Reobservations Report No. 1: Shifting Gears at Arecibo
Posted by Amir Alexander
2003/03/10 11:00 CST
In the next few days, SETI@home Chief Scientist Dan Werthimer, along with team members Eric Korpela and Paul Demorest, will head down to Arecibo in Puerto Rico. There, at the site of the largest radio telescope in the world, they will begin a new chapter in the short history of the project: the reobservation of SETI@home's most promising candidate signals.
This turning point comes after nearly four years in which the SETI@home receiver has been surveying the skies from its perch atop the Arecibo radio telescope. During this time millions around the world have been analyzing the raw data recorded at Arecibo on their personal computers, in search of unusual patterns that might represent deliberate transmissions. No less than 5 billion(!) different candidate signals have now accumulated, each of which just might be that one true signal from an alien civilization.
Now, after years of searching, gathering, and analyzing, SETI@home is preparing to revisit its most promising signals. For eight hours each day, on March 18th through the 20th, Dan Wethimer and his team will have full use of the Arecibo radio telescope. They will use the time to target between 100 and 200 locations in the sky where the strongest, clearest, and most promising candidate signals had been detected by SETI@home. Only a candidate signal that has been revisited and confirmed in this manner can be considered a potential intelligent transmission from the stars.
Why are these Days Different from all other Days?
Needless to say, these three days in March are not the only time SETI@home is searching for intelligent transmissions from the stars. The SETI@home receiver is, after all, a permanent fixture at Arecibo, and it collects data practically on every day of the year. What is so special, then, about the observations that will take place on these three days in the middle of March?
The answer is that throughout the year SETI@home operates in "piggy-back" mode. This means that SETI@home scientists have no control over the direction in which telescope is pointing. The job of pointing the telescope is left to the astronomers who were awarded special telescope time to pursue their own research. While the astronomers conduct their research, the SETI@home receiver simply sits in its place high above the giant Arecibo dish, recording the radio signals emanating from that part of the sky. Over time, as different astronomers pass through Arecibo and use the telescope for many different types of research, the great radio telescope covers practically every point in the sky visible from its location. And so, simply by "piggy-backing" on other scientists' research, the SETI@home receiver manages to survey the entire band of sky seen from Arecibo.
While this may seem like an inefficient way of conducting research, the "piggy-back" system is, in fact, perfectly suited for a SETI sky survey. A full sky survey would normally require an inordinate amount of time in which the telescope would be used only for the survey, to the exclusion of all other scientific research. Securing such a large chunk of time is impractical, given the high demand for observing time on the worlds large radio-telescopes. By "piggy-backing" on the normal operations of the Arecibo radio telescope, SETI@home is in fact able to survey a much larger part of the sky in a much shorter time than it would if it relied on exclusive telescope time.
The main drawback of this approach, however, is that it is practically impossible for SETI@home to linger on an interesting radio signal, or return to it later for a more careful observation. Since SETI@home does not have control over the radio telescope's movements, it simply surveys that part of the sky where the Arecibo dish happens to be pointing. All SETI@home scientists can do, even for the strongest, clearest, and most intriguing signal, is simply record it and hope that at some point down the road it may return to that position once more.
This inherent difficulty in SETI@home's approach is precisely what makes the reobservations at Arecibo so crucial: for the first time, SETI@home scientists will be able to return to their most promising signals and examine them carefully.
And so, from March 18th to the 20th, in addition to continuing its regular sky survey SETI@home will also be a highly sensitive targeted search. One after the other it will target the locations where the strongest, clearest, and most persistent signals have been detected in the past. This time, instead of piggy-backing, SETI@home scientists will have all of the telescope's resources at their command. They will be able to point the telescope wherever they wish, and linger on any interesting locations in the sky. What they will find, awaits to be seen.
Finalizing the Plans
When the SETI@home arrives at Arecibo, it will have with it a list of the locations of the 200 most promising signals detected so far by SETI@home. Given the limited time allotted - 24 hours in all - Dan Werthimer and his crew think it very unlikely that they will be able to observe all 200 locations. They do, however, intend to use their time as efficiently as possible, and revisit as many locations as possible.
