Amir Alexander • Aug 14, 2006
With Multi-Beam Receiver, [email protected] Takes Giant Step Forward
It has been two years now since the seven pronged multi-beam receiver was hoisted above the Arecibo radio telescope and installed in the Gregorian dome, 500 feet above the surface of the dish. During this time the new receiver had become an inseparable part of the regular operations at Arecibo, providing a new and indispensable resource for teams of scientists from around the world. But it was only two months ago that members of the [email protected] team headed down to Arecibo to join the fray. In seven intense days spent at the radio telescope Chief scientist Dan Werthimer and his colleagues completely overhauled the way SETI data is gathered at Arecibo, and ensured that [email protected] will henceforth enjoy the benefits of gathering data with the most advanced equipment anywhere in the world.
Unlike ordinary radio telescope receivers, which point and listen to a single point in the sky at any given time, Arecibo's new multi-beam receiver can point to seven different points simultaneously. Its seven receivers, arranged in a hexagon, operate independently, each collecting its own data separately from the others from a slightly different location in the sky. For many astronomers, interested in studying specific locations in the sky for extended periods of time, the multi-beam receiver offers no particular advantages. But for those interested in scanning the entire sky, the multi-beam arrangements is ideal: by pointing at seven points simultaneously, the new receiver can cover the entire surface of the sky many times faster than a traditional single beam.
And so it was that astronomers from several different fields joined forces and created the ALFA (Arecibo L-band Feed Array) consortium, charged with operating the new receiver. Each of the groups making up the consortium has its own scientific focus. One group is interested in mapping the distribution of hydrogen in the universe, another in the location of pulsars, and yet another is studying the structure of our galaxy, the Milky Way. The SETI group, of course, is searching for an intelligent signal emanating from somewhere in the universe. But all members of ALFA are united by a shared goal: to use the largest and most sensitive radio telescope in the world to conduct systematic surveys of the entire sky. In the competitive environment that governs telescope time allotments, no group by itself would be granted sufficient telescope time to conduct an efficient sky survey. But there is strength in numbers: working together, the ALFA consortium is granted as much as one third of all the observation time at Arecibo.
Because it is so hard to acquire sufficient telescope time for a sky survey, SETI observations at Arecibo were conducted up to now in the in the piggy-back mode. This means that the SETI receiver was perched permanently above the Arecibo dish at the base of the aging line feed antenna. Whichever point in the sky the telescope happened to be pointed at, that's where the SETI receiver would listen for intelligent signals. Over time, even without being granted dedicated telescope time, the piggy back SETI receiver managed to scan the entire sky visible from Arecibo.
While this arrangement made the best of a difficult situation, it was hardly ideal. Subject to the random vicissitudes of piggy-backing, a complete survey of the sky visible from Arecibo would take several years, and it was, furthermore, very uneven. Certain segments of the sky, interesting to some astronomers for their own reasons, were covered many times over before other locations were observed even once. This is a very inefficient way to conduct a SETI survey whose underlying assumption is that a signal from E.T. may come from anywhere in the sky.
Now, with the ALFA consortium operating the multi-beam receiver, things have become much simpler for SETI observers. Since all members of the consortium are interested in conducting an efficient sky survey, the multi-beam receiver will be scanning the entire sky systematically and relatively quickly. Technically, since Werthimer's group will at no time be controlling the telescope, SETI will still be "piggy backing" on the observations of others. But whereas previously SETI was forced to rely on the random meanderings of the telescope across the sky, it will now be able to look for signs of alien intelligence in a systematic survey of the entire sky. If before SETI was merely a passenger, going along for the ride, by joining forces with the ALFA consortium SETI has moved much closer to the driver's seat of observations.
This increases the efficiency of the [email protected] sky survey enormously, and would in itself have made the transition to the multi-beam receiver a giant step forward for [email protected]. But the benefits of the move do not end here: there are several additional reasons why collecting data on the multi-beam receiver represents a several-fold increase in the power and sensitivity of the search.
One factor is that unlike the old line feed antenna, which was hanging by itself, exposed to the tropical elements, the multi-beam receiver is located inside the distinctive-looking Gregorian dome. This is significant because of the unique geometrical features of the Arecibo telescope: whereas most radio dishes are parabolic in shape, the Arecibo dish is a giant half-sphere dug into the ground. Parabolic dishes have the advantage that they point their beams to a single point at their focus, creating a clear and strong signal. A sphere, in contrast, concentrates its beam to a line, which is why a "line feed" antenna was originally used at Arecibo.
