Amateur Optical SETI
Proceedings of SPIE's Los Angeles Symposium, OE LASE '93, Vol. 1867, The Search for Extraterrestrial Intelligence (SETI) in the Optical Spectrum, Los Angeles, California, January 21-22, 1993, pp. 178-208.
Copyright ©, 1993, The Columbus Optical SETI Observatory
Stuart A. Kingsley
In the companion review paper on so-called Professional Optical SETI, it was suggested that ETIs are more likely to use lasers to contact emerging technical civilizations, and that such optical ETI signals will have very high EIRPs. This paper further proposes, that it is a sensible activity for amateur optical astronomers to construct their own Optical SETI observatories. Details are given of the equipment required and the approximate costs. The author describes the Optical SETI Observatory which is presently under construction in Columbus, Ohio. A coordinated Amateur Optical SETI (AMOSETI) activity could make a useful contribution to SETI research by conducting a low-sensitivity Targeted Search in the visible and near-infrared spectrums. This could be done in parallel with the present NASA Targeted Search that is part of the High Resolution Microwave Survey (HRMS). Signal processing techniques and data-handling procedures developed for this AMOSETI research activity, would set the stage for NASA's eventual extension of HRMS into the optical regime.
In this paper, which is complementary to the Professional Optical SETI paper1, it is suggested that Amateur Optical SETI (AMOSETI) enthusiasts can make a useful contribution to the Search for Extraterrestrial Intelligence. This could be done by using medium-size amateur optical telescopes with photon-counting receivers having optical bandwidths in excess of 100 GHz. This paper is an expanded version of material that appeared in the original EJASA mini-book article.2
The word Amateur in the title of this paper is a bit of a misnomer, for there is nothing amateurish about what will be described herein. Rather, it is a reflection of the size of the receiving telescope aperture that warrants the use of the term Amateur. Indeed, the aperture size in the author's prototype Optical SETI Observatory or Optical Earth Receiving Station is 25.4 cm (10"); a little larger than the near-infrared ETI Uplinks proposed in NASA's 1971/73 Project Cyclops study described in the companion paper!1-2
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 used in the previous model.1-2 In practice, as Monte Ross3-9 has suggested, ETI signals are 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. It is believed that 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.
The high cost and technical complexity associated with optical heterodyne detection in the visible and near-infrared spectrum, as described in the complementary companion paper1-2, implies that the amateur receiver will most likely have to use photon-counting, a little cooling, and perhaps a monochromator (tunable optical filter). Unlike coherent receivers, incoherent receivers do not have the ability to reject Planckian starlight and daylight background noise if the signal is a weak continuous wave (cw) optical carrier.
1.1 Continuous Wave Beacons
Figure 1 results use slightly more conservative assumptions than modelled in Table 2 and Figure 2 to Figure 3 of the companion paper1; allowing this time for atmospheric transmission loss and deficiencies in the receiver, but with a dark-current that is assumed to be negligible. Again, the calculations are normalized to a reference range of 10 light years.
It is assumed that the amateur receiving telescope has a diameter of 30 cm (12"), uses a low-resolution scanning grating monochromator bandwidth of 100 GHz (0.143 nm) at 656 nm, and employs a direct-detection receiver consisting of a single avalanche photodiode (APD) or photomultiplier tube (PMT). The antenna efficiency = 0.7, monochromator efficiency = 0.5, APD quantum efficiency = 0.5, and the excess avalanche gain noise factor = 0. Now the Effective Isotropic Radiated Power (EIRP) of a solar-type star = 3.9 X 1026 W, and a diffraction-limited 10-meter diameter 1 GW transmitter (uplink) produces an EIRP = 2.3 X 1024 W. As previously shown1-2, this would appear to be 0.6 percent of the brightness of a second magnitude solar-type star (uncorrected for wavelength).
As in the complementary paper1-2, the Ha wavelength of 656 nm has been chosen for modelling purposes and does not represent a so-called "magic wavelength". In the region of the graph where the SNR is reduced due to the Planckian starlight background, daylight background further reduces the SNR by several dB, if attempts are made do to Optical SETI during the daylight hours.
Telescopes with apertures greater than about one meter diameter are only slightly affected by daylight when observing nearby stars for cw beacons, 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 has previously been suggested1 that the Great Telescopes of the world could be shared between conventional nighttime astronomy and daytime Optical SETI (Symbiotic SETI). There are, of course, concerns regarding the effects on conventional astronomical nighttime observations, of thermal currents caused by the observatory dome being open during the day, though adaptive optics might be able to overcome these problems1-2.
In the Microwave Cosmic Haystack, the flux densities of interest lie in the range of 10-27 to 10-20 W/m2. 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/m2. As indicated in Figure 1, an EIRP = 1023 W at a range of ten light years produces a received signal intensity Ir = 10-12 W/m2 (-120 dBW/m2), with an apparent visual magnitude of +11 if centered at the 500 nm peak of the night-adapted photopic response. Such an ETI signal would not be visible to the unaided eye even if it was not completely outshone by the second magnitude solar-type star.
