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EJASA - Part 7


                                                                     Page 42

                            AMATEUR OPTICAL SETI

        Working on the assumption that highly advanced ETI technology could
    appear to late Twentieth Century humanity like "magic", it is imagined
    that ETIs will be using much larger transmitting telescopes or arrays
    and transmitter powers far greater than 1 kW. [56]  In practice, the
    signal is likely to be pulsed, and, depending on the duty cycle, even
    less detectable by normal integrating detectors, i.e., the unaided
    eye, photographic plates, or standard CCDs.  Optical SETI is a branch
    of science to which the enthusiastic amateur astronomer may be able to
    make a useful contribution.  In so doing, this may increase public
    and scientific interest in all forms of SETI so that this field of
    scientific endeavor will at last get the financial support and effort
    it richly deserves.


                             Optional                     I
                            Intensifier         ----------<---------
                  --          ------           |
     Signal Pr   |  |        |      |          |
    -----------> |  | -----> |      | -----> -----   PIN Photodetector,
    -----------> |  | -----> |      | -----> /   \ APD or Photomultiplier
    -----------> |  | -----> |      | -----> -----
     Background  |  |        |      |          |        -----
         Pb       --          ------           |       |     |
             Narrow-Band                        --->---|     |----->
       Optical Bandpass Filter                         |     |
        (or Monochromator) Bo                           -----
                                             Low-Pass Electrical Filter
                                                          Be

    Figure 7 -

    Incoherent (direct) detection optical receiver.  The image or photon
    intensifier is only required if a zero-gain PIN photodetector is
    employed.  The narrow-band optical filter (Bo < 0.1 nm) is ideally
    a tunable device like a scanning grating monochromator.  The photo-
    detector current I is proportional to the received signal Pr.


        Figure 7 is a basic schematic of an incoherent photon-counting
    receiver for an Amateur Optical SETI Observatory.  The high cost and
    technical difficulties of optical heterodyne detection in the visible
    and near-infrared spectrum means that the amateur's receiver will most
    likely have to use photon-counting, a little cooling, and a mono-
    chromator.  Unlike coherent receivers, incoherent receivers do not
    have the ability to reject Planckian starlight and daylight background
    noise if the signal is weak.

        Figure 8 results use slightly more conservative assumptions than
    employed to derive Table 2 (Equ. 15, Page 78).  It is assumed that the
    amateur telescope has a diameter of thirty centimeters (twelve inches),
    uses a low-resolution scanning grating monochromator bandwidth of
    100 GHz (0.143 nm) at 656 nm, and employs a receiver consisting of a


                                                                     Page 43

    single perfect photon-counter.  For a received flux density of
    10^-12 W/m^2, the SNR is about 39 dB re 1 Hz (Equ. 31, Page 87).  In
    the region of the graph where the SNR is reduced due to Planckian
    starlight, daylight background further reduces the SNR by a few dB.

        In the Microwave Cosmic Haystack, the flux densities of interest
    lie in the range of 10^-27 to 10^-20 W/m^2.  It is suggested that the
    corresponding flux levels in the Optical Cosmic Haystack would be in
    the range of 10^-20 to 10^-10 W/m^2.  As indicated in Figure 8, an
    EIRP = 10^23 W at a range of ten light years produces a received signal
    intensity Ir = 10^-12 W/m^2, with an apparent visual magnitude of
    eleven.  This would not be visible to the unaided eye even if it was
    not completely outshone by the second magnitude star.

        This 39 dB Signal-To-Noise Ratio represents an SNR penalty
    compared to the performance of a 10-meter heterodyning array receiving
    telescope of about 34 dB.  This 34 dB SNR penalty figure should not be
    confused with the 34 dB CNR that was established in Table 2 (Page 22)
    for a 1 kW transmitter.  Starlight and daylight sky backgrounds only
    slightly affect the SNR for this range, intensity, and optical
    bandwidth.  The effect of the 10 to 20 dB Fraunhofer Planckian
    suppression factor has not been included in the graph of Figure 8;
    allowance for which would improve the night sky performance for weaker
    signals and/or larger optical bandwidths.

