Design for an Optical SETI Observatory
23rd Review Meeting of the Search for Extraterrestrial Intelligence (SETI), SETI: Science and Technology, 45th Congress of the International Astronautical Union, October 9-14, 1994, Jerusalem, Israel.
Copyright ©, 1994, The Columbus Optical SETI Observatory
Stuart A. Kingsley
This paper describes the design and construction of the first Visible Optical SETI Observatory in North America. The rationale supporting this activity is also given. The Columbus Optical SETI Observatory is designed to detect ultra-fast pulsed laser beacon signals in the visible and near-infrared. The design and construction of The Columbus Optical SETI Observatory has been underway for two years and employs the Meade LX200 10" (25.4-cm) Schmidt-Cassegrain Telescope. Recently, this observatory acquired a 240 sq. ft. control/conference room and a 10 ft. diameter astronomical dome to house the telescope. The targeted search will formally begin next year, and will examine the same stars in the northern hemisphere presently under investigation by Microwave SETI's Project Phoenix. A photon-counter will be used to look for the fast laser pulses. Eventually, the photon-counting system employed will be capable of detecting pulses as short as 1 ns. Each night's observations will be data-logged onto standard VHS video cassettes; the data rate and storage requirements being kept initially modest by setting a high discriminator threshold, in accordance with the Optical SETI rationale described. The observatory's control computer will be hard-wired to the Optical SETI Computer Bulletin Board (BBS). This will allow interested users to observe the control computer's screen in real-time, and the data being acquired. Internet access to this BBS should be available next year.
For 35 years now, The Search For Extraterrestrial Intelligence (SETI) has been conducted mainly in the microwave region of the electromagnetic spectrum.1 In particular, the so-called "Cosmic Water-Hole" around 1.4 GHz has received the bulk of researchers' attention. It is the belief of this author, and of several other scientists, that Optical SETI (OSETI) has more to commend it than Microwave SETI (MSETI).2-21
Note that it is important here to define the word "Optical". It is used in its broadest sense, being a superset of ultra-violet, visible, and infra-red SETI. While it is correct to speak of Visible SETI and Infra-red, nowadays it is not correct to imply that the word "optical" is a synonym for the word "visible". The definition of the word "Optical" is used here in its modern connotation, as adopted by electrical and photonics engineers over the past two decades. If we keep to this modern definition, we will avoid much confusion later between scientists and engineers from differing fields of expertise.
There are many advantages that lasers have over microwaves for SETI, and for interstellar communications in general. The important ones are listed in the next column:
For the past four years, this author has worked hard to convince the SETI community to revisit the optical approach, which has suffered severe neglect. Particularly, since the early 70's and the publication of The Cyclops Report.22 The reader should note that the author's observatory receiving telescope system, shortly to be described, is slightly larger than the ETI's transmitting telescope that was modelled in the flawed comparison table (the author's opinion) in The Cyclops Report.
Perhaps the strongest supporter of the correctness of the MSETI rationale is noted SETI pioneer Dr. Barney Oliver, the main author of that report. In scientific debate between our two camps, this author has argued the case for reconsideration of the merits of the optical approach to SETI. Over the past several years, Dr. Oliver has advanced several objections as to why ETIs would not use visible or infrared lasers for SETI purposes.
At one point the argument against lasers was that from the receiving point of view, an All Sky Survey wasn't practical as it would take far too long the pixelate the sky. The author's counter to that was that there was nothing particularly "holy" about being able to undertake an All Sky Survey. While it was practical at microwave frequencies, it wasn't at optical frequencies. It didn't necessarily follow that ETI's wouldn't employ high gain laser uplinks because that knew we couldn't use an All Sky Survey to detect their signals. The irony of this earlier objection to Optical SETI, is that for now, for political and practical reasons, the Microwave All Sky Survey part of the HRMS Project has been suspended!
As the main argument against lasers now stands, Oliver's principal objection to the use of very high gain transmitting telescopes or arrays is that the beams are too narrow. Hence, ETIs would not have the technical prowess to aim such beams at suitable stars like our own. He presently feels that an upper limit for uplink gain would be about 94 dB (2.5 billion), some 60 dB less than this author has conjectured. His argument is based on our present capabilities and knowledge of the peculiar proper motion of nearby stars - a moving target at best! If there is one cardinal rule in SETI, it is to NEVER constrain the capabilities of advanced technical extraterrestrial civilizations to that of late 20th century man. Time and again, throughout modern technological history, mankind has succeeded in seriously underestimating his own ability to produce quantum changes in technological development across time-frames, that on a geological evolutionary time-scale are vanishingly small.23
Oliver's current argument is more general, in that it not only applies to very high gain laser uplinks but also to their very high gain microwave counterparts. The idea that ETIs would be unable to achieve the point-ahead targeting required for these high gains cannot be seriously contemplated. They would be able to ensure that when their narrow beams of microwave or optical photons eventually arrive at the destination, that they would indeed illuminate the desired targeted star or planet.