At a received flux density of 10-12 W/m2 (-120 dBW/m2), the Signal-To-Noise Ratio (SNR) is about 39 dB normalized to a bandwidth of 1 Hz. The 39 dB normalized SNR represents a penalty compared to the performance of a 10-meter heterodyning array receiving telescope1-2 of about 34 dB. This 34 dB SNR penalty figure should not be confused with the 34 dB Carrier-To-Noise Ratio (CNR) that was established in Table 1 of the companion paper for a 1 kW transmitter.1 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 calculations for the graph of Figure 1; allowance for which would improve the night sky performance for weaker cw signals and/or larger optical bandwidths.
As can be seen from Figure 1, 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, they would seek to use pulses with low duty-cycle in preference to cw 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. This will be discussed in more detail later. From the Figure 1 graph of the modelled receiver, we can see that ETI signal detectability will not be substantially degraded for peak signal strengths higher than about 10-14 W/m2 (sixteenth magnitude) if the spectral resolution is less than about 0.01 nm.
In Table 1 of the earlier paper1, we showed that the modelled 1 kW signal at a wavelength of 656 nm and a reference range of ten light years, produces a received intensity of 2.04 X 10-17 W/m2. If this was received by a one-meter diameter incoherent ground-based telescope, the normalized SNR in a 100 GHz (0.143 nm) optical bandwidth (not allowing for Planckian Fraunhofer 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 mean ETI transmitter power was increased to 1 GW, thus producing an EIRP = 2.3 X 1024 W, this would lead to a received intensity of 2.04 X 10-11 W/m2, and the SNR would increase dramatically to about 63 dB re 1 Hz; essentially independent of Planckian background. This signal would stick out like a proverbial sore thumb. Another way at looking at this result is that the SNR = 33 dB in a 1 kHz post-detection bandwidth, or 3 dB in a 1 MHz bandwidth.
If such a powerful cw ETI signal is detected, 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! For the detection of a cw beacon and low-bandwidth modulation, a baseband photodetector and spectrum analyzer bandwidth of about 1 MHz should be more than adequate.
In the case of Professional Heterodyning Optical SETI1-2, we receive stronger detected signals and have a shot-noise limited noise floor set by the local-oscillator laser. Because we are dealing here with smaller, incoherent receivers that use an avalanche photodetector or photomultiplier tube, the precise analysis for the CNR or Bit-Error-Rate (BER) is extremely complex when the received signal power is very small and/or larger post-detection bandwidths are employed, and no account is made for 1/f noise. The reader is cautioned, that the above results may be somewhat optimistic.
Figure 1 forces us to consider whether such easily detectable signals could have been missed by professional optical astronomers with their larger telescopes? 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, and would be integrated into the noise by conventional photometers.
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.1-2
From what we have just seen, it would appear that as long as we can construct efficient incoherent receivers, that the sensitivity for small 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.
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. 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. As mentioned in the companion paper1-2, it is generally assumed that ETIs will remove their local line-of-sight transmitter Doppler shift (and chirp) with respect to their star.
1.2 Pulsed Beacons
Up to now we have been discussing the detectability of visible-wavelength (or near-infrared) cw beacons transmitted by ETIs. However, it is far more likely that ETI signals and associated beacons will be in the form of pulses, since that form of modulation makes their detection far easier. Monte Ross (conference co-chair) has pioneered the idea that pulses and pulse position modulation (PPM) are very effective for interstellar communications, details about which may be read elsewhere in these proceedings.1,3-9
Thus, it will suffice to make just a few comments here about the efficacy of pulse modulation techniques for interstellar communications. Figure 2 shows a pulse train, where by having a small duty cycle, the peak EIRP can be much higher than the mean EIRP. At the receiver end of the link, this makes it much easier to ensure that the optical front-end is quantum-noise limited. It also substantially reduces the requirement for optical predetection filtering, allowing the ETIs' signal to "outshine" their star with minimal optical bandpass filtering. The implication for so-called Amateur Optical SETI with small telescopes is that such small collecting areas may still be able to produce detectable signals for relatively low mean signal strengths.
Tne other advantage of using pulses is that at the receiver, photon-counting techniques can be effectively employed to yield far higher SNRs than are possible with conventional direct-detection receivers. As shown in the diagram of Figure 2, SNRpulse >> SNRcw. If the receiver's output is discriminated so that counts are made during small time intervals, then the ability to separate out real signal photoelectrons from Planckian background photoelectrons and photodetector dark-current is substantially improved. Indeed, the discrimination improvement is so high, that it may not be necessary to cool the optical receiver to reduce the dark-current count!
Figure 3 shows how a low duty-cycle pulse can represent a digital word by arranging short pulses to fall into different time slots.6-7,9 In the Professional Optical SETI (OSETI) paper1-2 and in most of conventional Microwave SETI (MSETI), the discussion of ETI signal detectability have largely concentrated on detecting cw beacons. However, if a wideband data-stream is transmitted by the ETIs, then it might be sensible to include a stronger pulsed beacon signal within that data-stream. This might be dual purpose - not only drawing attention to itself and acting the role of a cw beacon, but also acting as framing or synchronization pulses. It is this stronger pulsed beacon that amateurs are more likely to detect, though it is not without question that ETI signal strengths might be substantial enough for amateurs to detect the data-stream!
