained by the UC Berkeley Radio Astronomy Laboratory (RAL) until the array's hibernation in 2011. The RAL worked hand in hand with the SETI Institute during design and prototyping and was the primary designer of the feed, antenna surfaces, beam forming, correlator and imaging system for radio astronomy observations.

The astronomy decadal panel, Astronomy and Astrophysics in the New Millennium, endorsed SETI and recognized the ATA (then called the 1-Hectare Telescope) as an important stepping stone to the Square Kilometer Array (SKA). The most recent Decadal Survey recommended ending US financial support of the SKA.

The ATA aspires to be among the world's largest and fastest observing instruments. It will also permit astronomers to search for many different target stars simultaneously. If completed as originally envisioned, it will be one of the largest and most powerful telescopes in the world.

Although cost estimates of unbuilt projects are always dubious, and the specs are not identical (the conventional telescopes have lower noise temperature, but the ATA has a larger field of view, for example), the ATA has potential promise as a much cheaper radio telescope technology for a given effective aperture. For example, the amount spent on the first ATA-42 phase, including technology development, is roughly 1/3 of the cost of a new copy of a Deep Space Network 34 meter antenna of similar collecting area.[12] Similarly, the estimated total cost of building the remaining 308 dishes is estimated (as of October 2007) at about $41 million.[3] This is about a factor of 2 cheaper than the$85 million cost of the last large radio astronomy antenna built in the USA, the Green Bank Telescope, of similar collecting area. The contractor filed for a $29 million overrun, but only$4 million of this was allowed.[13]

## Status

Since its inception, the ATA has been a development tool for array technology (specifically, for the Square Kilometer Array). Future progress depends on the technical performance of the sub-array already under construction, and the procurement of additional funding.

The ATA was originally planned to be constructed in four stages, the ATA-42, ATA-98, ATA-206, and ATA-350; each number representing the number of dishes in the array at a given time (See Table 1).

Regular operations with 42 dishes started on 11 October 2007.[3] Funding for building additional antennas is currently being sought by the SETI Institute from various sources, including the United States Navy, Defense Advanced Research Projects Agency (DARPA), National Science Foundation (NSF) and private donors.

Astronomical data has been acquired since May 2005, utilizing a four-input correlator (four antennas, dual polarization) and then updated in January 2007 with two eight-input (16 antennas, dual polarization).[14] Scientifically useful data has been acquired and is helping commission the array.[15]

Correlator development continued, with deployment of one 32-input correlator in June 2007 and utilized as eight individual correlators with eight dual-polarization inputs each.

Beam-forming electronics utilizing the Berkeley Emulation Engine 2 (BEE2) were deployed in June 2007 and have been integrated into the system to allow for simultaneous astronomical and SETI observations.[16] As of April 2008, the first pulsar observations have been conducted using the beamformer and a purpose-built pulsar spectrometer.[17]

In May 2009, UC Berkeley announced it was performing all-sky surveys using the Allen Telescope Array.[18]

The ATA Team reported initial results from their survey of the Galactic Center Region at the June 2009 meeting of the American Astronomical Society (AAS Meeting #214).[19]

In April 2011, the ATA was placed in hibernation mode due to funding shortfalls, meaning that it was no longer available for use.[20] Operation of the ATA resumed on December 5, 2011.[9] The building efforts are now led by Dr. Jill Tarter.[21]

In 2012, the ATA was funded by a $3.6 million philanthropic donation by Franklin Antonio, cofounder and Chief Scientist of Qualcomm Incorporated.[22] This gift supports upgrades of all the receivers on the ATA dishes to have dramatically (2 − 10× from 1–8 GHz) greater sensitivity than before and supporting sensitive observations over a wider frequency range from 1–18 GHz, though initially the radio frequency electronics go to only 12 GHz. As of July, 2013 the first of these receivers was installed and proven. Full installation on all 42 antennas is expected as of June 2014. In July 2015 Russian billionaire philanthropist Yuri Milner announced that his Breakthrough Initiatives would donate US$100 million over the course of 10 years to fund SETI projects.[23] The Allen Telescope Array is being upgraded with more sensitive electronic radio receivers, capable at working at even higher frequencies.[24]

