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Amateur Optical SETI

Radobs 31

 
Is it sensible to suggest that Optical SETI is a branch of science to which
the enthusiastic amateur astronomer can make a useful contribution?  In so
doing, might this not so increase public and scientific interest in all
forms of SETI, that this field of scientific endeavor will at last get the
financial support it richly deserves.  In order to sensibly propose the idea
of "Amateur Optical SETI" it is assumed that ETIs will be using much larger
transmitting telescopes or arrays than we can presently construct on Earth,
and that their transmitter powers would be much greater than the 1 kW that I
usually use to normalize performance.
Unlike the proposed professional heterodyne receiving system with a CCD-type
PIN 128 X 128 photodetector array, an Amateur Optical SETI Observatory would
use a single Avalanche Photodetector (APD) in the Geiger mode as a photon-
counting receiver.  It would be interfaced to the focal plane of the
telescope via a fiber-optic umbilical.  This Optical SETI photodetection and
signal processing system could easily be connected to a variety of amateur
telescopes, including the 32" Perkins adjacent to "Big-Ear".
If we assume an amateur telescope of about 30 cm diameter (12"), the SNR
penalty compared to the professional 10 meter heterodyning receiving
telescope is 30.5 dB.  A perfect 656 nm space-based heterodyne system would
produce an SNR = 94 dB re 1 Hz for a 10 meter 1 GW transmitting telescope
(EIRP = 2.3 X 10^24 W) at a range of 10 light years.  The normalized ground-
based SNR = 88 dB re 1 Hz for the professional system, would shrink to a
level of about 58 dB re 1 Hz.  As described elsewhere, this enormous signal
is only 0.6% of the intensity of a 2nd magnitude star (our Sun at 10 light
years).  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 scanning monochromator driven by a PC.
Unlike large (adaptive) coherent receivers, small incoherent receivers do
not have the ability to reject Planckian starlight and daylight backgrounds,
which will cause fluctuation noise if the signal is weak.  Thus, at a
minimum, the amateur SETI receiver will be at least three orders of
magnitude less sensitive than a large professional receiving system.  In
practice, the sensitivity impairment may be significantly greater due to
less than ideal photo-detection efficiency, and the effect of Planckian
starlight.