Table 1 shows the projected performance data for the 25.4 cm (10") diameter Meade LX200 Schmidt-Cassegrain Telescope (SCT) which is at the heart of The Columbus Optical SETI Observatory, and which will be described shortly. Table 1 in this paper is a small telescope version of Table 3 given in the companion paper1 for a 10-meter diameter telescope. The system employs incoherent photodetection, but may use different receivers; one being optimized for low-bandwidth continuous wave (cw) direct-detection and the other for wide-bandwidth pulse detection by photon-counting.
The nomenclature for this table will be repeated here for convenience in a more extensive format. The shaded areas of Table 1 represent either SNRs that are negative or insufficient bandwidth (IB). For cw modulation at bandwidths above 1 kHz, the negative SNRs will be "optimistic" because the Noise Equivalent Power (NEP) for the chosen direct-detection receiver (10-4 pW//Hz) is too low and incompatible with the higher post-detection bandwidths. NEPs for wideband solid-state receivers are normally of the order of 10 pW//Hz, while photomultiplier NEPs are about 10-6 pW//Hz. However, all this for the moment is of academic interest since the shaded SNRs are negative anyway. Lines "a" to "c" are projections for detecting a cw optical carrier or a cw subcarrier modulation of the optical carrier. This signal could be the ETI "beacon" so favored by SETI lore. Subcarrier modulation may be preferred for the "beacon" in order to overcome 1/f noise. The "beacon" itself might contain low bandwidth modulation, being effectively the Rosetta Stone for the wideband channel. Lines "d" and "e" are estimations of the photon-counter detectability of 1 ns beacon pulses, transmitted at one second intervals. Lines "f" to "l" are the performance projections for various digital modulation schemes employing Pulse Position Modulation (PPM).3-9
Wavelength = 656 nm
Shaded areas denote undetectable
signals, i.e., negative SNRs, or insufficient bandwidth (IB).
The Mean Power is the average power of the transmitter, which range in the table from 1 kW to 1 GW. The Peak Power is the peak power during the pulse. In the case of cw signals, it is identical to the Mean Power. The Pulse Duration is the time that the signal is "on", and is of course not applicable (NA) for the cw situations. The M-ary PPM indicates the number of encoding levels or time slots for the Pulse Position Modulation schemes. The Bits Per Pulse is the number of bits that are encoded per pulse by the M-ary PPM modulation scheme. The Data Rate is the effective transmission rate with the M-ary modulation scheme. The Peak EIRP is the peak Effective Isotropic Radiated Power during each of the pulses, assuming a puny (for ETIs) 10-meter diameter uplink. The Mag is the apparent visual magnitude of the cw transmitter if its wavelength was centered on the peak dark-adapted responsivity of the human eye (500 nm), and is not applicable for the pulsed modulation scenarios. The Peak Signal is the peak value of the optical signal focussed onto the photodetector. The Photons Per Pulse is the number of photons that arrive at the photodetector during each pulse. The Post-Detection SNR is the signal-to-noise ratio in 1 Hz, 1 kHz, 1 MHz, and 1 GHz bandwidths. In the case of pulse detection not limited by background radiation or dark count, it is the photon detection rate per pulse, which differs from the Photons Per Pulse by the quantum efficiency. Generally, to satisfactorily detect pulses of 1 ns duration (with similar rise and fall times), the bandwidth should be at least 350 MHz.
It should be clear from Table 1, assuming the advanced technical prowess of ETIs in producing powerful cw and pulsed laser transmitters, that the cw (lines "b" and "c") and single-pulsed SNRs (lines "d" and "e") are adequate to allow detection by small receiving telescopes. Scenarios "d" and "e" might constitute a "pulsed beacon". It can be seen that cw SNRs are likely to be too small to allow for the successful demodulation of intelligence, unless the "beacon" signal itself has a very low bandwidth intensity modulation envelop. For the pulsed systems "f" to "i" and "k", signal levels for the various scenarios are of insufficient intensity to allow detection, let alone error-free demodulation.
The scenario according to the 1 GW transmitter of line "e", shows that the "beacon" actually outshines the solar-type star by nearly seven orders of magnitude during each 1 ns pulse! Even the very modest 1 MW transmitter of line "d" outshines the star by nearly four orders of magnitude, and would be easily detectable without any optical filtering. Present-day laboratory lasers are now capable of generating sub-picosecond pulses in the 50-Terrawatt range. Hence, assuming that an advanced technical civilization could transmit multi-terrawatt nanosecond-type pulses is not as far-fetched as might first be imagined. Moreover, the extremely high peak EIRPs could allow for relatively small telescopes to undertake observations during the day under a clear blue sky with relaxed optical filtering requirements!
Note that for the pulsed systems, the background radiation count due to the extra-solar background in a 100 GHz (0.14 nm) optical bandwidth is essentially negligible for the 25.4 cm diameter telescope, i.e., 6.6 X 10-4 counts per ns. Thus, speculating these high EIRPs, optical bandwidths can be made significantly larger than 100 GHz without impacting the SNR and BER. Conventional low-cost interference filters of 10 nm bandwidth would not impact the SNR or BER. Indeed, the optical bandwidth could be increased substantially above 100 nm before significant degradation occurred in the scenarios with positive SNRs. This is a major advantage over the cw approach and it also significantly cuts down the search time. For the amateur enthusiast, it can also lead to significant savings in the cost of the optical filters or scanning monochromator.