## Key science goals

The science goals listed here represent the most important projects to be conducted with the ATA. Each of these goals is associated with one of the four stages of development (see Table 1). The bulleted items are the projects that will be undertaken and the subtopics are some of the science that will be produced. The ATA will:

Table 1: Array performance and key science projects
Array Status Beam size (arcsec) Srms (mJy) Speed (deg²s−1) Key science
ATA-42 Dish construction complete; commissioning in progress with 32 input, dual polarization (64 total inputs) correlator. 245 x 118 0.54 0.02 FiGSS: 5 GHz Continuum Survey, Galactic Plane Molecular Spectroscopy, SETI Galactic Center Survey
ATA-98 Awaiting results ATA-42 for funding 120 x 80 0.2 0.11 ATHIXS† Trial Surveys, HI Stellar Outflows Survey, SETI Targeted Survey: 100 stars
ATA-206 TBD 75 x 65 0.11 0.44 ATHIXS, Map The Magnetized Galactic ISM, Pulsar Timing Array, Deep continuum and transient surveys, SETI Targeted Surveys
ATA-350 TBD 77 x 66 0.065 1.40 ATHIXS, Map The Magnetized Galactic ISM, Pulsar Timing Array Deep continuum and transient surveys, SETI Targeted Surveys
Note. Beam size and continuum sensitivity (Srms are estimated for a 6-minute, 100 MHz continuum snapshot observation at transit of a source at 40° declination at a wavelength of 21 cm. Speed is given for a survey at 21 cm observations with a bandwidth of 100 MHz that reaches 1 mJy rms.

ATHIXS is an all-sky deep HI extragalactic HI survey.

## Opportunistic science

After array construction, a few science goals that were not explicitly designed have been mentioned.

For a very different science goal, the Allen Telescope Array has offered to provide the mooncast data downlink for any contestants in the Google Lunar X Prize.[25] This is practical since the array, with no modifications, covers the main space communications bands (S-band and X-band). A telemetry decoder would be the only needed addition.

Also, the ATA was mentioned as a candidate for searching for a new type of radio transient.[26] It is an excellent choice for this because of a large field of view and wide instantaneous bandwidth. Following this suggestion, an instrument was custom-built for the ATA to search for bright radio transients, and observations were carried out between February and April 2008.[27]

## Instrument details

The ATA Offset Gregorian Design

The ATA-42 configuration will provide a maximum baseline of 300 m (and ultimately the ATA-350, 900 m). A cooled log-periodic feed on each antenna is designed to provide a system temperature of ~45K from 1 GHz to 10 GHz, with reduced sensitivity in the range 0.5 GHz to 1.0 GHz and 10 GHz to 11.2 GHz. Four separate frequency tunings (IFs) are available to produce 4x100 MHz intermediate frequency bands. Two IFs support correlators for imaging; two will support SETI observing. All tunings can produce four dual polarization phased array beams which can be independently pointed within the primary beam and can be used with a variety of detectors. The ATA can therefore synthesize up to 32 phased array beams.

The wide field of view of the ATA gives it an unparalleled capability for large surveys (Fig. 4). The time required for mapping a large area to a given sensitivity is proportional to (ND)2, where N is the number of elements and D is the diameter of the dish. This leads to the surprising result that a large array of small dishes can outperform an array with smaller number of elements but considerably greater collecting area at the task of large surveys. As a consequence, even the ATA-42 is competitive with much larger telescopes in its capability for both brightness temperature and point-source surveys. For point source surveys, the ATA-42 is comparable in speed with Arecibo and the Green Bank Telescope (GBT), but slower by a factor of 3 than the Very Large Array (VLA). The ATA-350, on the other hand, will be an order of magnitude faster than the Very Large Array for point-source surveys and is comparable to the Expanded VLA (EVLA) in survey speed. For surveys to a specified brightness temperature sensitivity, the ATA-98 will exceed the survey speed of even the VLA-D configuration. The ATA-206 should match the brightness temperature sensitivity of Arecibo and the GBT. The ATA, however, provides better resolution than either these single dish telescopes.

