Amateur Optical SETI - Basic Assumptions
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
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