Another advantage of the pulsed M-ary PPM approach, as mentioned earlier, is that it is much easier to make the effect of dark current insignificant, since as with stellar background radiation, the noise count during the short pulses will be very small, i.e., 2 X 10-8 counts per nanosecond time slot in the Table 1 model. Since photon counts are 250 counts per pulse for the "l" scenario, it can be seen that this level of dark-current can have no effect on SNR and BER.
The scenario of line "j" for the 25.4 cm (10") Meade SCT, indicates that a 8192 M-ary PPM transmitter of 1 GW mean power could send a detectable signal with a data rate of 1.6 Mbits/s, albeit with significant error; the number of photons per bit being 0.3. For the "l" scenario, BER . 10-8 at a data rate of 36 kbits/s would be obtainable for a mean transmitter power of 1 GW. The number of photons per bit for this case is 13. Thus, if ETIs choose to use a very high level of M-ary PPM, then it would be possible for small telescopes to successfully detect the data stream, though synchronization might be another problem! Because the EIRP of a solar type star is 3.9 X 1026 W but the EIRPs of scenarios "d", "e", "i", "j","k" and "l" are greater than this value, such signals could be detectable with little or no reduction in BER, without any optical bandpass filtering. Thus, the entire visible and near-infrared band could be monitored simultaneously, reducing the targeted search problem and search time to a relatively trivial level.
After finding indications of powerful pulsed ETI signals with a small telescope, bringing the Great Telescopes to search the same star with similar optical detection equipment might yield a signal that could be demodulated with little error. Whether we could "understand" the signal is altogether another matter!10
Now we come to the really interesting and exciting part of this paper. How easy and cheap will it be for amateur astronomy organizations to combine the efforts and resources of their members to participate in this Optical SETI activity? The answer to this question 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, sophistication and cost. A basic Amateur Optical SETI system may be 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 obviously 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. The reader should keep in mind that the system to be described may also be interfaced with large professional and semi-professional telescopes, with some adaptation.
2.1 Visible & Near-Infrared SETI
Figure 4 shows a schematic of the basic AMOSETI observatory. Unlike the case for the professional heterodyning system1-2, only a single photodetector is used, which can be either an APD or PMT. 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 to the rear of a small telescope would be excessive. 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.
In such a system, 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 field-of-view (FOV) may be increased to accommodate image scintillation and wander caused by typical atmospheric turbulence conditions and telescope tracking errors. Since "space" is largely empty, particularly as viewed when conducting a Targeted Search of stars within a few hundred light years, increasing the FOV about the diffraction limit (light-bucket mode) should not significantly degrade detection sensitivity for nighttime observations.
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. 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.
It is, of course possible, that ETIs might use linearly or circularly polarization-modulated signals, so some form of analyzer would then be required to detect the modulation. 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 (minimum) throughput loss of about 50 percent in the monochromator because high resolution diffraction gratings have a tendency to polarize light. However, because of energy efficiency considerations for pulsed transmissions, it is thought unlikely that polarization modulation would be employed. Of course, if the signals are frequency (or phase) modulated with relatively small deviations, then only the professional heterodyne receiver will be able to recover the modulation envelop, whatever the signal strength.1-2
The optical filter can be a computer-controlled scanning monochromator or a relatively inexpensive fixed interference filter. 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. Additional focal-plane optical fibers and photo-detectors may be employed for maintaining precise star-lock. Four additional optical fibers with unfiltered (4-quadrant) photodetectors might surround the ETI-detecting fiber for this purpose.
For cw beacon detection, an electronic mixer and filter may be included between the optical 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 Personal Computer (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.
In Figure 4, the purpose of the conventional CCD is just to display the star field on a TV or PC monitor and for precision star tracking. In this preferred design, it does not detect the ETI signal; that job is performed by the relatively fast single solid-state APD or PMT. APDs have the advantage of high quantum efficiency but usually have the disadvantage of higher dark current; the converse being the case for photomultipliers. With state-of-the-art solid-state photodetectors like the EG&G SPCM-200-PQ-F050 Single Photon-Counting Module with FC fiber-optic connector, the cooling to reduce dark current noise (25 cps) 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 would compromise detection sensitivity and bandwidth. It would also require a very high-quality and expensive CCD array. This would also be incompatible 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 if used in the cw detection mode.
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 (cps). 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 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 picture would be the most effective bridge between our cultures.10
Even as this is being written, substantial developments are being made in terrestrial video compression techniques for High Definition TeleVision (HDTV). Compression ratios as high as 100:1 have been achieved with only a small reported impairment in perceived video quality. A 100:1 compression ratio would reduced bandwidth required by the digitized video signal by a factor of 100. If it could be applied to an ETI interstellar communication system, the effective SNR would be increased by 20 dB. Of course, we cannot comment on whether ETIs would use such techniques, or what the level of compression would be. What we can say, however, is that optical communications technology, along with video compression techniques, would make it much easier to transmit high-quality "real-time" video signals over thousands of light years. What was previously thought difficult, though possible with old-fashioned analog TV signals and a 1 GW transmitter over ten light years, now becomes possible over one hundred light years or more.1-2
Table 2 indicates the approximate costs of putting together an AMOSETI Observatory. The low-end cost is approximately $8,400; less if telescope and computer system are already available. This is an affordable activity for many clubs and societies. Some of the equipment is optional and may be replaced by less sophisticated devices, e.g., the automatic scanning monochromator could be replaced by a manual monochromator or a series of discrete high-Q bandpass filters. By omitting the electrical spectrum analyzer and using a fixed optical bandpass filter, instead of a scanning monochromator, the cost of a rudimentary system adaptation to an existing telescope would fall to about $3,000. This figure will be affordable for some individual enthusiasts.