Two years ago, having become convinced that OSETI had considerable merit, even for small telescopes, the author embarked on designing and constructing his own Optical SETI Observatory. A Meade LX200 SCT was acquired first, followed by a dedicated control computer that was housed temporarily in a video center located in the author's living room. This year, a major upgrade to the facility occurred, with the construction of a dedicated 240 sq. ft. control/conference room and the purchase of an astronomical dome to house the telescope.
When the author first became seriously involved with SETI four years ago, his approach was to consider the detectability of continuous wave monochromatic laser ETI beacons. The receivers would employ optical heterodyne techniques. This approach does require some estimate of the "magic optical wavelengths" that have to be searched.
In previous publications, the author has referred to OSETI employing large telescopes and optical heterodyne detection techniques as being Professional Optical SETI (PROSETI), while that with smaller telescopes and incoherent optical detection techniques as being Amateur Optical SETI (AMOSETI).10,11 For the purposes of this paper, the main distinction between the two will be based on the sophistication of the signal processing - not on the telescope aperture size.
Although the author was aware of the benefits of using pulses, i.e., very high peak powers can override the stellar background, thus requiring little or no optical filtering, his true appreciation of the benefits didn't come to about two years ago. Since the late 60's, Dr. Monte Ross has been proposing that ETIs would use Pulse Position Modulation (PPM) and Pulse Interval Modulation (PIM) for interstellar SETI-type communications.3,15 Having accepted the logic of Monte Ross's arguments, this author became convinced that Optical SETI with small telescopes was now far more viable and attractive.
Figure 1 illustrates the basic pulsed modulation technique. In Figure 1(a) a pulsed wavetrain is shown, where the peak power, Ppk, can be much greater than the mean or average power, Pav. Such a waveform would make for an excellent "lighthouse" or pulsed beacon signal. Figure 1(b) shows the PIM concept of Monte Ross, where a digital word may be formed by choosing suitable time slots to place a pulse, and the peak power of any pulse is substantially greater than the mean power.
Figure 2 illustrates how the ETI Beacon photon count would fall off with range for both large and small receiving telescopes. It assumes an ETI uplink producing pulses of 1 ns duration at a repetition rate of one per second. The modeled laser transmitter operates at 550 nm, at a mean optical power of 1 GW. The wavelength of 550 nm has been chosen as a modeling convenience since it corresponds to the peak of the human visual (photopic) response, but does not actually represent a laser transition. The peak optical power according to this scenario is 1018 W. The ETI laser may be a nuclear or stellar-pumped spaced-based device. If it is assumed that ETI signals are designed to be detected within an atmosphere, the effects of dispersion within such atmospheres would likely limit the minimum pulse width to the order of 1 ns. Note that the pulse width ETIs may choose to use may be somewhat longer than 1 ns, with a commensurate reduction in peak power.
Concentric circles have been drawn indicating ranges from the laser transmitter of 10, 100 and 1,000 light years. The area between the 10 and 100 L.Y. range circles has been shaded to signify that this region approximately covers the range of Project Phoenix's Targeted Search. At a range of 100 light years, there are approximately 1,000 solar-type stars that would be the subject of this Targeted Search. The numbers enclosed in a thick outline box indicate the number of photons that would be detected per pulse at that respective range by a ground-based telescope of 10 meter aperture. Similarly, the numbers enclosed in the thin outline box indicate the number of photons per pulse that would be detected by a 25.4-cm aperture telescope, corresponding to the one employed at The Columbus Optical SETI Observatory.
It should be noted that Monte Ross has assumed more conservative figures for the EIRP of ETI beacons than this author, such that receiving light-buckets of about 1 meter aperture would be required for successful detection.15
At 100 light years, the 10-meter telescope would detect 680,000 photons per pulse, a weaker PPM/PIM signal buried between the beacon pulses should still be detectable. When the potential photon-count rate is very high, conventional direct detection, as used in digital fiber-optic systems, would be the preferred mode of photodetection. At a range of 100 L.Y., the 25.4-cm aperture telescope will detect 440 photons per pulse. As can be seen, even at a range of 1,000 L.Y., the small telescope can still detect on average, 4 photons per pulse. If such a signal was received, there would be no doubt that it would be detected and registered, since the detection of only one photon per time slot is needed to signify a pulse.