SNR dB re 1 Hz
 |
 |
100 |
    |
    |
    |
 90 |               10 m (394") Professional (Adaptive) Heterodyne Receiver
    |----------------------------------------------------------------------
    |  .        .        .
    |  .        .        .
 80 |  .        .        .
    |  .        .        .
    |  21.8 dB  30.5 dB  34.0 dB ------ Degradation
    |  .        .        .
 70 |  .        .        .
    |  .        .        .    0.81 m (32" Perkins) Photon-Counting Receiver
    |----------------------------------------------------------------------
    |           .        .
 60 |           .        .    0.30 m (12") Amateur Photon-Counting Receiver
    |----------------------------------------------------------------------
    |                    .    0.20 m (8") Amateur Photon-Counting Receiver
    |----------------------------------------------------------------------
 50 |
     ______________________________________________________________________
The graph illustrates the theoretical performance comparisons between
ground-based Optical SETI receivers at a range of 10 light years. 
EIRP = 2.3 X 10^24 W, as produced by a 1 GW transmitter, 10 meters in
diameter.  The size of the transmitter and mean transmitter power are not
important here, only the belief that ETIs have the capability of producing
Effective Isotropic Radiated Powers of about 2.3 X 10^24 W.
Wavelength = 656nm,
Atmospheric absorption = 4 dB,
Antenna Efficiency = 0.7,
Quantum Efficiency = 0.5,
Heterodyning Efficiency = 0.5.
From the above relative SNR graph, we see that it might not be ridiculous to
suggest that eliciting the help of thousands of enthusiastic amateur
astronomers would considerable aid the low-sensitivity targeted search of
the entire sky.  To the individual, the idea of Amateur Optical SETI only
makes sense if you believe that ETIs are not technically inept.  An All Sky
Survey of the type planned for the Microwave Observing Project (MOP), which
pixelizes the entire celestial sphere, does not make sense in the optical
regime.  There, the narrow diffraction-limited field-of-view means that for
most of the time (even for small optical telescopes) the optical detector(s)
would be viewing empty space.  It is estimated that a basic amateur system
could be constructed for about $6,000; less if a suitable telescope and
computer system are already available.  Who knows, perhaps ETIs don't expect
their signals to be detected until the targeted civilizations make a
collective, cooperative and systematic search of their home skies!  This
might be just the thing that enthusiastic members of astronomy clubs can
pool their resources to accomplish.  It goes without saying that should the
ETI signals be in the infrared, i.e., 10,600 nm, amateurs are going to be
out of luck with their conventional telescopes.
This author believes in putting his money where his mouth is, and to that
end has already started planning for construction of his own Amateur SETI
Observatory.  Will Bexley beat Delaware in being the first to detect ETI?
I have already purchased a number of components for the receiver and after
my return from the U.K., plan to begin its detailed design and construction.
The Optical SETI receiver will be capable of being interfaced with the
Perkins telescope at a later date.  I will also be building a visible-laser
diode (670 nm) communication system for demonstrating Optical SETI in the
presence of Planckian starlight.  An intense white light source (lamp) will
be used to simulate a star.  In view of complexity and cost factors, the
demo communication system will not be employing a heterodyne receiver but a
narrow-band interference filter centered on 670 nm.
* * * * * * * * * * * * * *
*         8" - 12"        * * * / Optional CCD Imaging Camera ------>------
*                         *   *->>                                         |
*    Schmidt-Cassegrain   * * *   |                                        |
* * * * * * * * * * * * * *       | Fiber-Optic Umbilical in Focal Plane   |
                *  *              |                                        |
                *  *              |   * * * * * * * * * *    * * * *       |
            * * * * * *           |   *    Scanning     *    *     *       |
            *  Drive  *<--         ->>*                 *-->>* APD *-->    |
            * * * * * *   |           *  Monochromator  *    *     *   |   |
                          |           * * * * * * * * * *    * * * *   |   |
                          |                ^                           |   |
      * * * * * * *       |                |      * * * * * * * *      |   |
      *           *       |                |      *  Optional   *      |   |
      *           *       |                |      *  Spectrum   *<-----|   |
      * * * * * * *       |                |      *  Analyzer   *      |   |
          *   *           |                |      * * * * * * * *      |   |
     * * * * * * * *      |                |                           |   |
     *      PC     *------------<->---------------------<->------------|   |
     * * * * * * * *                                                   |   |
                                     Audio                             |   |
                                    <----------------------------------|   |
                                                                       |   |
                                    * * * * * * *                      |   |
                                    *   Video   *<---------------------    |
                                    *  Monitor  *<-------------------------
                                    *   Or TV   *
                                    * * * * * * *
Basic Amateur Optical SETI or Poor Man's SETI.  For simplicity, some
components have been omitted.  For instance, an electronic mixer and filters
may be included between the Avalanche Photodetector (APD) photon-counting
receiver and the display/audio devices to beat down the detected spectrum to
lower frequencies.  The 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 optional CCD imaging camera, and the detected signal.
In keeping with the great tradition of OSU's Radio Observatory, in doing
excellent work on a shoe-string budget (or no budget) and making scrounging
a fine art, I to would like to make an appeal for donations of equipment. 
In particular, I have need of a scanning monochromator, covering the entire
visible and near-infrared part of the optical spectrum, with about 0.1 nm or
better spectral resolution, and being capable of being driven via an RS232
or IEEE 488 interface.  Other than the actual telescope, this would be the
major expense.  In addition, the availability of an R.F. spectrum analyzer
(> 100 MHz) would be useful.  I have a spare PC, and I will be building the
photon-counting receiver.  If anyone can point me in the direction of a very
cheap (or even zero cost) Schmidt-Cassegrain of about 8" to 12" aperture,
and with drives that can also be interfaced to a PC, don't be reticent to
let me know!
Several weeks before my talk at NASA-Ames I wrote to the Presidents of both
Meade and Celestron, thinking that they would be interested in the
commercial aspects of Optical SETI, and might like to attend my NASA talk. 
So far I haven't had a response - probably thought I was a nutter!
May 3, 1991
RADOBS.31
BBOARD No. 497
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley                       Copyright (c), 1991        *
* AMIEE, SMIEEE                                                           *
* Consultant                            "Where No Photon Has Gone Before" *
*                                                   __________            *
* FIBERDYNE OPTOELECTRONICS                        /          \           *
* 545 Northview Drive                          ---   hf >> kT   ---       *
* Columbus, Ohio 43209                             \__________/           *
* United States                            ..    ..    ..    ..    ..     *
* Tel. (614) 258-7402                     .  .  .  .  .  .  .  .  .  .  . *
* skingsle@magnus.ircc.ohio-state.edu         ..    ..    ..    ..    ..  *
* CompuServe: 72376,3545                                                  *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *

 

 


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