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EJASA - Part 6
Page 35
years, with a power of 16 dB re 1 kW, i.e., 40 kW. This is a trivial
amount of power for an ETI.
On the basis that the author thinks that ETI transmitter powers
will be in excess of 100 MW and perhaps even substantially in excess of
1 GW, we could decide to lower the detection sensitivity and go for a
faster sampling rate, thus speeding up the search. For the purposes of
this analysis we will stick to the 100 kHz pixel sampling rate. As
previously stated, we will assume that we are doing our single star
signal processing in real time, with 100 kHz minimum bin bandwidths.
This means that the entire array would take 0.164 s to scan. If we
assume no scan dead time, then to scan the entire visible band between
350 nm and 700 nm at a sensitivity level of about -150 dBW/m^2
(10^-15 W/m^2), would take about two hours (Equ. 21, Page 82). An All
Sky Survey of this type would take at least 136 million years! If a
survey of this type could have been started when the dinosaurs roamed
Earth, we would be just about reaching the end of the first scan!
(Don't anyone accuse the author of lacking a sense of humor).
On the other hand, for a sensitivity of -150 dBW/m^2, a Targeted
Search scan of a single star over the 280 GHz effective bandwidth of
the 656 nm Fraunhofer line (Table 4, Page 30) with a 10 GHz MCSA, with
on-line data storage, and a 10 microsecond pixel sampling time, would
take 4.6 seconds. This is a very reasonable time, so that a slower
scan at selected laser and Fraunhofer lines could be performed to
reduce the minimum detectable flux levels.
PROFESSIONAL CO2 SETI
Just as this paper was being completed, the author received a copy
of Albert Betz's (University of California, Space Sciences Laboratory,
Berkeley, CA 94720) latest paper on Optical CO2 SETI. [57] For the
sake of completeness, because there is currently so little Optical SETI
literature available, and because Betz's paper is a very up-to-date
account of the only observational Optical SETI work presently being
done in the United States, a short description is now given. The work
of Townes and Betz is supported by a NASA grant NAGW-681. As mentioned
on Page 5, this low-profile SETI work is being done on Mount Wilson, and
is piggy-backed onto a much larger NASA program to investigate astro-
physical phenomena at the galactic center, e.g., a possible black hole.
To start with, here now is a complete quote of the abstract from
Dr. Betz's paper, which was presented in August of 1991 at the Santa
Cruz, California USA-USSR SETI Meeting:
"In an effort complementary to NASA's search for microwave signals
from an extraterrestrial intelligence, we are searching for possible
laser signals of a similar origin. We are surveying approximately 300
nearby stars in a multi-year effort to detect narrowband laser signals
in the 10 um wavelength region. For this directed search, we are using
an available 1.7 m telescope and a heterodyne receiver tuned to
discrete CO2 laser frequencies between 26-30 THz. The bandwidth of the
heterodyne allows us to analyze a Doppler velocity range up to
Page 36
+/-60 km/s around selected laser lines, and thus accommodate the
velocity dispersions of hypothesized laser sources orbiting nearby
stars. The resolution of the spectrometer is currently 2.4 MHz
(24 m/s), with 10^3 spectral channels available. Although this
resolution is somewhat coarse, any indication of a signal could be
subsequently analyzed at much higher resolution with the type of signal
processor (MCSA) now being developed for the microwave survey."
Betz uses a slightly different transmission throughput
relationship to that employed by this author (Pages 77-78). For his
parameters: Pt = 1 kW, D = 10 m, R = 10 L.Y. (9.461 X 10^16 m), and
Wl = 10.6 um (see Appendix A for parameter definitions):
Pr = 9.9 X 10^-18 W
This figure for received power is about 2.1 dB greater than given
in Table 2, Line 13 on Page 22 (6.1 X 10^-18 W). The reason for the
slight discrepancy is that Betz uses an approximation by omitting a
PI^2/16 factor (see Equs. 13 and 14 on Pages 77 & 78 for more details).
Earlier it was stated that the minimum beam divergence thought
possible by Townes and others was about one second of arc. However,
this recent paper by Betz indicates a new, more optimistic limitation
of about 0.1 second of arc. This is only a factor of 7.25 greater than
the 0.0138" diffraction limited beamwidth for the visible system (as
shown in Table 2, Line 5 on Page 22, and on Page 73). By assuming that
the nearest stars to be targeted are around 50 parsecs (163 L.Y.) away,
a beam divergence of 0.1 arcsecond is compatible with the expected
zones of life. Because of this increase in beam directivity, Betz gets
an infrared SNR improvement over the 300-meter diameter Arecibo system
of about 3 dB (a factor of 2). Figure 4 on Page 28 shows that the
microwave system has a CNR of 20 dB, while the infrared system has a
CNR of 22 dB; a 2 dB difference in favor of the infrared system. Thus,
taking into account the slightly different assumptions made in this
analysis, i.e., the transmission relationship, the microwave front-end
temperature and quantum efficiency, the theoretical results for the CO2
system in this paper are in very close agreement with that of Betz's
paper.