In planning to make the optimal use of their observation time, SETI@home scientists have to consider the two distinct types of motion that direct the radio telescope's beam. The first of these is simply the result of the Earth's diurnal rotation - every 24 hours, Arecibo's beam completes a full cycle of the night's sky. This is a natural movement that takes place without any human intervention. The second type of motion is brought about by shifting the location and angle of the feeds (line-feed or Gregorian dome) at the focus of the radio telescope's dish (see image). By doing so, the radio telescope's operators can point its beam at a wide band of the sky. This, however, is a time consuming mechanical process. Shifting the telescope from pointing due south to due north, for example, takes over 7 minutes. Changing the angle of observation in the sky (the "zenith") is even slower, with every 2 degrees shift occupying a full minute.
The few minutes it takes to adjust the direction of the telescope's beam may not seem like much to an outside observer. Even for most scientific purposes, which focus on a few celestial objects for long periods of time, taking a few minutes to point the telescope is time well spent. For SETI@home, however, the time it takes to mechanically move the feeds from one point to another is simply time wasted. The shorter the amount of time spent on adjusting the telescope, the more time will be left for actual observations, and more candidates that will ultimately be revisited.
In order to minimize the amount of time spent on mechanically adjusting the telescope, the SETI@home team had to decide in advance on the precise order in which the candidate signals will be revisited. For this purpose, team member Paul Demorest wrote a computer program that sorted through the list of candidates and came up with the optimal order, which would minimize the "dead" time spent in between each observation. The final list, however, takes into account not only the optimal observation order, but also the ranking of the different candidates. The SETI@home team must make sure that the best and most promising candidates are visited, and do not slip down the priority list due to time constraints. After all, we would not want to miss that one true signal simply because it happened to originate from an inconvenient location.
Meanwhile at Arecibo
Meanwhile, in preparation for SETI@home's three-day targeted search, other changes are taking place at Arecibo. First and most importantly, during the reobservations the SETI@home team will use a different feed than it usually does. The regular SETI@home receiver is located at the base of the needle-shaped "line feed." This is an older antenna that is rarely used today by astronomers, a fact that makes it ideally suited for a piggy-back year-round sky-survey. For the reobservations Dan Werthimer and his team will shift to the much newer receiver located in the Gregorian dome. This semi-spherical structure hangs 500 feet above the giant Arecibo dish and contains the telescope's newest and most sensitive receivers. Given the choice of the several receivers located in the Dome, the SETI@home team selected the highly sensitive "L band receiver" as the most suitable instrument with which to conduct the reobservations.
The new arrangements offer some clear advantages to Dan Werthimer and his crew. The Gregorian dome system has a better focus and higher resolution than the line feed normally used by SETI@home. Furthermore, the L band receiver is cooler, generates less noise, and as a result is more sensitive than the SETI@home receiver. This means that during the reobservations SETI@home will cover its most promising candidate signals with greater detail and accuracy than they were observed before.
At the same time, the change of venue also raises a significant problem: Because of the higher resolution of the Gregorian system, its observation beam is significantly narrower than that of the line-feed. At any given moment, the line-feed scans an arc 5 minutes wide (0.08 degrees) of the sky. At the same time, the Gregorian system scans an arc of only 3 minutes (0.05 degrees).
This raises the possibility that the "Gregorian" beam, even when pointed at the precise direction of a candidate signal, might miss a signal even if it is actually present. It is, after all, possible that the candidate signal was originally detected on the margins of the wider line-feed beam. If that is the case, it will remain outside the coverage area of the narrowly-focused "Gregorian" beam.
Because of this problem, SETI@home scientists concluded that it would not be enough to simply point the Gregorian beam in the direction of each candidate signal. Instead, they decided that the location of each candidate signal must be scanned, by making five passes in the area with the telescope's beam. Each pass will take 40 seconds, making for a total of 200 seconds that will be spent on each candidate location. The total area scanned around each candidate will form a square of 1/6th by 1/6th of a degree.
Scanning the location of each candidate signal, rather than simply pointing at it, has other advantages as well for the reobservations process. It should be remembered that each candidate signal is composed on not one, but at least two separate observations, each of which detected a signal coming from the same direction. Except, of course, that it was never EXACTLY the same direction - only very close, close enough that the different events could be plausibly considered the same signal. By scanning the area around a candidate signal, SETI@home researchers make sure that they cover all locations at which the candidate signal had been located, as well as others locations in the immediate vicinity.
Finally, scanning each candidate's location will help the SETI@home crew to distinguish between a real signal from space and Earth-generated radio interference. If a signal remains constant no matter which direction the beam is pointed to, there is a good chance that is represents a strong radio source on Earth. If however, the signal gains and loses strength depending on where the telescope is pointing, then it does, most likely, originate in the stars.