        If a powerful ETI signal is detected, given an adequate SNR, it
    might even be possible for an amateur observer to demodulate a signal
    of moderate bandwidth, not just detect the presence of an excess
    number of photons arriving in a given time!  A photodetector bandwidth
    of about 1 MHz would probably be desirable, and well as a spectrum
    analyzer covering a similar frequency range.

        As can be seen from Figure 8, the SNR is degraded by Planckian
    starlight at low signal intensities and larger optical bandwidths.  In
    this regime, if the signal flux drops by 20 dB, the SNR falls by 40 dB
    because the receiver is no longer signal quantum noise limited.
    Clearly, if ETIs want their signals to be detected by relatively small
    incoherent receivers, it pays to use pulses with low duty-cycle in
    preference to C.W. signals.  High peak EIRPs can override all external
    and internal noise sources and thus make their signals as detectable
    as possible for a given mean EIRP.

        In Table 2 we showed that the 1 kW signal at a range of ten light
    years produces a received intensity of 2.04 X 10^-17 W/m^2.  If this
    was received by a one-meter diameter incoherent adaptive ground-based
    telescope, the normalized SNR in a 100 GHz (0.143 nm) optical bandwidth
    (not allowing for Planckian dark line continuum suppression) would be
    about -42 dB re 1 Hz.  In this situation it would indeed help to
    operate the transmitter within a Fraunhofer line.  The SNR would be
    increased to -32 dB re 1 Hz for a 10 dB Fraunhofer line contrast
    factor.  Either way, the presence of the signal would not be detectable
    without considerable integration.  However, if the ETI transmitter mean
    power was increased to 1 GW, leading to a received intensity of
    2.04 X 10^-11 W/m^2, the SNR would increase dramatically to about


                                                                     Page 44

    Postdetection Normalized SNR, dB re 1 Hz
       |
       |
      80 |
         |
         | Ir = 10^-10 W/m^2       EIRP = 1.1 X 10^25 W (6th Magnitude)
      60 |* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 
         |
         | Ir = 10^-12 W/m^2       EIRP = 1.1 X 10^23 W (11th Magnitude)
      40 |* * * * * * * * * * * * * * * * * * * * * * * * * * * *
         |                                                           *
         | Ir = 10^-14 W/m^2       EIRP = 1.1 X 10^21 W (16th Magnitude)
      20 |* * * * * * * * * * * * * * * * * * * * * *
         |                                               *
         | Ir = 10^-16 W/m^2                                  *
       0 |.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*....................*.....
         |                                       *                    *
         | Ir = 10^-18 W/m^2                         *
     -20 |* * * * * * * * * * * * *                      *
         |                            *                      *
         | Ir = 10^-20 W/m^2              *                      *
     -40 |* * * * * * *                       *                      *
         |                *                       *
         |                    *                       *
     -60 |                        *                       *
         |                            *                       *
         |                                *                       *
     -80 |         Night Sky                  *                       *
         |                                        *
         |                                            *
    -100  --------------------------------------------------------------
       10^0      10^2      10^4      10^6      10^8      10^10  ^  10^12
                                                                |
                             Optical Bandwidth, Hz              |
                                                      100 GHz (0.143 nm)

    Figure 8 -

    Signal-to-noise ratio versus optical bandwidth for (perfect) Photon-
    counting 656 nm receivers.  Range = 10 light years, diameter = 30 cm,
    antenna efficiency = 0.7, spectrometer efficiency = 0.5, quantum
    efficiency = 0.5, excess avalanche gain noise factor = 0, dark
    current = 0.  EIRP of a solar-type star = 3.9 X 10^26 W.  A
    diffraction limited 10-meter diameter 1 GW transmitter produces an
    EIRP = 2.3 X 10^24 W, and appears to be 0.6 percent of the brightness
    of a second magnitude solar-type star.


