Publications DepartmentABSTRACT
For many years the microwave Search for Extra-Terrestrial Intelligence (SETI) held the spotlight, while the number of optical SETI observatories on this planet could be counted on the thumbs of one hand. In the five years since the last Optical SETI Conference, that has begun to change, with optical SETI finally emerging into the scientific mainstream. Advancing technology is only partly responsible for OSETI's change of fortune. This author believes that the ultimate acceptance of the optical search strategy can be attributed to the tireless efforts of a single pioneer.
Recent pioneering efforts in microwave SETI have met with similar resistance from the established SETI community, reminding one of an adage from the American West: pioneers end up with arrows in their backs. The SETI League's Project Argus sky survey, for example, which seeks to do credible science with modest amateur equipment, designed, built and operated by dedicated non-professionals, continues to draw criticism from the SETI establishment. Many traditional radio astronomers still believe that SETI requires the kinds of facilities which only governments can afford.
This paper explores optical SETI's recent move from the sidelines to center stage, in search of lessons which the world's amateur microwave SETIzens can learn from our dedicated optical brethren.
KEYWORDS
Optical SETI, Microwave SETI, Amateur Radio Astronomy, Project Argus, sky survey
1. THE DOMINANCE OF MICROWAVE SETI
Guglielmo Marconi, arguably the father of radio, stated in a paper presented to the Institute of Radio Engineers in 1922: "…the study of short electric waves, although sadly neglected practically all through the history of wireless, is still likely to develop in many unexpected directions, and open up new fields of profitable research." [1] Though he could not possibly have known it at the time, those unexpected directions and new fields most certainly could be said to include radio astronomy and SETI. The study of the cosmos at microwave wavelengths is both beholden to and partly responsible for the tremendous explosion in electronics technology which characterized the latter half of the twentieth century.
One document which has dominated microwave SETI research for most of the discipline's existence is the almost legendary Project Cyclops [2] design study. In Table 5-3 its chief author, the late Dr. Bernard M. Oliver, compared the effectiveness of various competing technologies, and concluded that as interstellar communications could best be accomplished in the microwave spectrum.
Searching for intelligently generated signals in the optical spectrum had been proposed by Schwartz and Townes as early as 1961. [3] Townes, the physicist credited with sharing in the development of the MASER and the LASER, has subsequently maintained that short GigaWatt pulses of coherent light could propagate readily through the interstellar medium, briefly outshining stars by a factor of a billion. These pulses, if they exist, should be visible to Earth's most powerful optical telescopes, were suitable photon-counting pulse detectors to be developed.
Nevertheless, Oliver's computations in Project Cyclops showed optical SETI (OSETI) to be hopelessly inadequate for the task. For a quarter-century thereafter, whenever an OSETI experiment was proposed, SETI professionals pointed to the Cyclops calculations as justification for concentrating their limited resources solely in the microwave spectrum.
2. THE EMERGENCE OF OSETI
Historically, the most frequently voiced objections to the optical SETI paradigm have dealt with perceived limitations in human technology. That is, OSETI was deemed incapable of traversing interstellar distances, based upon calculations predicated on Earth-level technology. As our technology has improved, those arguments have become moot. Significant advances in Earth's optical communications capabilities were announced at the previous two OSETI meetings [4,5], and doubtless more will be announced at this one. I will cite a single example.
During the past seven months, the United States Air Force has deployed an airborne infrared gas laser producing over 1 MegaWatt of continuous wave (CW) power at a wavelength of 1.6 microns. [6] Although shockingly inefficient (it requires a two-stage gas turbine just to power it), this laser is the first practical example on Earth of a potential interstellar optical transmitter. True, it was designed strictly for a military purpose, and also true that, being carted around inside a Boeing 747, its beam is unlikely ever to be directed at distant suns. But clearly its existence invalidates the argument that a civilization cannot generate sufficient laser power to communicate across interstellar distances.
Perhaps because of such technological breakthroughs as the one just cited, SETI professionals are only recently beginning to embrace lasers as a valid interstellar communications medium. Table 1 summarizes past and current OSETI searches. It is significant to note that the half a dozen OSETI projects currently underway represent more such simultaneous activity than ever before attempted. Several of these searches are being conducted by individuals and organizations formerly regarded as OSETI's harshest critics. Clearly, OSETI has gained enough legitimacy to enter mainstream scientific thinking. As noted in the following Section, the author credits a single colleague, Dr. Stuart A. Kingsley, with bringing about this change in attitude.
