The Planetary Society Optical SETI Telescope Opens
Posted by Bruce Betts
28-04-2006 12:00 CDT
On April 11, 2006, a new era dawned in the search for extraterrestrial intelligence (SETI) with the dedication and beginning of operations of The Planetary Society Optical SETI Telescope in Harvard, Massachusetts. It is the first devoted optical SETI telescope in the world. The telescope was constructed by Paul Horowitz and his group at Harvard University using funding from Planetary Society members.
An unassuming structure, looking something like a small storage building, sits in a clearing at the Oak Ridge site of the Harvard College observatory. The building's only unusual features are a small weather station and a mysterious rod on top and, more unusual, a strange looking set of metal beams on one end of the building, making it look like the builders ran out of money and didn't cover the second half of the building structure. On a pleasant spring day in April 2006, 75 people gathered at this site. With a remote computer command, the unusual began, and the roof of the building began to slide off onto the mystery beams, revealing cutting-edge technology -- the great new hope in the search for extraterrestrial intelligence.
Radio "Versus" Optical
Much has changed on our world, including in the field of optics, since 1959, when Giuseppe Cocconni and Philip Morrison first suggested SETI be carried out at radio wavelengths, particularly at the 21-centimeter emission line of hydrogen. At the time, there were no operational lasers. Only 2 years later, R. N. Schwartz and Charles Townes suggested that perhaps SETI should also be carried out at optical wavelengths. (Townes won a Nobel Prize in 1964 for co-inventing the laser.) The technology was still in its infancy, however, and it was hard to imagine where it would develop. Thus, SETI began at radio wavelengths and continued that way for 40 years. In that time, lasers have become much more powerful, and they continue to improve rapidly. Optical light communication began to replace other forms of communication in our world, primarily through fiber optics, and the SETI community began to look again at optical SETI.
How plausible is it that an alien civilization could use optical wavelengths to communicate across the galaxy? Well, for reference, using 2006 Earth technology, one could construct a device that is capable of outshining our Sun by a factor of more than ten thousand for a brief instant, or a set of repeatable instants. Pretty amazing -- and eye opening, so to speak. And that is just using 2006 Earth technology in a field that is rapidly growing and developing. If alien civilizations are communicating, they are likely more advanced than ours and easily could be outshining their parent stars by factors of 100,000 or more.
Advantages of Optical Communication
So, communicating across the galaxy with lasers is plausible, but would ET want to use optical laser pulses? Radio certainly has its advantages, particularly its ability to penetrate through intervening clouds or other material in the interstellar medium. But optical communication has some distinct advantages as well for galactic communication, many of which are the same reasons that on Earth we now use light for communication more and more and why NASA is developing the techniques necessary for optical communication with deep space spacecraft. Optical provides the following advantages over radio:
- Higher frequencies mean optical signals can carry far more information, the same reason we are using fiber optics and pursuing optical spacecraft communication.
- Optical signals can form a more tightly focused beam, enabling better targeting.
- Dispersion, which broadens radio wavelengths, is negligible at optical wavelengths.
- Radio transmitters have reached a stable maturity, whereas lasers continue to develop rapidly.
- Radio observations are plagued by interference by radio, tv, cell phones, etc.
But, if ET sends a laser pulse into the galactic forest, and no one is there to see it, does it really . . . well, you get the idea. ET could have been pulsing our planet constantly for aeons and we wouldn't have known, until recently.
A Brief History of Optical SETI
In the late 1990s, The Planetary Society began funding groups at both Harvard University and the University of California at Berkeley to do targeted optical SETI research. They would either target a select number of stars and then observe them, or piggyback on other observations already being carried out. What they were looking for were sudden rapid spikes in the light coming from a star system. To match typical pulse times and to reduce the effects of the parent star, they tried to measure pulses that were only a billionth of a second.
The problem with targeted approaches is that we can sample only a limited number of stars. Searchers have to make a guess as to where to look or take their chances with piggyback observations. They have also assumed that communication would come from a star system. What we'd really like to do is survey the entire sky. Then, no assumptions need to be made about where a signal may come from, and, at least spatially, nothing is overlooked.
The challenge of all-sky optical SETI is considerable. In fact, various astronomers told Paul Horowitz that it was impossible. Part of Horowitz's inspiration was to prove them wrong. The big challenge is that the sky is a very big place to cover, and we need to collect samples at billionths of a second (a nanosecond). This means we need a really fast and capable customized system of electronics. As we’ll see, Horowitz and his students created a cutting-edge, custom-designed system of electronics to process unbelievable quantities of data. This is perhaps not too surprising when you realize that Horowitz co-authored the standard college-level electronics book. (I can hear all the physics and electrical engineering majors out there saying, "Oh, that Horowitz!") With support coming from The Planetary Society, Horowitz and his team set out to do "the impossible."