    63 dB re 1 Hz; essentially independent of Planckian background.  This
    signal would stick out like a proverbial sore thumb.  In the case of
    Professional Heterodyning Optical SETI, we were dealing with stronger
    detected signals and a local-oscillator produced shot noise floor.
    Because we are dealing here with smaller, incoherent receivers that use
    an avalanche photodetector, the precise analysis for the CNR or BER is


                                                                     Page 45

    extremely complex when the received signal power is very small and/or
    larger post-detection bandwidths are employed.  The reader is
    cautioned, that the above results may be somewhat optimistic.

        Figure 8 forces us to consider whether such easily detectable
    signals could have been missed by professional optical astronomers?
    Perhaps, because there are so many stars and frequencies to search,
    and with the limitations of conventional spectrographic equipment, we
    can hope that these signals have been missed or overlooked.  Again, if
    the signals have low duty-cycle, the mean signal powers detected by
    integrating detectors would be considerably less.

        Scanning grating monochromators/spectrometers are available with
    ten times the resolution previously quoted, i.e., 10 GHz (0.0143 nm)
    optical bandwidths.  High-Q Fabry-Perot spectrometers with bandwidths
    as small as 1 MHz are perhaps less useful here because of their free-
    spectral range and multiple response characteristics, requiring
    additional broadband filtering.  However, the tandem combination of a
    scanning grating monochromator and a Fabry-Perot would form a very
    powerful optical filtering and spectral analysis system, comparable in
    many respects to what could be achieved with a heterodyne system.

        For the thirty-centimeter diameter telescope system, ETI signal
    detectability will not be substantially degraded for peak signal
    strengths higher than about 10^-14 W/m^2 (sixteenth magnitude) if the
    spectral resolution < 0.01 nm.  If the EIRP was about 10^25 W, the
    received signal flux would be at the threshold of unaided eye
    visibility of about 10^-10 W/m^2, and yield an SNR of 60 dB re 1 Hz.
    This would give an SNR = 30 dB in a 1 kHz post-detection bandwidth, or
    a just detectable 0 dB in a 1 MHz bandwidth.

        It would appear that as long as we can construct efficient photon-
    counting receivers, that the sensitivity of small incoherent receiving
    telescopes will not be unduly affected by the relatively large optical
    bandwidths of such receivers, though their sensitivity will be degraded
    if operated in daylight.

        There was no particular reason in choosing the 656.2808 nm
    (457.1214 THz) H_alpha line for the purposes of modelling the visible
    system.  While it could be considered a "magic wavelength", it does
    not coincide with a known laser transition.  It has an effective band-
    width of about 280 GHz, though its half-power bandwidth is somewhat
    smaller (Table 4, Page 30).  A less expensive way of undertaking
    Amateur Optical SETI observations at this single wavelength, instead of
    using the more flexible scanning grating monochromator, would be to
    employ a standard narrow-band H_alpha solar filter.  To further reduce
    costs, a photomultiplier could be used in place of the state-of-the-art
    cooled avalanche (geiger-mode) photodetector.

        It may be possible for amateur astronomy groups to "steal a march"
    on NASA as far as the low-sensitivity search for ETI in the visible
    and near-infrared spectrum is concerned.  For Amateur Optical SETI
    to be a sensible pursuit for the astronomical and space enthusiast
    requires the belief that ETI technology would appear to emerging


                                                                     Page 46

    technical civilizations comparable to ourselves to be like "magic".
    The demands placed on assumed ETI technical prowess are even greater
    than when considering the practicality of Professional Optical SETI.
    The onus would be on ETIs to make their signals easily detectable.

