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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.


Continued


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