The Townes and Betz CO2 telescope is computer driven, with the
ability to point blind to approximately one arcsecond, both during the
day and night. As indicated on Page 23, CO2 SETI is just as effective
during the day as at night, since, whatever the limitations of the sky
background, it is essentially constant over the 24 hour day.
The reader should note that the 128 X 128 pixel array specified for
the Professional Visible SETI system has a field of view of about
2.1 X 2.1 arcsec (Figure 10, Page 81), and thus is semi-compatible with
the pointing accuracy of Betz's system. Note that a medium size
visible wavelength telescope with a single incoherent photodetector
system, may have to be steered and pointed during daylight hours with
point blind accuracy better than 1 arcsec. If the pixel size and FOV
are increased to accommodate steering inaccuracies and atmospheric
turbulence, the daylight background would increase and degrade the SNR.
Page 37
INCOHERENT OPTICAL SETI AT 10,600 nm
In a later section, we will describe an incoherent Optical SETI
receiver for visible and near-infrared wavelengths, with Amateur
Optical SETI application. For the sake of completeness, Figure 6 has
been included here to demonstrate the relatively poor response of a
small incoherent (photon-counting) CO2 receiving system. This should
be compared to Figure 8 (Page 44), given for the case of incoherent
Optical SETI at visible wavelengths. Identical signal flux levels and
telescope apertures have been employed in both graphs. These graphs
have been located at the top of their respective pages to allow the
pages to be flicked back and forth for easier comparison.
In the incoherent CO2 system, where the signal-to-noise ratio (SNR)
is quantum noise limited, the SNR it is greater than in the visible
spectrum because "hf" is smaller. However, where the SNR is background
noise limited, the SNR is severely degraded. For a high signal
intensity of 10^-14 W/m^2, as produced by a transmitter at a distance
of ten light years with an EIRP of about 10^21 W, the SNR for a 30 cm-
diameter CO2 receiving telescope begins to degrade for optical band-
widths greater than about 1 MHz.
The infrared telescope's photodetector must be subject to
considerable cooling, e.g., using liquid nitrogen, to avoid high dark-
current, and it must be provided with a cold-shield to restrict its
field-of-view (FOV) to background thermal radiation. Note that the
performance of an amateur CO2 system could well be much worse than
shown in Figure 6, because CO2 transmitter gains and EIRPs are likely
to be much less than available at visible wavelengths. Unfortunately,
high-Q optical filters centered on the CO2 wavelength are not available
with wide tuning characteristics, although a small degree of tuning may
be obtainable by tilting the filters. Fixed optical filters with
100 GHz bandwidths at 10,600 nm are available for several hundred
dollars. The cost of a extremely high-Q 10 GHz (0.035 percent
bandwidth) interference filter may run into several thousand dollars.
Even then, the thermal background detected is excessive, and the filter
itself must be cooled.
As has been pointed out repeatedly and demonstrated by Equ. 32
(Page 88), the optical heterodyne receiver has the great advantage over
its direct detection counterpart (Equ. 31), in that the effective
optical bandwidth through which background radiation is received is
determined by the small electrical I.F. bandwidth. Also, because of
the excessive dark-current characteristics of 10,600 nm photodetectors,
there is considerable merit in using a local-oscillator laser to swamp
out these noise sources, though coherent detection would not
necessarily obviate the necessity to employ some cryogenic cooling.
Thus, there is much truth in the observation that as far as ground-
based CO2 SETI receivers are concerned, only coherent receivers are
practical, such as the interferometer system presently being employed
by Townes and Betz on Mount Wilson, and described on the previous two
pages. [57]
Page 38
Postdetection Normalized SNR, dB re 1 Hz
|
|
80 | Ir = 10^-10 W/m^2 EIRP = 1.1 X 10^25 W
|* * * * * * * * * * * * * * * * * * * * * * * * * *
| *
60 | Ir = 10^-12 W/m^2 EIRP = 1.1 X 10^23 W *
|* * * * * * * * * * * * * * * * * * * *
| *
40 | Ir = 10^-14 W/m^2 EIRP = 1.1 X 10^21 W
|* * * * * * * * * * * * * * * *
| * *
20 | Ir = 10^-16 W/m^2 * *
|* * * * * * * * * * * *
| * *
0 |.Ir = 10^-18 W/m^2.........*...................*..............