The aim of the Optical SETI receiving system described herein, is not to detect the expected rich but weaker ETI data-stream, but the strong ETI beacon pulse. Anything more is probably too much to hope - but one never knows! If such a beacon is detected, then the great telescopes of the world could be brought to bear on the signal, employing suitable electronics to yield the information-rich data stream. Of course, the problem of actually demodulating the signal is left to others. It is very likely that such a signal would contain real-time video images. It has been proposed that this is the only efficient way that our dissimilar culture would be able to develop a lexicon that will allow us to understand them.17
Present Terrestrial Laser Capabilities
It is instructive to understand what is the present state-of-the-art for terrestrial laser systems. Positive Light manufactures a laser capable of producing Terrawatt pulses in a laboratory environment, albeit at only femtosecond (10-15 s) pulse durations. Lawrence Livermore National Laboratories has recently announced a future upgrade to its NOVA laser fusion facility.24 By early next century, the new National Ignition Facility (NIF) will be capable of producing 1 ns with peak powers of 1,000 TW, though at a pulse repetition rate of only one per day.
Pulsed ETI Beacon Detectability & Background Noise
Figure 3 is rather busy graph which helps explain why the author believes that small amateur-type optical telescopes can make major contributions to the science of Optical SETI. The top solid line represent the "signal" photon-count from the pulsed ETI laser. Again, it is assumed that the pulse is of 1 ns duration with a repetition rate of 1 Hz. The photon-count rate is thus described in terms of the count per pulse, equivalent in this case to the count per ns. For this graph, it has been assumed that the laser transmitter frequency is centered on the peak of the human visual (photopic) response, i.e., at 550 nm, and that the spectral response of the optical detector is similar to that of the photomultiplier tube presently being employed at The Columbus Optical SETI Observatory.
If we assume that ETI stars are very similar to our sun, than the stellar magnitude scale can also be approximately calibrated in distance, where at a range of 10 light years the ETI's star would appear as a 2nd magnitude object, falling to the 7th magnitude at 100 light years and to the 12th magnitude at a range of 1,000 light years. Since the Targeted Search covers the range 10 to 100 light years, this range is shown in a lighter shade.
The parallel broken line some 68 dB below the "signal" line represents the stellar background "noise" from the ETI's star. Again, its count rate is described in terms of the number of photons detected per ns. For illustrative purposes, we have shown a pulse sandwiched between these two lines at each extreme of the graph. The broken horizontal line at 2 x 10-5 per ns count rate represents the average measured value of the number of sky light-polluted background noise photons at the zenith, detected per ns in the City of Columbus. It is equivalent to a count rate of 20 kcps. The SNR margin is essentially represented by the shaded area between the two sloping lines.
The graph is based on stellar and sky background performance data recently obtained during shakedown measurements. The Field-Of-View (FOV) was constrained by a 1.2 mm pinhole to be about 100 arcseconds. This FOV should be compared to the theoretical diffraction-limited pixel size of about 0.5 arcseconds for a 25.4-cm aperture telescope. Since the SNR is substantial, the FOV could be enlarged so that significant tracking (guidance) error can occur without the image of the star missing the optical detector during the observation period. Because it is plausible to utilize large FOVs, it is possible to employ lower cost "light-bucket" receivers.
For visually dimmer stars, the sky background noise level will set the limits on signal detectability. At an equivalent noise level of about 5 x 10-8 photons per ns, we find the noise due to photomultiplier (PMT) thermal dark current is essentially negligible. This count rate is equivalent to 50 cps.
Eventually, what we would like to do is have the signal processing electronics basically measure the count in every 1 ns increment of time and decide whether or not an ETI photon has been detected. However, for the moment, we are limited by signal processing and data storage costs. Thus initially, we will do the much simpler operation of setting a very high discriminator threshold and detecting only those photon pulses that exceed that level. The photon-counter discriminator level is set so that it essentially suppresses the counting of all but the most energetic of photon "events".
It is clear from this model and the shaded area of Figure 3, that there is a considerable signal-to-noise ratio count margin. Thus, ETI pulsed beacon signals could be significantly weaker than proposed here, and yet still be detectable with small telescopes. This signal might be weaker because the ETI's beam was less powerful, less focused, of greater pulse width or higher repetition rate. It might also be weakened by being time and spatially multiplexed over many targeted stars.
Such is the SNR margin, that it can be said that with larger, well-controlled telescopes and reduced FOV, Optical SETI is the one branch of optical astronomy, save for solar astronomy, that can be done during the day under a clear blue sky!
Although it is somewhat like comparing apples and oranges, we can contrast the sensitivity of the laser receiver for pulses with that of a large microwave receiver for continuous wave signals:
Amateur Optical SETI System
For a 25.4-cm aperture telescope AMOSETI system with gain and discriminator threshold backed off for 100 detected photons required per pulse:
Pulse Sensitivity ~ 10-4 W/m2.