As indicated earlier, there was no particular reason in choosing the 656.2808 nm (457.1214 THz) Ha 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 bandwidth of about 280 GHz, though its half-power bandwidth is somewhat smaller. 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 Ha solar filter.
Often, monochromators use triple gratings in order to obtain spectral resolutions of 0.01 nm or better. For the highly sophisticated approach, costs rise to in excess of $40,000. For instance, the Hewlett-Packard 71451A Optical Spectrum Analyzer13 is probably the most versatile, low-loss optical spectrum analyzer on the market today, albeit very expensive. One of the unique aspects of its construction is that the fiber-optic connectorized monochromator section may be employed independent of the internal laser and photodetector, and the double-pass monochromator is essentially insensitive to polarization effects. Note the cost of this particular unit is not modelled in Table 2, as it would more than double the cost of the high-end approach.
Instead of a scanning grating monochromator, a scanning grating spectrometer might be used, where a linear CCD array is employed to produce an essentially instantaneous display of optical spectra on a monitor. However, this does not allow for the flexibility of employing a single photodetector optimized for bandwidth and photon-counting sensitivity, and thus this approach will be more expensive and less sensitive.
2.2 Infrared SETI
If the analysis summarized in the graph of Figure 1 is repeated for an incoherent CO2 (10.6m m) system, where the receiver is quantum noise limited, the SNR 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/m2 (-140 dBW/m2), as produced by a transmitter at a distance of ten light years with an EIRP of about 1021 W, the SNR for a 25.4 cm (10") diameter CO2 cw receiving telescope begins to degrade for optical bandwidths greater than about 1 MHz.
For such a receiver, 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 FOV to background thermal radiation. 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 demonstrated in the companion paper1-2, the optical heterodyne receiver has the great advantage over its direct detection counterpart, 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 in the earlier paper1-2 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.
In celebration of the official switch-on of the HRMS Project last October, the author started designing and constructing his own Optical SETI Observatory. Figure 5 shows a photograph of the f/10 25.4 cm (10") Meade LX200 SCT that forms the heart of The Columbus Optical SETI Observatory. After careful investigation, it was decided that Meade LX200 represented the state-of-the-art in small computer-controlled SCTs.
Back in 1987, when the author installed a TVRO (Television Receive Only) 10 ft. diameter satellite dish to receive Intelsat news feeds from the United Kingdom, he had the foresight to install a spare TVRO cable. This spare TVRO cable is now used to interface the Meade LX200 with its control center, the latter being presently housed in the author's living room, within a modified video-center cabinet. The spare TVRO control cables are used for RS-232 serial port interfaces, and the two 75 W coaxial leads (fomer C- and Ku-Band feeds) are available to convey the detected photon signal to the instrumentation in the control center and to provide a CCD and/or TV link.
To convey an idea as to the part of the sky that can be seen from this site, the reader should note that the picture is taken pointing to the north, and that the satellite dish is shown pointing to the Intelsat at 53o W; looking over his ranch house which is off to the right. The sign on the inside of the fence says OPTICAL SETI OBSERVATORY - Site of the World's First Amateur Optical SETI Observatory.
Interfacing and mains power (110 V) for the Meade LX200 is provided at the foot of the pole supporting the satellite dish. For the present time, a 9 ft. square plinth consisting of concrete patio squares has been constructed in front of the dish to provide a level surface for the tripod. It takes about twenty minutes to set up the system for an observing session, but plans are in hand to either deploy a small permanent dome, such as the HOME-DOME, on the plinth, or to relocate the observatory and dome to the rear roof of the author's home.
The picture shows the telescope head with its dew-shield extension section, attached to the equatorial mount (Superwedge) on a fully-extended tripod. In practice, for Optical SETI observations, there is no need to use the equatorial mount, since field rotation is not a problem as it would be for astrophotography. Setting up the system as shown, is a two-man job because of the weight of the telescope head (61 lbs). However, since the SCT can be used in the altazimuth-mode and the tripod legs do not have to be fully extended, it is possible for one person to set up the system.
Figure 6 shows a close-up of the SCT without its dew-shield. One of the main benefits of using the equatorial mount is when the object to be observed is at a high declination, close to the local zenith. In such situations, the equipment mounted at the back of the telescope may collide with the base of the fork mount. Unfortunately, Meade did not provide sufficient rear clearance or means of programming a maximum declination limit, so it is quite easy to do serious damage to the telescope if it is driven too far via its hand control or under remote computer control. Indeed, EPOCH 2000TM, the astronomy software being employed to control the telescope has a warning about this. One advantage of the equatorial mount is that it allows the zenith to be reached without obstruction problems from the base of the fork mount. Of course, the use of a fiber-optic umbilical would overcome this potential damage problem.