3. THE ROLE OF THE DEDICATED AMATEUR
While some scientific organizations tolerate grudgingly the involvement of amateurs, astronomy and SETI depend upon it. It is the radio amateur Grote Reber, after all, who built the first modern radio telescope, and with it produced the first radio maps of the Milky Way galaxy. It is amateur optical astronomers who discover the majority of asteroids, supernovae, and other highly intermittent astrophysical phenomena. It is the global cadre of amateurs forming the grass-roots, nonprofit SETI League who have collectively dedicated one hundred small radio telescopes (more than operated by the rest of the world's SETI community, combined) to the search for intelligently generated microwave signals in space. [7]
Perhaps the most vocal proponent of OSETI is similarly a dedicated amateur, Stuart Kingsley of Columbus OH. Dr. Kingsley's has been a voice in the wilderness for years, his optical observatory among the first to search for laser communications from space. The scientific establishment is only now beginning to embrace OSETI, due in large part to Kingsley's research, publications and conference presentations.
Since 1990 Stuart Kingsley has been conducting what has become the world's longest-running optical SETI program, from an observatory dome behind his home in Columbus, OH. (It is perhaps significant that Columbus was also home to the late Big Ear radio telescope, and the world's longest running microwave SETI project.) Kingsley's modest 25 cm diameter reflector telescope searches the 550 nm spectrum for pulsed lasers emanating from nearby stars. He has also produced and maintains the world's most comprehensive Optical SETI website, [8] all while holding down a "day job" as Director of Engineering for a respected photonics company. While most SETI scientists concentrated on the more conventional microwave spectrum, Dr. Kingsley's optical search has received support from such visionaries as Nobel laureate Dr. Charles Townes and novelist Sir Arthur C. Clarke.
As vindication of Kingsley's vision, the past five years have seen the launch of a number of ambitious OSETI projects at the Harvard-Smithsonian Astronomical Observatory; the University of California, Berkeley; Princeton University, New Jersey; the Lick observatory in California; on the Keck telescope in Hawaii; at Perth and Sydney, Australia; and in the Czech Republic (see Table 1). Kingsley now chairs the SETI League's Optical SETI Committee, through which he encourages other experimenters to embrace OSETI.
If Townes is the Moses of OSETI, having wandered in the desert for forty years, then Kingsley could certainly be considered our Joshua, leading us into the promised land. But like most prophets, Kingsley has been long without honor. Often, an established science's failure to embrace the contributions of amateurs has less to do with the merit of their ideas than it does with competition for limited resources.
At a SETI conference three years ago [9] no less a SETI icon than Dr. Frank Drake cautioned the author against promoting small-dish SETI, lest the public might judge more ambitious facilities unnecessary, and cease funding them. It is important to note that Drake, a respected member of The SETI League's advisory board, was not questioning the technical capabilities of our Project Argus stations, but rather their impact upon public perceptions as to funding needs. Could it be that for years OSETI's detractors similarly feared diversion of limited funds away from microwave SETI projects? One wonders if Kingsley's small amateur telescope is similarly drawing fire from the emerging OSETI establishment, not because it is inadequate to the task, but rather because advertising the virtues of such modest facilities might impede the development of more costly and capable ones.
In fact, as will be shown next, Kingsley's prototype amateur OSETI station, like the inexpensive amateur telescopes which many of us possess, is truly an optical Arecibo, fully capable of performing meaningful research at the very highest level.
4. COMPARATIVE ANTENNA GAIN: MICROWAVE VS. OPTICAL
A student recently presented me with this interesting question: "I understand that light is a form of electromagnetic radiation. So are radio waves. Now if a telescope can magnify light 400x, couldn't it also do the same with radio waves? It seems to me you should be able to focus your telescope on a star, put your reciver at the eye piece, and get a signal 400x more powerful! If this would work, it would cost less than a dish, and take up less space. So, why don't radio astronomers do this?"