|* * * * * * * *
| * * *
-20 | Ir = 10^-20 W/m^2 * * *
|* * *
| * * *
-40 | * * *
| * * *
| * * *
-60 | * * *
| * * *
| * *
-80 | Day & Night * *
| * *
| * *
-100 --------------------------------------------------------------
10^0 10^2 10^4 10^6 10^8 10^10 ^ 10^12
|
Optical Bandwidth, Hz |
100 GHz (37.5 nm)
Figure 6 -
Signal-to-noise ratio versus optical bandwidth for (perfect) photon-
counting CO2 receivers. Range = 10 light years, wavelength = 10,600 nm,
diameter = 30 cm, antenna efficiency = 0.7, spectrometer efficiency =
0.5, quantum efficiency = 0.5. Dark current is assumed to be
negligible, though in practice it will impact the above sensitivity
curves at lower flux levels, even more than the sky background.
Needless to say, the construction cost of a heterodyning CO2 SETI
telescope/receiver is likely to be excessive for the amateur
enthusiast. For this reason, CO2 SETI is not being proposed for the
amateur. This activity is best left to NASA and the professional
observer.
Page 39
ADAPTIVE TELESCOPE TECHNOLOGY
Perhaps one of the most exciting developments in modern optical
astronomy is the subject of adaptive telescope technology. The author
believes that this not only has profound implications for conventional
optical astronomy but also for Optical SETI. In particular, for what
we call Symbiotic Optical SETI. What follows is a description of the
technique obtained from the tutorial introduction to reference 69.
"Earth-based telescopic adaptive-optics systems need a reference
(guide) star which is near objects of interest and bright enough to
provide information on the wavefront distortion. But natural guide
stars for a usable portion of the visible spectrum are few and far
between, allowing glimpses of just 0.003 percent of the night sky.
Rather than cursing the darkness, astronomers and engineers are
lighting some celestial candles of their own.
To create the artificial guide stars, a laser is beamed into the
sky, which answers back inflamed. The laser energy creates Rayleigh
backscattering in the stratosphere (10 - 40 km up) and resonance-
fluorescence backscattering in the mesospheric sodium layer
(80 - 100 km). No radically new technology is required for the lasers,
although the breadth of capabilities is large for a single laser. For
zenith viewing of a 20-cm atmospheric patch using the Rayleigh
approach, the laser must put out 82 watts; for the sodium-
backscattering approach the required exciting power is 14 watts. At
the sodium layer, which results from meteor ablation, the beam must be
0.5 meter in diameter, with a pulse rate of 100-200 pps and 100
millijoules per pulse.
The laser guide-star concept was first put into practice by Chester
Gardner and Laird Thompson, who in 1987 created, photographed, and
measured their own glowing beacon, shot like some giant flare above the
Mauna Kea Observatory in Hawaii. [69]
The basic system requirement is that the distortion of the guide
star must be measured and the adaptive mirror adjusted in the time it
takes for a star to twinkle, or, depending on how you look at it, the
time between twinkles. This window of visibility known as twinkle time
(also called scintillation coherence time) is open for a scant
10 milliseconds."
The requirements to produce a diffraction limited image over the
entire focal image plane are rigorous. It could be that the criteria
for Optical SETI are rather less demanding. The requirement here is
for imaging the ETI signal onto a two-dimensional photodetector array,
where the primary purpose (neglecting Planckian suppression needs) of
the array is to detect ETI photons, not to produce a super high-quality
extended image. As described on Pages 10 and 83, it is shown how
efficient detection of an ETI signal might be obtained with a simple
passive technique, if ETIs cooperate by transmitting a signal
accompanied by a pilot-tone beacon. Such a technique automatically
makes any telescope adaptive, without the need for deformable mirrors
and laser guide stars.
Page 40
THE COLUMBUS TELESCOPE PROJECT
As this paper was nearing completion, the author learned that a
decision had been made to terminate Ohio State University's
participation in The Columbus Project, the construction of a twin
8-meter diameter interferometric telescope to be built on Mount Graham
in southeastern Arizona. The instrument, which is supposed to see
"first light" in 1994, will have the light gathering power of a single
11.3-meter (448-inch) mirror and the resolving power of a 22-meter
(866-inch) telescope.
The project was a joint venture between OSU, the University of
Arizona, and Italy's Arcetri Astrophysical Observatory. The reason
given for OSU's pulling out of the project was a lack of privately
donated funds. Within these pages, this author has suggested the
possibility of a future symbiotic relationship between Professional
Optical Astronomy and Professional Optical SETI. During the early part
of this study, an idea was formulated that plans for The Columbus
Telescope might be changed, so that both scientific activities could be
undertaken at that site; Professional Optical Astronomy being done at
night, and Professional Coherent Optical SETI mainly during the day.
