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Planetary News: 2004

Multi-Beam Receiver Promises New Vistas for SETI Research

The view greeting morning hikers on the hills surrounding the Arecibo observatory on Wednesday, April 21, was unusual, to say the least. Down below, as always, was the giant 1000 foot reflective dish of the Arecibo radio telescope, the largest in the world; hanging 500 feet above the dish, at its reflective focus, was the familiar triangular platform that carries both the Gregorian dome and the needle shaped antenna used by SETI. But there, between them, was something else: a cylindrical metallic object, about the size of a large washing machine, hanging in mid-air and slowly making its way upwards from the surface of the dish towards the Gregorian dome.

The early risers viewing this scene may not have known this, but they were witnessing a crucial moment in the history of astronomical research at Arecibo. The strange gold and white object dangling before them was a multi beam receiver, one of the first in the world, and it was being installed within Arecibo’s Gregorian dome. With it, the radio telescope will be capable of looking at not one, but seven different points in the sky simultaneously.

Like other radio-telescopes, Arecibo works by pointing its beam at a particular point in the sky and listening intently. Being the largest radio telescope in the world, Arecibo is also the most sensitive, and it can “hear” things other telescopes cannot. But in principle it works in the same way.

This arrangement works well for most scientific purposes: astronomers studying the radio signatures of particular stars and galaxies, for example, do so by focusing on their target for extended periods of time, and recording the data they collect. But there is one type of study for which the single point approach is remarkably inefficient: general sky surveys, which look at the entire observable band of sky. In order to cover all the sky with a single beam, astronomers must piece it together from millions of separate observations, each of a single point.

Because they are so time-consuming, it has been practically impossible to conduct systematic sky surveys at major observatories such as Arecibo. Given the intense competition for observing time among scientists, no observatory could devote the months of observing time required for an all-sky survey simply for the benefit of a single project.

The Multi-Beam Solution
This has been a serious problem for SETI scientists such as Dan Werthimer, who believe that the best chance to discover ET is by looking at the entire sky. To get around this, Werthimer and his team have devised their ingenious “piggy-back” strategy. They placed a receiver permanently above the Arecibo dish, at the base of the needle-shaped antenna. Though the SETI team cannot direct the telescope, the SETI receiver collects data from whichever point in the sky the beam happens to be pointed at. Given enough time, the telescope will cover the entire band of sky visible from Arecibo, and the SETI receiver will consequently complete an entire sky survey. Using this innovative strategy, the SETI receiver has completed four sky surveys since its installation six years ago.

But SETI scientists are not the only ones interested in general sky surveys. Other groups of astronomers, researching the structure of the universe and the distribution of objects within it, have also been hoping to scan the entire sky. Unfortunately, given the time constraints of work in a major radio-observatory, their opportunities were few and far between,

A multi-beam receiver at Arecibo can change all that: by looking at seven different points in the sky simultaneously, it can cover an area seven times larger than an ordinary single beam receiver. Naturally, this reduces the time it takes to cover the entire visible sky to a fraction of what it was.

Just as importantly, the various groups interested in radio sky surveys have now banded together. Pulsar researchers hope to gain insights about the origins and history of these unique objects by mapping out the distribution of the different types of pulsars in interstellar space. Other researchers are interested in mapping out the distribution of hydrogen in inter-galactic space, by searching the sky at the hydrogen absorption frequency of 1.4 GHz (21 cm.). Since hydrogen is by far the most prevalent element in the universe, comprising around 71% the mass of all existing matter, scientists believe that such a survey can teach us a great deal about the history and formation of galaxies and stars. Yet other scientists are interested in using the array for detailed surveys of our own galaxy, investigating the structure of the Milky Way, and - last but not least - SETI scientists will use the new array to survey the skies for signals from alien civilizations.

All these different groups of scientists interested in conducting radio sky surveys joined forces to bring the multi-beam receiver to Arecibo. Together, the scientific consortium is known as ALFA – the Arecibo L-Band Feed Array.

The advantages of forming the Alfa consortium are obvious to any scientist who has attempted to secure observing time on a major observatory such as Arecibo knows. To put it simply, there is strength in numbers. Normally, an astronomer working on a given project can hope to be granted an observing session of no more than a few hours, or days at the most. But when several groups, all interested in a particular type of observation, come together, they are granted much larger chunks of observing time. According to plans, once the multi-beam receiver becomes operational, around one third(!) of the total observing time at Arecibo will be dedicated to the ALFA sky survey. When you add to this the fact that each survey of the sky will take only a fraction of the time it did with a single beam, the end result is clear: the new scientific consortium along with the new multi-beam technology represent a quantum leap forward for radio frequency sky-surveys at the worlds largest and most sensitive radio telescope.

What it All Means for SETI
How will all this affect SETI research? In several important ways, all of which add up to a major increase in the programs scope and sensitivity.

First of all, Werthimer and his colleagues will be able to move beyond the “piggy-back” strategy. For while the “piggy-backing” is undoubtedly ingenious, and makes the best of a difficult situation, it nevertheless has some serious drawbacks. For one thing it is slow –it takes over a year on average for the piggy-backing receiver to complete an entire sky-survey. The coverage of the sky in such a survey is also very uneven – some patches of the sky, which happen to be of interest to astronomers, will be observed many times over before other areas are targeted even once – regardless of which region is of interest to SETI researchers.

As part of the Alfa consortium SETI will now be part of systematic sky surveys that will take place regularly at Arecibo. And with the multi-beam receiver, these surveys will also be much faster than any single beam survey could ever be.

In addition, the SETI surveys will now be conducted at a much increased level of sensitivity. This is because the seven receivers that comprise the multi beam array are newer and much “quieter” than the old SETI receiver. This means they produce far less electromagnetic noise of their own, enabling them to distinguish much fainter signals from the stars.

Finally, explains Chief Scientist Dan Werthimer, due to the increased capabilities of the new receiver, SETI@home will be able to search at a much wider frequency band than is currently the case. Whereas the old SETI receiver has a bandwidth of 100 MHz around the hydrogen absorption frequency of 1.4 GHz (21 cm), the new receivers will listen for signals at a range of 300 MHz around the same frequency.

The multi-beam receiver at Arecibo has been in place for several months now and scientists are currently using it to conduct preliminary test observations. Once regular observations begin, the new array will constitute a giant step forward for SETI. The faster and more regular sky surveys, at an increased sensitivity and broader bandwidth, will push the boundaries of SETI to new and unexplored territories.