Meade will probably provide a software fix to this later, but for the moment, the author has fitted an adjustable magnetic switch, which is wired into the base unit. This cuts the power to the declination motor should the declination exceed a certain value. This reed-relay system is situated under the declination circle on the right of the picture; the cable linking it to the base altazimuth drive unit can just be seen. During early shake-down experiments, a PMT was mounted at the rear of the telescope, and this can be seen in the picture.
Recently, a Zeos 486DX2 66 MHz Tower PC has been purchased to act as the control and signal processing center for the observatory. This PC is presently running Windows 3.1, but will probably be upgraded to Windows NT. Plans are being made to acquire a 17" - 20" Super VGA monitor for displaying many Windows simultaneously. The large screen is not only for displaying a lot of information, such as the star map, video, frequency and/or time-domain data, but also for educational purposes. It is appreciated that "SETI" can have a major impact on the scientific interests of young people, since it involves all the "ologies". Thus, it is expected that tours of this facility will become a regular feature of the observatory routine.
Figure 7 shows dump of the screen produced by the EPOCH 2000TM program. This program can control the Meade LX200 via its RS-232 serial port. The SCT can be controlled by simply telling EPOCH 2000TM to go to a certain object, or using the mouse, clicking on an object on the displayed map. Recent tests on the performance of the LX200 under control by EPOCH 2000TM indicate that generally, the object selected will be found within a half a FOV of the telescope using the 26 mm eyepiece. This corresponds to a positioning accuracy of about a quarter of a degree. The manual control buttons on the display can then be used to bring the object into the center of the FOV. Figure 8 shows part of the Orion printer plot corresponding to the star map in Figure 7.
Version 1.03 of EPOCH 2000TM can also access the CD-ROM based Hubble Guide Star Catalog, and process CCD images. The software will also be compatible with the future Remote Telescope Network that is presently being developed at the Mount Wilson Institute. The author has been working with Greg Fisch, the President of Farpoint Research, making suggestions for improvements to the program which will make it more useful for both professional and amateur SETI enthusiasts. With the work being done on the Remote Telescope Network and the author's own Optical SETI computer bulletin board system (BBS) [Tel: 614-258-1710], it is possible that within a year or two, The Columbus Optical SETI Observatory may be accessible for remote operation by modem.
Several major items of equipment have yet to be acquired, including a CCD star guidance camera system, like the Santa Barbara Instrument Group's (SBIG) ST-4. This can be directly interfaced to the Meade SCT drive and allows the telescope to keep the star under observation in the center of the field-of-view for much longer periods of time. Star guidance accuracy to about one arc second should be possible - this being only twice the theoretical diffraction-limited performance of the telescope, and much better than typical seeing conditions. To maximize the ETI detection sensitivity of the system, a separate off-axis guiding telescope will probably be employed, rather than share photons with the APD or PMT. This secondary guidance telescope will have to be strapped down very rigidly to the main tube to avoid differential flexing effects.
Plans are also being made to incorporate a high-sensitivity TV camera on the system so that a real-time video image of the star field can be displayed on a monitor in another Window, alongside the star map produced by EPOCH 2000TM.
The most important piece of equipment in the OSETI Observatory, after the SCT, is the photon-counter. This photon-counter will be Hamamatsu Type No. H4730-01, with a C4710-52 high voltage power supply. For those amateur astronomy groups and societies wishing to replicate the AMOSETI Observatory described herein, you should note that this photon-counter system costs about $1,300. It has a rated optical bandwidth of 185 to 850 nm, and a peak quantum efficiency of about 23% at 440 nm. The PMT pulse rise-time is about 2 ns. At 25oC, the dark-count is about 100 cps, and the photon-counter is capable of a count rate up to 3 Mcps, with a pulse pair resolution of 50 ns, and a TTL positive logic output.
It is expected that by this autumn, The Columbus Optical SETI Observatory will be counting interstellar photons with the new Hamamatsu PMT. Up to now, a number of limited shake-down observations have been done using a old "flying-spot" scanner PMT in direct-detection mode, without any optical filtering. Not surprisingly, no ETI signals have been detected!
One major issue which will shortly have to be faced, will be the question of signal processing, data reduction and data logging. At the time of writing, proposals are being written to address these issues and obtain the first funding for this OSETI activity. Up to now, the author's three years of SETI research has been self-funded out of his other optoelectronic consultancy activities. Just for this observatory to date, about $10k has been invested. The present thinking is that VHS or S-VHS video cassettes might form an economic way of storing the reduced data obtained from a night's observation, at least for low-cost amateur approaches to OSETI. This could allow for a replay of sessions for demonstration purposes during non-observing hours, and for ETI signal confirmation purposes, as suggested by "The SETI Protocols", Clause No. 6 (see the appendix to these proceedings):
The discovery should be confirmed and monitored and any data bearing on the evidence of extraterrestrial intelligence should be recorded and stored permanently to the greatest extent feasible and practicable, in a form that will make it available for further analysis and interpretation. These recordings should be made available to the international institutions listed above and to members of the scientific community for further objective analysis and interpretation.
