Last November I went to a talk on the Allen Telescope Array (ATA). I had written the first draft of an article some time ago, but I hadn’t managed to finish it until now. The ATA is a radio telescope currently under construction in Northern California.
Before I discuss the ATA, I want to talk about radio telescopes in general. The first radio telescope was built in the early 1930’s. Over time, more radio telescopes were built. The early telescopes all had a single antenna and produced blurry images. Astronomers wanted sharper images. They realized building bigger antennas would improve the resolution but this was expensive. In the early 1950’s, Martin Ryle and Antony Hewish took four antennas and combined the output with electronics to produce one signal. This became known as an “array.” Arrays were a cost effective way to increase resolution. (Ryle and Hewish won the 1974 Nobel Prize in Physics “for their pioneering research in radio astrophysics: Ryle for his observations and inventions, in particular of the aperture synthesis technique, and Hewish for his decisive role in the discovery of pulsars.”)
Other arrays have been constructed, the most well known is the VLA, located in New Mexico. It is an array of 27 antennas arranged in a “Y” shape.
Suppose we want to build a new array, how do we decide the number of antennas to use? Typically the array will be built so it can produce images with a specific resolution or better (call this resolution R). This resolution is picked to support specific research goals. Now suppose our array has N antennas (N is an unknown, we don’t know its value yet). Given N, we can calculate the diameter of each antenna (call this D) that would result in the given R. We pick N to minimize the total cost of the project. The cost is the sum of two items. First, the cost of the antennas is proportional to the volume of material. That is a function of both N and D, but as N increases the cost goes down (that’s because D decreases faster than N increases). Second, the cost of the electronics is a function of N. As N goes up the cost of the electronics goes up. To reduce the overall cost, you have to balance these two items. This means there is an optimal value of N for a specified resolution. N should not be too big nor too small.
When the VLA was designed in the early 1970’s, the decision was made to built the array with N=27 and D=25 meters, which were reasonable choices at the time. Since the 70’s, electronics have steadily become less and less expensive. So the optimal solution today would involve an array with more than 27 antennas, each of which is smaller than 25 meters.
So let’s return to the Allen Telescope Array. It is currently under construction in Northern California and operated by the University of California Berkley. When completed, it will consist of 350 antennas each of which is 6.1 meters in diameter. At the end of 2006, 42 antennas were operational. Over time additional antennas will be added until all 350 have been completed. Each antenna is relatively inexpensive and approximately 3 antennas can be constructed each week.
Unlike the VLA, which has 27 antennas arranged in a “Y” shape, the ALA antennas are arranged in a seemingly random manner. However it only looks random, they are carefully arranged so that there is a well defined region of the sky where the telescope is sensitive. The telescope is able to reject signals outside this region much better than traditional telescopes. And as a side effect, it is possible to add antennas one at a time. Even with 42 antennas, the array can be used for research and as more antennas are added, the resolution gradually increases.
One expense in traditional radio telescopes are refrigerators. Refrigerators are used to keep the electronics cool which reduces noise. The ATA uses sterling-cycle refrigerators which were obtained at low cost. Each antenna produces an RF (radio frequency) signal which is converted to a digital signal that in turn is processed by digital IF processors. The signals from the various antennas are combined by a phase array back end. This arrangement provides a great deal of flexibility. It is possible to have four different experiments operational simultaneously. And the ATA electronics can do some simple calculations itself that support different types of experiments. For example, it is possible to configure what is known as a selective null. This is used to remove sources such as GPS satellites that otherwise might add unwanted noise to the signal. This and other operations can be configured in a flexible way in real time (unlike more traditional radio telescopes, where these kinds of operations require time consuming post processing).
The arrangement also allows different parts of the sky to be examined simultaneously. The ATA can be used to take “snapshots” (traditional radio telescopes need to point at a target for long periods to form an image, the ATA can do the same thing much faster). It is possible to use the ATA to do some simple analysis directly without producing an image (though more complex analysis will require post processing). And the ATA is able to process a wide range of frequencies simultaneously. Some of the applications of the ATA include:
This was the first part of the talk. Before I go on to the second part, I should tell you about the presenter. The talk was given by Dr. Jill Tarter, the Director of the Center for SETI Research at the SETI Institute located in Mountain View, California. However Dr. Tarter is best known because of the motion picture “Contact.” In the movie, Jodie Foster plays an astronomer searching for extra-terrestrial radio transmissions. Jodie Foster’s character was based on Dr. Tarter.
Dr Tarter spent the rest of the talk discussing SETI. SETI is a program to look for evidence of intelligent life and developed from ideas that are at least one hundred years old. In the 19th century, many scientists believed if there is life on the earth, it must exist on other planets. At the time, Mars and Venus were assumed to be inhabited. Other scientists argued that life was created on the earth, and that is the only place it exists. However no one really knew. While our knowledge has grown over the past hundred years, we still don’t know if there is life elsewhere in the universe.If there is life elsewhere in the galaxy and if that life has built a civilization, we might be able to detect evidence of that civilization. One way to do that was to search for radio signals (which can be detected by a radio telescope). The late Phil Morrison (physicist and team member of the Manhattan Project) said that if we detect a signal from a civilization it tells us about their past (since the signal takes time to reach us) and our future (presumably the civilization will be more advanced than us).
The SETI program was originally funded by NASA; however NASA dropped its funding in 1993. Dr. Tarter had the responsibility of looking for new funding. Soon, through the generosity of Microsoft co-founder Paul Allen, funding arrived. That funding allowed the ATA to be built.
The ATA will be used partly for radio astronomy applications (as mentioned above) and partly for SETI. We don’t know what frequencies an extra-terrestrial civilization might use (if they exist at all), nor do we know where they might be located. So any search involves looking at a wide range of frequencies and looking at every point in the sky. This is very time consuming process. The ATA promises to make the process faster. However, even with the ATA there are no guarantees of success.
While in the early days SETI was limited to radio signals, more recently researchers realized that an advanced civilization might communicate using lasers. A narrow beam could be sent in a specific direction in the form of a short bright pulse of light. Up until recently searching for such pulses was not practical. However recent improvements in devices known as photodiodes have made searching for these signals a possibility. However this cannot be done with the ATA and would require additional equipment.
Whether there are extraterrestrial civilizations or not, the search is likely to continue for many years.
The information in this article came from the following sources.