The antennae for the ATA are 6.1 by 7.0 metres (20.0 ft × 23.0 ft) hydroformed offset Gregorian telescopes, each with a 2.4 meter sub-reflector with an effective focal length/diameter (f/D) ratio of 0.65. (DeBoer, 2001). The offset geometry eliminates blockage, which increases the efficiency and decreases the side lobes. It also allows for the large sub-reflector, providing good low frequency performance. The hydroforming technology used to make these surfaces is the same technique used to generate low-cost satellite reflectors by Andersen Manufacturing of Idaho Falls, Idaho. The unique, interior frame rim-supported compact mount allows excellent performance at a low cost. The drive system employs a spring-loaded passive anti-backlash azimuth drive train.

## Data Management

As with other arrays the huge amount of incoming sensory information requires real time array processing capability to reduce data volume for storage. For ATA-256 the average data rates and total data volume for the correlator are estimated to be 100 MByte/s and 15 PByte for the 5-year survey period.[28] Experiments such as transient surveys will exceed the rate significantly. The beamformers produce data at a much higher rate (8 Gigabytes per second (Gb/s)) but only a very small fraction of this data is archived. In 2009, the signal detection hardware and software was called Prelude, composed of rack mounted PCs augmented by two custom accelerator cards based on digital signal processing (DSP) and field-programmable gate array (FPGA) chips. Each Programmable Detection Module (one of 28 PCs) can analyze 2 MHz of dual-polarization input data to generate spectra with spectral resolution of 0.7 Hz and time samples of 1.4 seconds.[28]

In 2009, the site had a 40 Mbit/s internet connection, adequate for remote access and transferring of data products for ATA-256. An upgrade to 40 Gbit/s was planned, which would enable direct distribution of raw data for offsite computing.[28]

### Computational complexity and requirement

Like other array system the ATA has a computational complexity and cross-connect which scales as O(N2) with the number of antennas N. The computation requirement, for example, for correlating the full ATA bandwidth (B = 11 GHz) for the proposed N = 350 dual-polarization antenna build-out, using an efficient frequency-multiply (FX) architecture, and a modest 500 kHz channel width (with number of channels F = 2200) is given by:[29]

2B \langle N \log_2(F)(10 OPs)+(N \frac{N +1}{2})\times 4(8 OPs) \rangle = 44 Peta-OPs per second

where Ops is an operation. Note that since each dish has a dual polarization antenna each signal sample is actually a two data set, hence 2B.

## References

1. ^
2. ^ John Johnson, Jr. (1 June 2008). "Aliens get a new switchboard: a SETI radio telescope in Northern California". The Los Angeles Times. Archived from the original on 4 October 2008. Retrieved 2008-09-29.
3. ^ a b c Dennis Overbye (11 October 2007). "Stretching the Search for Signs of Life". The New York Times. Retrieved 2009-04-14.
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6. ^ Hardy, Michael (2011-04-29). "Federal Computer Week". Fcw.com. Retrieved 2011-09-19.
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9. ^ a b "SETI Search Resumes at Allen Telescope Array, Targeting New Planets" (Press release). SETI Institute. December 5, 2011. Retrieved 29 December 2012.
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19. ^ Results from the Allen Telescope Array: SETI Survey of the Galactic Center Region [2]
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25. ^ "Google Sponsors Lunar X PRIZE to Create a Space Race for a New Generation" (Press release). X PRIZE Foundation and Google Inc. 13 September 2007. Archived from the original on 11 May 2009. Retrieved 2009-04-14.
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28. ^ a b c A Radio Sky Surveys Project with the Allen Telescope Array - Response to the Request for Information Part 2 Geoffrey C. Bower, October 15, 2009
29. ^ Aaron Parsons; et al. (Oct 29, 2006). "PetaOp/Second FPGA Signal Processing for SETI and Radio Astronomy". Signals, Systems and Computers: 2031–2035.