In many ways, The Columbus Optical SETI Observatory or AMOSETI Observatory is a small version of the late Shvartsman's "MANIA" system that is now being duplicated in Argentina.1,2,11 Thus, this observatory, when it comes on-line later in the year, will be simultaneously the only amateur observatory in the world and only the second (or third) Visible Optical SETI Observatory of any description!
Table 3 is an extract from the list (provided by the SETI Institute) of the closest stars that form the group of 800 stars which are subject to HRMS's "Targeted Search". The stellar positions are given for Epoch 1950. Presently, the list covers stars in the range of 4.39 to 81.5 light years from Earth, but is subject to review.
The column, second from right lists the spectral type, according to the following nomenclature:
UD = White Dwarf
The catalog numbers in the 8,000 object internal database of the Meade LX200 SCT corresponding to the listed stars, are shown in the far right column. Note, that future versions of the EPOCH 2000TM program may contain this 800 targeted star search database. This will then be updated as NASA publishes revised lists. (See the Latham/Soderblom paper elsewhere in these proceedings).
The idea that ETIs would use powerful fast beacon and signal pulses instead of cw beacon or modulated carrier signals has profound implications for both Professional (as defined by this author) and Amateur Optical SETI. It can substantially affect the complexity and cost of putting such an observatory together, and the time required to undertake targeted searches. It might even allow for the optical equivalent of the "All Sky Survey", where the entire celestial sphere is pixelized.
If we speculated that it indeed possible (and affordable) for ETIs to continuously transmit mean powers greater than 1 MW towards suitable targets - perhaps hundreds or thousands of beams simultaneously or time sequentially, then the amateur Optical SETI enthusiast or even the professional researcher might decide to dispense with the ability to detect cw signals. While the receiver for cw beacon detection is relatively inexpensive, a high-resolution scanning optical monochromator is not.
The implications of eliminating the capability to search for relatively weak cw signals are the following:
After listing the above advantages of the pulsed approach, the reader would be forgiven for wondering why any sensible ETI would even bother to send cw signals. The pulsed attention-drawing beacon approach of lines "d" and "e" of Table 1, would replace the need for a cw beacon and in addition, could be combined with large M-ary PPM to act as framing (synchronization) pulses. This would satisfy terrene SETI lore for the presence of a "Signpost".1-2 Perhaps Dr. Monte Ross (conference cochair) had it right all along! Certainly, in keeping with the author's philosophy of putting the onus on the more technically advanced civilization to make their signals easily detectable, the pulsed approach has much to commend it. These scenarios are so very different from the assumptions that went into the Cyclops Report1,2, where the onus was placed on us to detect very weak signals. Pulse durations as small as 1 ns duration should not be significantly affected by dispersion within the atmosphere. Dispersion would increase the WER as pulses would appear in the wrong time slots.
The one disadvantage of restricting the amateur search to short pulses is that the amateur will need a very sensitive wideband photon-counter, and a means of electronically displaying and recording the signals. This capability is not inexpensive. In terms of state-of-the-art photon-counting technology, it would appear that photomultipliers have an advantage over their APD counterparts when the latter are used in the Geiger mode because of their smaller response time, even though their quantum efficiencies are lower, i.e., less than 25% as compared to 75% for APDs. Because PMT responsivities drop drastically in the near-infrared, where silicon APDs peak up, both types of detector are likely to be used to provided wider spectral coverage.
The Amateur Optical SETI system just described is quite capable of being upgraded in sensitivity by slaving "n" similar telescopes together, and combining the photons from the "n" optical fibers through a single monochromator or filter and photon-counter. In this way, ten telescopes of 25.4 cm (10") aperture would have approximately the same sensitivity as a single 81-cm (32") telescope, but in a more cost-effective manner. Of course, ten small telescopes would not have the same ability as a 32" (81 cm) telescope to reject the effects of daylight, should daylight Optical SETI be desired. The approach could be adopted, as with the original Cyclops Study proposal, to gradually increase the number of telescopes as the need arises and with the availability of funding, assuming that ETI signals are not detected soon after system activation. This would, of course, require accurate time synchronization between the telescopes, though this should not be much of a problem. However, the requirement to match the wavelength accuracy of the optical filter or monochromator to within 100 GHz is probably a more severe obstacle for cw OSETI - not a problem for pulsed OSETI. In the case of co-site slaving, where pre-detection combining of photons would occur, the SNR would increase at a rate proportional to the number of identical telescopes. For remote site slaving, where only post-detection electrical signal combining could be employed, the SNR would increase at a rate proportional to the square root of the number of identical telescopes.
A large, single barrel, telescope could be constructed using several smaller mirrors, each with its own focus and optical fiber. In this way, only one drive system would be required. A much simpler construction is possible because we do not need to image a star field, just collect as many photons as possible from the region around a single star (light-bucket mode of operation). This could be somewhat like the Multi-Telescope Telescope (MTT) that has been designed by Georgia State University's (GSU) Center for High Angular Resolution Astronomy (CHARA).1,2
The author's study of the optical approach to SETI, appears to indicate that the amateur SETI enthusiast could make a useful contribution to the search using medium-size amateur optical telescopes with photon-counting receivers. It is certainly more debatable whether Optical ETI signals are present at sufficient flux intensities to be detectable by small incoherent telescopes. However, although the theoretical SNRs described for small direct-detection receiving telescopes are not particularly impressive, even if very high mean EIRPs are assumed, it must be remembered that ETI signals are likely to be pulsed and far more detectable than the cw signals.