On Columbus Day, October 12, the Microwave Observing Project will
commence its search of the sky. As we in Columbus, Ohio, approach the
quincentennial of Columbus' discovery of the Americas, what more
fitting way could there be to celebrate the first encounter with the
New World than if OSU's participation in The Columbus Project was
resumed and the telescope's purpose modified to include the search for
extraterrestrial intelligence. The New World would be looking for
other, perhaps older worlds, with more mature technical civilizations.
OSU is already home to the "Big Ear" Radio Observatory, which under
the guidance of Professor John D. Kraus (Director) and Dr. Robert Dixon
(Assistant Director), has been undertaking conventional microwave
SETI for many years. On the same site in Delaware (a little north of
Columbus), and close to "Big Ear", is the Perkins Optical Observatory.
At the moment, the author is working on ideas to upgrade the Perkins
Observatory for Semi-Professional Incoherent Optical SETI. This
observatory presently contains a 81-cm (32-inch) Cassegrain.
OPTICAL SETI RATIONALE
SETI would not seem so mysterious to the average person if it was
recognized that this is yet another communications problem, albeit
complicated by the fact that we do not know where or when to look, the
transmission frequency, the bandwidth, or the modulation format. In
many ways it is just another aspect to our manned and unmanned space
program, but one that has received relatively little funding. It took
many years before SETI was recognized as a legitimate science and not
pseudoscience. The technology described here for Optical SETI is more
than just a means of contacting emerging technical civilizations. If
intelligent life is not uncommon in the galaxy, and if electromagnetic
waves are still the primary means of interstellar communications, the
Page 41
ability of optical relays to form a galactic network might obviate the
necessity to use low-loss microwaves or the far-infrared in order to
propagate across the entire galaxy in one go. After all, it is very
difficult to have a snappy conversation when communicating over one
hundred thousand light years!
Earlier, we showed that our "perfect" 10-meter diameter symmetrical
656 nm heterodyning system was capable of yielding over a range of
10 light years, a CNR of about 34 dB re 1 kW re 1 Hz, for a diffraction
limited EIRP of 2.3 X 10^18 W (see Table 2 and Figure 4). Since a
solar-type star has an EIRP of 3.9 X 10^26 W, we pose the question:
What is the communication capability of such a communications link when
the mean EIRP of a large transmitter array is 2.5 times that of the
star, i.e., when the mean EIRP is about 10^27 W? This condition
corresponds to the transmitter appearing as a 1st magnitude object; a
situation which would produce a noticeable (2.5 times) brightening of
the ETI's star. Since the ratio of EIRPs {10^27/(2.3 X 10^18)} is
4.4 X 10^8, the CNR will be improved by 86 dB, resulting in a CNR of
about 120 dB re 1 Hz, and a photon detection rate of about 10^12 s^-1
(Equ. 36). If the bandwidth is increased to 10 GHz, the CNR falls to
about 20 dB. Thus, this just naked-eye noticeable transmitter would be
just about capable of sending a 10 Gbit/s data stream across 10 light
years with low bit-error-rate {BER} (Equ. 37). This would allow a
hypothetical Encyclopedia Galactica to be uploaded or downloaded rather
efficiently!
This might give new meaning to Arthur C. Clarke's "Extra-
Terrestrial Relays", which in the October, 1945 issue of WIRELESS WORLD
described the basic idea for the present terrene geostationary (the
Clarke Belt) satellite system. [67] Clarke had originally given his
article the title "The Future of World Communications". Perhaps this
paper should be titled "The Future of Interstellar Communications"?
In many ways, Arthur C. Clarke and "Extra-Terrestrial Relays" has
done more to shape what we now call the "Global Village" than any
other single factor on our planet. Indeed, the spreading of the ideas
of democracy, and freedom, and the breakup of the Soviet Empire have
more to do with former Soviet President Mikhail Gorbachev, Russian
President Boris Yeltsin, and author Clarke than any other factor. The
latter is perhaps the unsung hero here. The failure of the August 1991
Soviet Coup was facilitated by the ease with which it is now possible
to communicate. Those readers who own TVROs (TeleVision Receive Only)
satellite receivers will especially appreciate the power of this
technology. We can be sure that the reception, demodulation, and
decoding of the first ETI signal - be it microwave, millimeter-wave, or
optical - will have an immense effect upon our civilization. Just the
act of detecting a carrier signal will forever change our view of the
Universe and humanity.
The following section deals with the amateur approach to Optical
SETI, showing how an amateur observatory can be constructed. This is
based on the more controversial assumption that optical ETI signals may
be present in the visible spectrum, and of sufficient intensity, to
yield detectable signals with relatively small receiving telescopes.
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