        Since peak EIRPs > 10^23 W are thought possible, which lead to peak
    intensities at a range of ten light years greater than 10^-12 W/m^2
    (eleventh magnitude), the detectability of such signals with amateur
    equipment is imaginable.  Telescopes with apertures greater than about
    one meter diameter are only slightly affected by daylight when observing
    nearby stars, indicating that Daylight Professional/Semi-Professional
    Optical SETI may be feasible for larger telescopes with incoherent
    receivers.  It should be realized that even during the day, the sky is
    essentially black when viewed with artificial narrow bandwidth eyes!

        It is not yet clear whether the 81-cm (32-inch) Perkins Telescope
    in Delaware, could be upgraded with a precision-drive system that would
    allow for satisfactory image-tracking during the night and day.  Image-
    tracking difficulties at night might be mitigated by using a photon-
    counting array or image intensifier (or microchannel plate) instead of
    a single photodetector.  There are also some concerns, regarding the
    effects on conventional astronomical nighttime observations, of thermal
    currents caused by the observatory dome being open during the day.

        Because optical bandwidths of these incoherent Amateur Optical SETI
    receivers will be much wider than the effective optical bandwidths in
    coherent Professional Optical SETI receivers, there is no concern for
    anticipating or removing local line-of-sight Doppler chirps (drifts).
    These chirps can be as high as 50 kHz/s (Table 2 and Equ. 40).  Such
    drifts are insignificant for optical bandwidths of the order of 100 GHz
    in any reasonable amount of observation (dwell) time.  Allowance should
    be made for Doppler shifts of the ETI transmitter and Fraunhofer lines
    when making a detailed search of specific frequencies, since these
    shifts can be comparable to the width of a Fraunhofer line (Table 2
    and Equ. 39).  For specific laser frequencies not coinciding with
    Fraunhofer lines, this requires knowledge of our line-of-sight velocity
    relative to the star being observed.  However, for transmissions and
    observations within Fraunhofer lines, the receiver could simply be
    tuned for minimum Planckian starlight noise.  As before, it is assumed
    that ETIs will remove their local line-of-sight transmitter Doppler
    shift (and chirp) with respect to their star.

        It should be noted for the record that thermoelectrically-cooled
    CCD (Charged Coupled Device) cameras are now available to the amateur
    which allow the sixteenth magnitude to be reached in under one minute
    of integration, with negligible threshold effects.  Even the fastest
    photographic film has such low quantum efficiency that only a few
    percent of the photons are converted to exposed film grains.  The dark
    current count for the photon-counter should ideally be kept below about
    five hundred counts per second if the SNR of a potential ETI signal is
    not to be excessively degraded.  It may be reasonable to suggest that
    eliciting the help of thousands of enthusiastic amateur optical
    astronomers might considerably aid the low-sensitivity Targeted Search
    of the entire Northern and Southern Hemisphere skies.


                                                                     Page 47

            HOW TO BUILD YOUR OWN AMATEUR OPTICAL SETI OBSERVATORY

        How easy and cheap will it be for amateur astronomy organizations
    to combine the efforts and resources of their members to participate in
    this activity?  The answer to this is that there is no hard figure.  It
    depends very much on how sophisticated and sensitive one is prepared to
    be.  There will always be tradeoffs between sensitivity and cost.
    Figure 9 shows a basic Amateur Optical SETI system based on the use of
    twenty-centimeter (eight-inch) or larger telescopes.  While smaller
    telescopes (reflectors or refractors) may be used, the potential
    detectability of ETI signals will be degraded.

        However much the reader may be excited by the statements made
    herein, the reality of the situation is that SETI, be it conducted in
    the microwave or optical spectrums, can become a rather monotonous
    endeavor.  It is an activity well-suited for automation.  Hence, the
    system to be described makes extensive use of computer-driven hardware.
    The same computer can be used to analyze the spectral (optical and
    electrical) data obtained with various signal processing algorithms to
    see if there is a weak ETI signal hidden within the noise.