Perhaps one of the interesting aspects of the Amateur Optical SETI concept using incoherent detection is that not only may there be a useful contribution made by the enthusiast, but that such activities may occur before Professional (Visible) Optical SETI and its coherent detection systems get established. A low-level search by amateurs might help set some of the criteria for later professional searches, even if the results are negative. Amateur optical SETI has the potential to bring SETI to the masses, something that has not really been possible at microwave frequencies, except in a limited way for a few enthusiastic radio hams with modified satellite receivers (AMSETI).14 It also has the power to cause a renaissance in public interest for astronomy and the night sky. It is an activity in which amateur optical astronomers who live in big cities can participate, unincumbered by light pollution, the bane of conventional amateur astronomers. This could be the opportunity to dust off those old telescopes and put them to use again. It is clear, that today there is an enormous interest in SETI amongst the population. Professional SETI scientists could tap into that interest to receive increased SETI funding and the cooperation of enthusiastic amateurs.
It may thus 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 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. As previously indicated, the philosophy of the author's approach to SETI is that the onus would be on ETIs to make their signals easily detectable, though whether their criteria for doing so would be based on small receiving telescopes is quite another matter.
It does not appear that Amateur Optical SETI at the infrared Carbon Dioxide (CO2) wavelength at 10.6 m m would be very sensible because of the limitations set by the essentially 24-hour day, 300 K temperature background of the atmosphere, particularly for small apertures. Professional Optical SETI in the visible and near-infrared can use coherent or incoherent optical receivers. The coherent approach is generally more sensitive but far more complex and expensive. However, based on performance considerations, both ground-based Professional and Amateur Optical SETI in the infrared would have to be restricted to coherent receivers. This represents a complexity and cost problem for the amateur. Of course, there could be very powerful cw or pulsed CO2 ETI transmitters present, as powerful as conjectured for Visible SETI that have so far escaped detection; for we may not have been looking in the right direction at the right moment, with suitable detection equipment.
Of course, the efficacy of the optical approach versus the microwave approach to SETI hinges on the assumptions of the cw or pulsed laser power that ETIs can expend. The author thinks it not unreasonable to believe that an advanced extraterrestrial civilization (Type II)1-2 will have command of energy resources that make the dedication of 1 MW of mean optical power (whatever the size of the transmitting antenna array) to a single targeted solar system a trivial matter. It can be further speculated that dedicating 1 GW of mean power to be shared by many targets is still a trivial "low-cost" matter for such a civilization. On the other hand, some would argue that to allocate a mean power of 1 GW to Communications with Extraterrestrial Intelligence (CETI) for targeting one planet would appear excessive. But it is impossible for late 20th century man to make any "sensible" estimation of the desire of ETIs to both communicate and expend such resources on a CETI project.
Within this account of Amateur Optical SETI is the further ambitious desire to detect the modulation envelop. For weak cw 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, allowing for more powerful professional receivers to be built later for detecting the modulation envelope. As a spin-off from HRMS, electronic Multi-Channel Spectrum Analyzers (MCSAs) could be developed for the Amateur Optical SETI market, eventually making Amateur Optical SETI an even more affordable activity for optical astronomy clubs and societies.
Today, the technology is available to construct efficient, highly-sensitive photon-counting receivers for the visible and near-infrared regimes. For several thousand dollars, top-of-the-line amateur optical telescopes could be equipped with the instrumentation to make unattended frequency searches of selected targeted stars. If this new scientific endeavor really takes off, market growth will lead to considerable reductions in hardware and software costs, making this activity more affordable.. Perhaps ETIs do not expect their signals to be detected until the targeted civilizations make a collective, cooperative, and systematic search of their home skies!
The work reported here is described in much greater detail and supported by extensive calculations in the original EJASA2 article on Optical SETI which was published on the Internet. See the Preface for details on how to obtain a copy.
When the author first became involved with Optical SETI, particularly so-called Professional Optical SETI, he felt that the late Dr. E. F. Schumacher's "Small Is Beautiful" philosophy was appropriate. Today, the author is tempted to go further, and suggests that as far as Amateur Optical SETI Observatories are concerned, "Very Small Is Exquisite" as long as ETI lasers are powerful, pulsed and visible! 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. Would it not be ironic if we discovered that for millennia ETI photons have been raining down on this planet, but we have been too blind to see!
The author wishes to thank Farpoint Research for the donation of a copy of their EPOCH 2000TM astronomy software package. He is also pleased to state that Hamamatsu has agreed to help in this enterprise, and will be donating the PMT photon-counting system described in this paper. Other companies who might wish to assist this activity by making tax-deductible donations of equipment and software to this prototype AMOSETI Observatory, are welcome to do so through the North American AstroPhysical Observatory (NAAPO). Contact the author for details.