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Amateur Optical SETI - Basic Assumptions

Radobs 37

 

He's Back!  While in England, I developed the rationale for Amateur Optical
SETI, and showed that it may not be such a "crazy idea".  You may recall
that I had originally intended to devote some of the time to drafting
technical papers and glossy articles on Optical SETI.  However, I
subsequently decided to give priority to finding out if the exciting idea
of Amateur Optical SETI was sensible.  Over the next few weeks I will be
uploading a series of documents aiming to prove that it is, with the caveat
that if the reader doesn't believe that ETI technology will be found to be
vastly superior to our own, then this material is only of academic
interest, and there is no need to read any further.  Some of the statements
made below are supported by the results of this analysis which will be
described in RADOBS.43A, RADOBS.43B and RADOBS.43C.  The following is a
list of upcoming documents:
RADOBS.37      AMATEUR OPTICAL SETI - BASIC ASSUMPTIONS (this file)
RADOBS.38      SNR/CNR & INCOHERENT/COHERENT DETECTION
RADOBS.39      SNR VERSUS DETECTION SYSTEM (spreadsheet)
RADOBS.40      SNR VERSUS OPTICAL BANDWIDTH (spreadsheet)
RADOBS.41      PHOTON-COUNTING SNR VS OPTICAL BANDWIDTH (spreadsheet)
RADOBS.42      PHOTON-COUNTING AND DARK CURRENT
RADOBS.43A     FEASIBILITY OF AMATEUR OPTICAL SETI- PART A
RADOBS.43B     FEASIBILITY OF AMATEUR OPTICAL SETI- PART B
RADOBS.43C     FEASIBILITY OF AMATEUR OPTICAL SETI- PART C
Here now is the first of these files, delineating the underlining
assumptions that lead me to think that Amateur Optical SETI is a worthy
pursuit:
1.   ETIs find it is easy to communicate within their local group of stars 
     (within a few thousand light years) using visible and near-infrared
     optical techniques.  We assume that ETIs are using lasers for
     interstellar communications, and that their principal communication
     wavelengths lie in the visible or near-infrared.  They do not or not
     often operate in the far-infrared, e.g., the Carbon Dioxide wavelength
     of 10,600 nm, for interstellar communications over distances of less
     than 1,000 light years.  If we wish to look out more than several
     thousand light years, particularly in the galactic plane where
     interstellar dust and gas severely attenuates visible and near-
     infrared radiation, we would more profitably devote the search to the
     far-infrared.  The one million solar-type stars within a sphere 1,000
     light years in diameter, gives us plenty of work for the visible and
     near-infrared search.  Amateur Optical SETI at 10,600 nm is likely to
     be very difficult, and would require special (cooled) telescopes and
     detectors optimized for that wavelength region.  The atmospheric
     transmission window at 10,600 nm is about 50%, and the 300 K
     temperature of the sky presents a considerable noise background, day
     or night.  In due course, I intend to undertake a similar analysis for
     Amateur Infrared SETI in the 10,600 nm region of the spectrum.
2.   Amateur Optical SETI in the visible and near-infrared regime is an
     affordable activity for enthusiastic amateur astronomers groups or
     clubs.
3.   Amateur Optical SETI Observatories should conduct a targeted search of
     about 800 stars within 1,000 light years of the Earth, the same stars
     that are the subject of investigation in the Microwave Observing  
     Project (MOP).
4.   Initial frequency bands to be searched will be those in the visible
     and near-infrared spectrum associated with Fraunhofer dark absorption
     lines, and major (efficient) laser transitions presently known to
     mankind.  Although Fraunhofer dark lines should be search first, known
     laser frequencies not coinciding with Fraunhofer lines should also be
     inspected.  As will be shown, even for incoherent receivers, operation
     of strong ETI transmitters within a dark line is not mandatory, though
     for weaker signals the increased contrast ratio would improve signal
     detectability.
5.   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),
     which can be as high as 50 kHz/s.  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.  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.  It is assumed that ETIs will remove their
     local line-of-sight transmitter Doppler shift with respect to their
     star.