        Particularly for an optical receiver with a wide tuning range,
    i.e., one that uses a grating monochromator, the mass of the
    additional equipment required to be attached would be excessive for a
    small telescope.  Hence, the preferred way to couple the SETI receiver
    to the telescope would be via several meters of a single strand of
    low-loss multimode optical fiber.  The output face of the fiber-optic
    umbilical replaces the slit normally found in a monochromator/
    spectrograph.  This approach is additionally useful if cryogenic
    cooling techniques have to be employed at the optical front-end.

        The optical fiber is positioned to be centrally placed in the focal
    plane and the fiber input arranged by suitable imaging, i.e., SELFOC
    lens (GRIN rod), to match to the telescope's diffraction limited spot
    size (Airy disk).  In practice, if daylight SETI is not attempted, the
    optical fiber's aperture and FOV may be increased to accommodate image
    wander caused by typical atmospheric turbulence conditions.  The
    diagram shows a beamsplitter sharing the image with the CCD, though the
    CCD might make use of off-axis guiding to avoid light loss, i.e., for
    locking onto a guide star.  The graded-index lens also serves to
    convert the focal ratio of the telescope to one that matches the fiber
    for maximum throughput, this operation being equivalent to matching
    numerical apertures.  Some mode scrambling may be required to ensure
    that the output numerical aperture (N.A.) of the fiber is fully
    illuminated at all times, whatever the light launching conditions.
    This ensures that amplitude fluctuations do not occur in the slitless
    monochromator or spectrometer as the image of the star and transmitter
    dances around the entrance (input end) of the fiber.

        Multimode optical fiber essentially depolarizes light, so that any
    polarization analysis equipment must be situated at the input, focal
    plane end of the fiber.  There will be an inherent throughput loss of
    about 50 percent in the monochromator because high resolution
    diffraction gratings have a tendency to polarize light.


                                                                     Page 48

     --------------------   Beamsplitter/Off-Axis Guiding CCD Imaging/
    |      8" - 14"      |-- _  Tracking Camera------------->--------------
    |                    |  |_|->-                                         |
    | Schmidt-Cassegrain |--      | Optional Polarizing Optics & Multimode |
     --------------------         | Fiber-Optic Umbilical in Focal Plane   |
                  |  |            |                                        |
                  |  |            |    -----------------     -----         |
               ---------          |   |     Scanning    |   | APD |        |
              |  Drive  |<>-       ->>|     Grating     |->-| or  |->-     |
               ---------    |         |  Monochromator  |   | PM  |   |    |
                            |          -----------------     -----    |    |
                            |                 ^                       |    |
                            |                 |                     -----  |
                            |                 |                    | Amp | |
                            |                 |                    |     | |
                            |                 |                     -----  |
                            |                 |                       |    |
       -----------          |                 |    -------------      |    |
      |           |         |                 |   |   Optional  |     |    |
      |    VDT    |         |                 |   |   Spectrum  |<----|    |
      |           |         |                 |   |   Analyzer  |     |    |
       -----------          |                 |    -------------      |    |
          |   |             | RS-232/IEEE-488 |                       |    |
      ------------- <>------------<>----------    Baseband Signal     |    |
     |     PC      |<-------------<----------------------<------------|    |
      ------------- <-----                            -------         |    |
        Optional          | CCD Video                |  Low  |        |    |
      FFT Spectrum        |                Audio <---|  Pass |<-------|    |
      Analyzer Card       |                          | Filter|        |    |
                          |                           -------         |    |
                          |          -----------                      |    |
                          |         |   Video   |<--------------------     |
                          |         |  Monitor  |<-------------------------|
                          |         |   Or TV   | CCD Video                |
                          |          -----------                           |
                          |                                                |
                           ------------------------------------------------