Covering the Sky
In order to cover the entire sky, the telescope will look at one elevation (altitude, in telescope terms) each night. For a night, the telescope remains fixed, pointing at a particular angle. Meanwhile, you and I and the telescope rotate under the stars, thanks to Earth's rotation. As Earth rotates, it sweeps the telescope's field of view across the sky. In this way, a stripe of the sky is covered. The next night, the altitude is changed, and a new stripe is covered. Each clear night, one can cover about one third of the sky in any given stripe . As Earth orbits the Sun, new pieces of sky are visible at night, and one can reobserve each altitude and fill in the other parts of the stripe. The search can cover the entire sky in about 200 nights, and we predict it will take 1 to 2 years to get those 200 clear nights.
At any one moment, the camera images a 1.6 by 0.2 degrees rectangle of the sky with a pair of 512-pixel nanosecond-speed photodetectors. It has a minimum viewing time of about 1 minute per target -- that is how long the system has to detect a signal from any given point in the sky, at least the first time through on the survey. One minute may not seem very long, but remember that the system is taking data every nanosecond, so each minute gathers a lot of data for every point on the sky. The system does rely on ET to be putting out a signal at least once every minute, or it requires getting lucky, but that is the trade-off for covering the entire sky.
The Observatory and Telescope
Part of the brilliance of this search strategy is that the telescope's mechanical systems can be simplified. The telescope needs to move in only one axis: altitude. That also means a dome isn't necessary -- a clear opening up and down in one direction in the sky is sufficient. This simplification created significant cost savings in the construction of the new optical SETI observatory compared with a normal observatory. This is why the observatory could be a "regular" building, with an unusual sliding roof.
The telescope is also unusual in its look. The builders were able to use an easier-to-construct boxlike structure, giving the frame a square cross section -- unusual looking, but just as effective for this purpose as a traditional-looking telescope.
The telescope has a 72-inch primary mirror, eclipsing its 61-inch neighbor 100 yards away and earning the title of biggest primary mirror in the East. Unusual for a telescope, the 72-inch mirror is spherical rather than parabolic, because spherical mirrors are easier and cheaper to produce. One of the advantages of a dedicated optical SETI telescope is that one can "simplify" in various areas compared with a typical astronomical observatory. The optics, too, can be simplified because we don't need to produce the highest-resolution images possible. Fundamentally, we are counting photons. We need to know where those photons came from, but we don't need to produce pretty pictures or know where they came from to an arcsecond resolution. For these reasons, Horowitz says the "telescope" is more properly called a light bucket -- but, The Planetary Society Optical SETI Light Bucket just didn't have a very nice ring to it.
The Camera and Electronics
One important thing in science -- as with much of life -- is knowing where you can simplify without harming the result and where you can't. Where this system can't skimp is the camera and the electronics. This telescope needs to be able to image an area of the sky every nanosecond. To do so, it utilizes a pair of 512 photodetectors (each pair observes the same location in order to get rid of spurious results that may occur in just one detector). The system has to use photomultiplier tubes, an older technology, rather than charge-coupled devices (CCDs) because CCDs just can't begin to count photons at nanosecond intervals -- they are a million times too slow.
The signals are then fed into 32 identical processors. Horowitz's graduate student Andrew Howard designed these amazing chips specifically for this task. Each contains more than 250,000 transistors. Together, these processors process 3.5 terabits (3.5 trillion bits) of data per second! That is the equivalent of the information in all the books in print . . . every second.
The processors highlight and keep track of any possible event -- a large increase in photons occurring in at least one of the detectors. For precise and accurate timing, the system uses the Global Positioning System (the extra mast near the weather station on the building).
In the modern era, I believe no piece of electronics is truly complete without a remote control. The telescope and even its building are no exception. The entire system can be operated remotely over the Internet, whether the operator is in Cambridge, Massachusetts (as Horowitz's group is likely to be) or in Madagascar (less likely to find them there). Information about the observatory comes from the weather station mentioned earlier, a suite of webcams, a number of status sensors on the electronics (such as temperature and humidity), and the control systems that open the roof, move the telescope, and handle all the other operations. After the system is thoroughly checked out, it will move to fully automated control, and humans won't need to intervene. The system will decide when to observe and then do it. It will then pass all the possible ET events on to the humans. Ah, a remote control that knows what you want without you having to touch it.
The first steps are to work all the kinks out of the system, fully understand its data, and move to fully automatic control over the next few weeks to months. Assuming that goes swimmingly, and ET isn't discovered immediately, where do we go from here? The answer probably is a second identical observatory a few miles away. With the current setup, every trigger -- that is, every time you get a spike in the signal that might indicate ET -- needs to be thoroughly checked out, then reobserved to make sure it was not due to a cosmic ray exciting the electronics or some other non-ET source. With targeted SETI, Horowitz's group collaborated with Princeton University and did some simultaneous observations of the same objects. Requiring that both observatories see a signal at the same time creates much more confidence that with even one observation, you may be seeing ET. It will greatly speed the weeding out of all the false events.
For now, we are ecstatic that the great effort to produce The Planetary Society Optical SETI telescope is complete and that the new search has begun. The opening of this telescope represents one of those rare moments in a field of scientific endeavor when a great leap forward is enabled. We also keep our eyes looking to the future, when we may find out the answer to the question, are we alone?
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