6.   The analysis is based on the ambitious desire to detect the modulation
     envelop.  Hopefully, the ETI signals will be intensity or
     polarization-modulated so that the modulation can be detected by an
     incoherent receiver.  For convenience, all signal-to-noise ratios in
     the following analyses are normalized to a 1 Hz post-detection
     bandwidth, but they imply no preconceived notion as to the ideal
     detection bandwidth.  For weak signals, we may only be able to detect
     the presence of an optical carrier or beacon (perhaps Signpost SETI),
     and then, only after some signal integration.  However, this would be
     a significant achievement by itself, and would allow for more powerful
     professional receivers to be built later for detecting the modulation
     envelop.
7.   Whereas a large Professional Optical SETI Receiver would employ
     coherent heterodyne detection, linear and/or circular polarizers, and
     at least two PIN photodetector arrays, on the grounds of simplicity
     and cost, a small Amateur Optical SETI Receiver would employ a
     scanning spectrometer, optional linear polarizers and circular
     waveplates, incoherent photon-counting, and a single cooled avalanche
     photodetector (APD) operated in the Geiger mode.
8.   Whereas a large Professional Optical SETI Receiver might employ a
     10 GHz Multi-Channel Spectrum Analyzer (MCSA), on the grounds of
     simplicity and cost, a small Amateur Optical SETI Receiver might
     employ detection/display devices as simple as an audio output and
     video monitor, or a conventional low-frequency spectrum analyzer, or a
     1 MHz MCSA with 100 kHz, 10 kHz, and 1 kHz bins.
9.   ETI technology is vastly superior to that available to 20th Century
     man, i.e., they are not technical inept.  Their technical prowess will
     allow for the transmission of very high power narrow beamwidth optical
     signals, to selected targeted star systems.  The onus will be on them
     to ensure that their signals are detectable by emerging technical
     civilizations such as our own.  We need make no specific assumption
     about the power, size or vastness of ETI transmitting telescopes or
     arrays, or the modulation bandwidths, only that they will have the
     desire and capability to be able to afford to send very high EIRP
     signals on a semi-continual basis to many carefully selected targets.
10.  The existence of very large optical ETI EIRPs (of the order of
     10^23 W) is postulated, up to a mean level that is likely to have been
     discovered accidently by optical astronomers.  We note here that the
     Sun's EIRP = 3.9 X 10^26 W.  As a reminder, a puny diffraction limited
     10 meter diameter telescope with a 1 GW transmitter can produce an
     EIRP = 2.3 X 10^24 W, equivalent to 0.6% of the Sun's brightness.  Of
     course, low duty cycle signals with much higher EIRPs are possible
     that still wouldn't be detectable by casual optical observations using
     integrating detectors.  The existence of "mean" ETI flux levels in the
     Optical Cosmic Haystack in the region of 10^-20 W/m^2 to 10^-10 W/m^2
     is conjectured.  An EIRP = 1.13 X 10^23 W will produce a flux
     intensity at a range of 10 light years of 10^-12 W/m^2.  The solar
     flux level at this range is 3.5 X 10^-9 W/m^2.  The Microwave Cosmic
     Haystack generally covers flux levels in the region of 10-^30 W/m^2 to
     10^-20 W/m^2.
11.  As suggested by Dr. John Rather of NASA-HQ (see the upcoming paper
     "Laser Revisited: Their Superior Utility for Interstellar Beacons,
     Communications, and Travel" - August 1991, Vol 44, No. 8, Journal of
     the British Interplanetary Society) ETI transmitting arrays may be so
     large that they have an extensive Rayleigh near-field range. 
     Alternatively, as I have suggested, their beamwidths may be modulated
     to take account of the distance of the target, such that flux
     densities for targets at 1,000 light years are similar to flux
     densities at 10 light years.  If flux densities at a range of 10 light
     years of 10^-12 W/m^2 are postulated, similar flux densities at 1,000
     light years may be possible, leading to even less of a problem from
     Planckian starlight at the extremes of our search range.  The backing
     off of nearby ETI transmitter EIRPs may be another explanation as to
     why powerful nearby transmitters have not been accidently detected.