My response, though presented at the layman's level, underscores one of the key advantages of optical over microwave SETI. In his question, the student described exactly how both (Newtonian) optical telescopes and (parabolic dish) radio telescopes work. The reason the optical telescope magnifies light hundreds or thousands of times is that its mirror is large relative to the wavelength of the light being gathered. A radio telescope similarly "magnifies" its "light" hundreds or thousands of times, because its mirror (the parabolic dish -- which focuses light to its eyepiece, the feedhorn) is large relative to the wavelength it is focusing. The only problem is, the radio telescope is dealing with electromagnetic radiation about half a million times longer that visible light wavelengths, so for equivalent performance, its "mirror" needs to be about half a million times larger than the equivalent optical telescope's.
Now that we agree on the basics, I told my class, let's run the numbers: A reflecting telescope (optical or radio, it really doesn't matter) has a "magnification" which can be described in terms of power gain. At 100% efficiency (which we can never achieve, because the real world isn't perfect), we can calculate that power gain. It's actually easier to calculate voltage gain, and then square it, since power ratio varies with the square of voltage ratio. The relationship is:
Voltage gain ~ (Reflector circumference) / (wavelength)
where both are measured in the same units. Of course, circumference equals diameter times pi (for a round mirror), and diameter is twice radius, which is why all the textbook formulae contain a (2 pi * r) factor.
Next, power gain = (voltage gain)^2. Think of this as your "magnification" of light.
Finally, in radio we usually convert power gain to dBi, a logarithmic shorthand. dBi means deciBels compared to an isotrope. An isotropic radiator is a theoretical (can't actually build one, buy one, or find one in nature) ideal omnidirectional antenna. Omnidirectional means it radiates equally poorly in all directions. Anyway, the conversion is:
dBi = 10 * log (power ratio)
where we use a base 10 logarithm.
So let's put all this together and run some examples.
First: my optical telescope (a Celestron model C-8) has a 100 mm radius reflector. That mirror has a circumference of (2 * pi * 100 mm) ~ just over a half meter. I use it to magnify visible light which has a 500 nanometer wavelength.
The voltage gain is (1/2 m)/(500 nm) = 1,000,000!
Theoretical power gain is (1,000,000)^2 = 1,000,000,000,000!
Converting to dBi, that's 10 log (10^12) = +120 dBi
(only I won't really get anywhere near that performance, because my eyepiece and mirror are quite imperfect.)
Next: let us consider the world's largest radio telescope, the Arecibo radio observatory, at the 1420 MHz hydrogen line. The mirror at Arecibo has a 152 meter radius. Its circumference is (2 * pi * 152 meters) ~ just under 1 kilometer. I use it to receive "light" at a wavelength of 21 cm. So:
Voltage gain is (1 km)/(21 cm) ~ 5,000.
Theoretical power gain is (5,000)^2 = 25,000,000.
Converting to dBi, that's 10 log (2.5 * 10^7) ~ +74 dBi
(only Arecibo won't really get anywhere near that performance, because its eyepiece {the feedhorn} and mirror {the reflector} are quite imperfect.)
Well, actually I've misled you a little here, because I'm comparing apples to kumquats. If we level the playing field by restricting each instrument to its intended application, we see that my Celestron telescope and my 12 foot radio telescope are just about equivalent in performance (see below). But this doesn't invalidate this rather startling discovery:
In terms of power gain, my Celestron telescope is tens of thousands of times more sensitive than Arecibo. Which explains why optical SETI is so appealing to the budget-limited amateur.
The foregoing discussion left my class wondering why the typical amateur optical telescope has a magnification of about 400, while we have just computed its power gain at 1,000,000,000,000. The apparent discrepancy is because antenna gain is calculated relative to that elusive isotrope, while optical magnification is generally specified relative to the naked eye. So if we know the optical telescope's gain, and want to know its magnification, we need also to compute the "gain" (relative to isotropic) of the human eye. Fortunately (although optical physics has its own formulas) straight antenna theory can apply here as well.