    Figure 9 -

    Basic Amateur Optical SETI or Poor Man's Optical SETI.  Only a single
    photodetector is used, which can be either an avalanche photodiode
    (APD) or a photomultiplier (PM).  The optical filter can be a computer-
    controlled scanning monochromator or a relatively inexpensive fixed
    interference filter.  Additional focal-plane optical fibers and photo-
    detectors may be employed for maintaining star-lock.  An electronic
    mixer and filter may be included between the photon-counting receiver
    and the display/audio devices to beat down the detected spectrum to
    lower frequencies.  This electrical local-oscillator would likely be
    driven by the PC.  The TV (video) monitor can be used both to display
    the star field via the CCD imaging/tracking camera and the detected
    signal, or these could be displayed on the PC.  Later, several
    telescopes could be slaved together to increase light gathering power,
    sensitivity, and SNR of a would-be ETI signal.


                                                                     Page 49

        The output of the fiber is expanded and collimated in the usual way.
    However, if a single photodetector is employed, as indicated in
    Figure 9, some form of cylindrical output lens will be required to
    match the aspect ratio of the beam from the diffraction grating(s) to
    the photodetector.  For this reason, some investigators may prefer to
    use a photomultiplier with a large cathode to collect all the photons.

        As this document was nearing completion, the author's attention was
    drawn to a recent report by Douglas et al [93] on an astronomical
    heterodyned spectrometer.  The title of the report is somewhat
    misleading as this author feels that the word "homodyned" would have
    been more applicable.  Unless fringes actually move across a photo-
    detector at an interference beat rate, a system cannot be said to
    really employ heterodyne techniques.  However, the report does describe
    a high resolution spectrometer using a fiber-optic umbilical, and in
    that respect is relevant to the discussion here.

        In Figure 9, the purpose of the conventional CCD is just to display
    the star field on a television (TV) or personal computer (PC) monitor
    and for precision star tracking.  In this preferred design, it does
    not detect the ETI signal; that job is performed by a relatively fast
    single solid-state Avalanche photodetector (APD) or photomultiplier
    (PM).  APDs have the advantage of high quantum efficiency but the
    disadvantage of higher dark current; the converse being the case for
    photomultipliers.  With state-of-the-art solid-state photodetectors
    like the RCA SPCM-100-PQ Single Photon-Counting Module, the cooling to
    reduce dark current noise is applied via Peltier (thermoelectric)
    coolers, and their mass is relatively insignificant.  Though the
    imaging CCD can itself be used as the ETI detector, this approach might
    compromise detection sensitivity and bandwidth.  It would also require
    a very high-quality and expensive CCD array.  This would be incompa-
    tible with the use of the device for star field imaging and fine
    guidance because of the narrow-band optical filtering requirements of
    the SETI receiver.  The input end of the fiber-optic umbilical might be
    dithered in the focal plane to aid guidance, and to ensure fine
    dynamic-tracking on a star's image.  Indeed, four additional optical
    fibers with unfiltered photodetectors might surround the ETI-detecting
    fiber and be used for this purpose.

        Note that the audio monitor in the schematic is for listening to
    the hiss of stellar noise and perhaps audibly detecting the presence of
    a strong artificial signal.  The Planckian background in a 100 GHz
    optical bandwidth for a 2nd Magnitude star, produces a photon-count
    rate of about 18,000 s^-1, which should be compared to the dark-current
    count rate for a high-quality cooled photodetector or photomultiplier
    of less than several hundred counts per second.  An essential component
    will be a variable threshold detector connected to an alarm system.
    The TV or PC monitor could also serve to display a noisy raster and the
    presence of any coherent signals.  It is unlikely though, that an ETI TV
    picture will pop up (in any TV standard), considering the deficiencies
    in SNR and bandwidth with amateur receivers!  However, if high SNR and
    bandwidth can be supported by ETI transmitters and terrene professional
    receivers over interstellar distances, a sequentially scanned TV [36]
    picture would be the most effective bridge between our two cultures.


Continued


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