12.  Optical bandwidths of the order of 100 GHz (0.143 nm), typical of
     relatively simple incoherent photon-counting receivers with scanning
     grating spectrometers, are compatible with signal flux levels of the
     order of 10^-12 W/m^2 from nearby stars.  At this and greater flux
     levels, background Planckian starlight does not degrade the signal-to-
     noise ratio.  At further distances, assuming the inverse square law,
     signal flux levels may be decreased at the rate of 20 dB per decade of
     range without SNR degradation from Planckian starlight, though of
     course, the SNRs (signal-to-quantum noise ratios) will fall due to the
     reduction in signal level.  By compatible, I mean to suggest that the
     SNR re 1 Hz bandwidth is substantially positive at low optical
     bandwidths, and is not reduced by the Planckian background for the
     moderate optical bandwidths that are provided by conventional
     spectrometers.
13.  Although the following analyses and previous ones have been based on
     the reception of CW signals, it is more likely that such signals will
     be pulsed.  As indicated above, some ETI transmitters may have peak
     EIRPs higher than solar-type stars , i.e., > 3.9 X 10^26 W, yet the
     mean EIRPs will be substantially less than produced by such stars, and
     thus not detectable on a casual basis by conventional integrating
     detectors, i.e., the naked eye, photographic film or CCD cameras.
14.  Very high peak EIRPs ensure that the transmitters considerably
     outshine their stars when viewing such signals with moderate bandwidth
     optical filters.  The received signals will then be quantum shot-noise
     limited throughout the duration of the pulse, overriding all other
     external and internal noise sources, and ensuring maximum
     detectability.  Since ETIs might expect that their signals may
     initially be detected with incoherent optical receivers (square-law
     detection), the 20 dB SNR decrease per decade of peak signal power
     reduction which occurs when signals are background or internally noise
     limited, is another reason why a pulsed signalling technique would be
     preferable.
15.  Larger Amateur Optical SETI Receivers (apertures > 0.8 m) may be used
     in daylight, under a clear blue sky for the targeted search of nearby
     stars without degradation in sensitivity.  However, accurate tracking
     of a star in daylight may be difficult, and SNR degradation due to
     daylight may be increased if diffraction limited pixel size and FOV
     has to be enlarged above that of the Airy disk [(2.44)Wl/d] to
     accommodate tracking errors and atmospheric turbulence.
16.  The photon-counting receiver will be substantially cooled to reduce
     dark-current noise.  Hopefully, cryogenic systems can be avoided by
     the use of multi-stage Peltier devices (see RADOBS.42).
17.  The analysis on the effect of Planckian starlight and daylight on ETI
     detectability in incoherent receivers is based on unpolarized light,
     so that these backgrounds may be reduced by 3 dB if a polarizer is
     employed.
18.  If the dwell time per optical frequency is significant, i.e., several
     seconds, slowly spinning retardation plate/polarization analyzers may
     be used to check for the presence of polarization-modulated signals or
     to improve carrier detectability if a discovered ETI signal is
     background limited.
19.  Should an Amateur Optical SETI group detect optical ETI signals, the
     announcement of the discovery (post-detection protocol) will first be
     conveyed to Bob Dixon and the SETI Institute, and independently
     confirmed, prior to any public release of the news.
Postscript
I will remind the reader here that the intention is to design and construct
a suitable cooled photon-counting receiver that can be used with any
optical telescope.  The receiver will be interfaced via a fiber-optic
umbilical.  I will probably propose that a prototype receiver be first
designed to work with a 12" telescope, with the intention of later fitting
it to the 32" Perkins telescope adjacent to "Big Ear".  Other than the
specific computer interface for the close-looped control of the telescope's
motion, the receiver will be of such generic design as to have universal
application.
Over a dozen high-quality viewgraphs summarizing the results of this
investigation will be sent to Dr. Kent Cullers at the SETI Institute within
the next few weeks.  A couple of these graphs will be (crudely) reproduced
in documents RADOBS.43A, RADOBS.43B and RADOBS.43C.
June 30, 1991
RADOBS.37
BBOARD No. 584
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley                       Copyright (c), 1991        *
* AMIEE, SMIEEE                                                           *
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