Let's consider the human eye to be an antenna, whose aperture to electromagnetic radiation is the pupil. Say the radius of the dilated pupil (it is dark when we use a telescope, after all) is on the order of 2 mm. We can use the same equations we use for a parabolic antenna:
Voltage gain ~ (pupil circumference) / (wavelength)
Voltage gain ~ (2 pi * 2 mm) / (500 nm) = 25,000
Power gain = (voltage gain)^2 ~ 600,000,000 = +88 dBi
which makes the naked human eye an optical Arecibo!
Now, as to the question of the magnification of my Celestron, let's assume I have an eyepiece which perfectly couples to my pupil (I don't, which means it's efficiency is less than 100%, but this is the ideal case.) Theoretical magnification would be the ratio of the antenna's power gain to that of the naked eye. That comes to (1,000,000,000,000) / (600,000,000) = 1667. The optics people have simpler formulas for calculating magnification, to be sure (ratio of mirror to eyepiece dimentions being the favored one). We would expect the results obtained that way to correlate well with antenna theory.
Next, let's calculate the power gain of that Celestron telescope, not in dBi (deciBels compared to isotropic), but rather in a new unit which I'm going to call dBe (deciBels compared to the human eye). The relationship should be (and in fact is):
Telescope Gain (dBe) = Telescope Gain (dBi) - Human Eye Gain (dBi)
Telescope Gain (dBe) = (+120 dBi) - (+88 dBi) = +32 dBe
[notice that the i's in the dBi units above cancel]
It has been previously shown 10 that the "standard" 12-foot dish used in Project Argus amateur SETI stations achieves a gain of +31.8 dBi. Comparison to an isotrope is as appropriate for radio telescopes as comparison to an eyeball is for optical telescopes. Thus it appears that, as far as their intended applications are concerned, the average amateur radio telescope (typified by the Argus station) and the typical optical telescope (exemplified by the Celestron C-8) are roughly equivalent in performance.
Whereas the human eye is the optical equivalent of an Arecibo, we see here that the standard Project Argus station is the microwave equivalent of a Celestron! And since significant optical astronomical discoveries have been made with Celestron-class telescopes, we have every reason to expect significant microwave astronomical discoveries to be within the grasp of our Project Argus-class amateur radio telescopes.
More to the present point, if we assume amateur microwave SETI stations to be scientifically capable, how can we consider amateur optical SETI, with readily available instruments, any less valid?
5. CONCLUSION
Technologists have been known to reject the unconventional. Futurists embrace it. There may be false starts and wrong turns, but on the whole, the opportunity for serendipitous discovery and unanticipated spin-off applications outweighs the risk of the blind alley. This is true in the burgeoning field of ETI research, and applies equally to amateur microwave SETI, the search for self-replicating interstellar probes, infrared scans for artifacts of massive technological development, and, of course, Optical SETI. To exclude even one of these fields of related research is to constrain our own success, snatching defeat out of the jaws of victory.
To paraphrase Marconi, the study of optical electromagnetic waves, although sadly neglected practically all through the history of SETI, is still likely to develop in many unexpected directions, and open up new fields of profitable research.
6. REFERENCES
The author, an aerospace engineer credited with the design of the world's first commercial home satellite TV receiver, received his Ph.D. in Engineering from the University of California, Berkeley, and taught for 24 years. He is the founding Executive Director of The SETI League, Inc., a membership-supported educational and scientific nonprofit which has emerged as the leader in a grass-roots Search for Extra-Terrestrial Intelligence.
Paul is the author of over 275 publications. He is a Fellow of the British Interplanetary Society, serves as a fellowship interviewer for the Hertz Foundation, a manuscript reviewer for several peer reviewed journals, has been an advisor to the National Science Foundation, and is a military program evaluator for the American Council on Education. Paul's honors include the Robert Goddard Scholarship, the Hertz Fellowship in the Applied Physical Sciences, the Horonhjeff Grant, the Hertz Doctoral Thesis Prize, the EAA Safety Achievement Award, the John Chambers Memorial Award, the ARRL Technical Achievement Award, and the Dayton Hamvention Technical Excellence Award.
AUTHOR AFFILIATION
Executive Director, The SETI League, Inc.
PO Box 555, 433 Liberty Street
Little Ferry, NJ 07643 USA
Phone: (201) 641-1770
Fax: (201) 641-1771
Email: n6tx @ setileague.org
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