When using a line feed, telescope operators produce a unified signal by adding up the different parts of the signal, but the results are never as strong and clear as they are with a parabolic dish. The Gregorian dome, due to its unique geometrical shape, focuses the signal reflected by the Arecibo spherical dish on a single point inside the dome. In effect, the Gregorian dome transforms Arecibo's spherical dish into the equivalent of a parabolic dish.
To take advantage of this, the receiver has to be positioned at the focus point inside the dome. The dome therefore contains several such receivers, each with different characteristics, which rotate around, taking turns at the focal point. The multi-beam receiver is the most recent addition to this exclusive club, and it provides SETI scientists with a level of sensitivity that could never be achieved with the line feed antenna.
Another advantage is, most obviously, that the multi-beam antenna looks at seven different points simultaneously. This makes it possible to cover the entire sky far more quickly than when working with a traditional single-beam receiver. When combined with the fact that all ALFA members are interesting in conducting a systematic sky survey, the result is far faster and more even coverage of the heavens.
Then there are the advanced characteristics of the seven receivers themselves. Each of them is about 5 times more sensitive than the old receiver, located at the base of the line feed. Add to that the fact that the new version of the [email protected] client program -- the one users run on their computers -- is also 5 times more sensitive than the old program. Together, the new receiver and the new software bring about a 25-fold increase in sensitivity for [email protected]!
The multi-beam receiver is also uniquely suited for resolving one of the nagging problems of modern radio-telescopy: screening out radio frequency interference (RFI). It is an unavoidable fact that the radio frequencies in which radio telescopes listen for faint signals from the sky are flooded with not-at-all faint man-made radio signals that originate here on Earth. Separating RFI from true signals from the sky is a persistent challenge for all radio-telescopic observations, including SETI.
In this, however, a multi-beam receiver offers a unique advantage. Since each of the beams is looking at a different section of the sky, then a true signal, coming from space, would show up in only one of the receivers. Therefore, if a signal appears on more than one of the receivers, it is a sure sign that it is not coming from space at all, but most likely from the area around Arecibo. This simple screening of signals is an enormous help to the [email protected] team, by keeping out of consideration a mass of RFI signals, which might otherwise be subject to careful and time-consuming analysis.
With all these advantages in mind, the [email protected] team, comprising Dan Werthimer, Jeff Cobb, and Josh Von Korff, headed down to Arecibo to make the switch. First, they disconnected the old faithful SETI receiver that was attached to the line feed, and then they set about establishing the connection between the [email protected] network and the multi-beam receiver.
A chief component of this transition was the brand new data recorder, designed and built by Dan Werthimer and Aaron Parsons, with software designed and implemented by Jeff Cobb. Despite its modest name, the so called "recorder" is, in a way, the brains of the SETI observations. It does, of course, record the data, and unlike the old recorder, which listened and recorded on a single channel, the new recorder is tuned to no less than 14 channels. This means that it records the seven receivers that comprise the "multi-beam" separately, each of them at 2 different polarizations.
But the recorder has other responsibilities as well. It is, for instance, constantly monitoring the automated ongoing transmissions at Arecibo, reporting which of the telescope's receivers is currently at the focal point, the frequency of the current observations, and where the telescope is pointing. If the multi-beam receiver is not at the focus, or if for other reasons the telescope is not generating data relevant for [email protected], the recorder immediately suspends its recording operations. In fact, explains Jeff Cobb, a good part of his time at Arecibo was spent working closely with the telescope's technicians on integrating the recorder into Arecibo's network, and making sure that the communications flow smoothly.
Because all the members of the ALFA consortium share an interest in efficiently surveying the sky, the multi-beam receiver generally does not dwell on a single point for long. But on the occasions when this does happen, the recorder is programmed to take advantage of the situation. Normally, the SETI recorder is listening for signals at a band centered 1420 MegaHerz (MHz). But when the telescope is dallying at a specific point in the sky, there is no point in recording on a single frequency for an extended period of time. Instead, the recorder will begin listening at other, neighboring frequencies, jumping to 1422.5 MHz, 1425 MHz, etc, on one side, and to 1417.5 MHz and 1415 MHz on the other. Overall there are 21 different frequencies the recorder can tune into given sufficient time, and who knows? Maybe E.T.'s signal is being delivered in one of them.
Overall, during their week at Arecibo Wertheimer, Cobb, and Von Korff put in the pieces that will make possible an enormous leap forward in the efficiency and sensitivity of [email protected]. And in the very near future, [email protected] users will begin processing data collected by Arecibo's cutting-edge multi-beam receiver.
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