Professional Optical SETI System
For a 10-m aperture telescope with a PROSETI signal processing system giving 1 detected photon per pulse:
Pulse Sensitivity ~ 10-9 W/m2.
Professional Microwave SETI System
These above figures may be compared to that for Microwave SETI's Targeted Search:
CW Sensitivity ~ 10-27 W/m2.
Since the assumed overall ground-based basic photon detection efficiency of only 1% is very conservative, the actual pulse sensitivities might be improved by an order of magnitude. Of course, in terms of mean powers, the optical pulse sensitivities above are one billion times greater than indicated, i.e., 10-13 and 10-18 W/m2, respectively.
The relationship peak power Ppk and average power for a pulsed laser is given by:
Pav Ppk = ------ W (1) tau.r
Pav = average power (1 GW),
tau = pulse width (1 ns),
r = pulse repetition rate (1 Hz).
Substituting the values in parentheses for a pulsed ETI laser beacon system with a 1 Hz repetition rate, we find that:
Ppk = 1018 W.
Diffraction-Limited Telescope Gain
The gain of a diffraction-limited dish or telescope is given by:
pi.D G = [----]2 (2) Wl
D = diameter of transmitter aperture (10 m),
Wl = wavelength (550 nm).
Substituting the values in parenthesis for an ETI uplink transmitting at the center of the human photopic response, we find that:
G = 155.1 dB.
Peak Effective Isotropic Radiated Power
The Effective Isotropic Radiated Power is the power that the transmitter appears to have if it radiated isotropically. It is given by:
EIRP = P.G W (3)
Substituting the values for Ppk
and G given above, we find that:
Note that for a star like the sun:
EIRPstar = 3.9 x 1026 W.
The intensity of the received signal and stellar background noise is given by:
EIRP I = -------- W/m2 (4) 4.pi.R2
where R = range (9.461 x 1016 m).
Substituting the values in
parenthesis for a range of 10 light years, we find that just outside the atmosphere:
Istar = 3.5 x 10-9 W/m2.
Detected Signal Power
The signal power appearing at the photodetector through a V-type optical filter is given by:
Tatm = atmospheric transmission (0.25),
Aeff = telescope aperture efficiency (0.5),
Feff = optical filter efficiency (0.5),
d = diameter of receiver aperture (0.254 m).
Substituting the values in parentheses, we find that:
Slaser = 8.9 x 10-5 W.
For a solar-type star:
Sstar = 1.1 x 10-11 W.
For a solar-type star and a laser centered on the human visual response, the apparent magnitude may be expressed in terms of its intensity I:
m = -[19+2.5log(I)] (6)
Ilaser = 2.8 x 10-2 W/m2,
Istar = 3.5 x 10-9 W/m2.
Substituting the above values for a range of 10 light years, we find that:
mlaser ~ -15.
mstar ~ 2.
Photon Detection Rate
The signal photon detection rate is given by:
eta = photodetector quantum efficiency (0.17),
h = Planck's constant (6.63 x 10-34 J.s),
f = optical frequency (5.45 x 1014 Hz).
Substituting the values in parentheses for a center wavelength of 550 nm, we find that the "signal" photon detection rate Nlaser:
Signal = 44,000 counts per pulse.
For a solar-type star, we find that the stellar background "noise" photon detection rate Nstar:
Noise = 6,000,000 counts per second.
The "signal" is buried in the noise and the ratio between the "signal" and "noise" photons is approximately -20 dB. However, during each one nanosecond laser pulse, the SNR is positive and nearly 70 dB! This is the very important benefit of searching for very short pulses in adjacent time slots corresponding to the expected pulse duration, even if the "signal" consists of only one or two detected photons per pulse. Another important benefit is that knowledge of the "magic frequency" is not required.
The philosophy that has governed the design and construction of The Columbus Optical SETI Observatory is summarized below. Some of the issues listed have already been mentioned. The construction of this prototype observatory was started in October 1992, coincident with the official switch-on of the HRMS Project:
Figure 4 shows the basic schematic of an Optical SETI Observatory orientated towards detecting fast ETI pulsed beacon signals.
The telescope employed is the Meade LX200 Schmidt-Cassegrain Telescope (SCT) which is now housed in an astronomical dome. The 10 ft. diameter, 6 ft. high fiber-glass PRO-DOME 10 is manufactured by Technical Innovations, Inc. The system includes motors for the dome rotation and shutter operation. An infra-red sensing system that tracks the position of the telescope tube and keeps the shutter opening synchronized with movement of the telescope will be installed later. For the moment, the dome is mounted on a wooden deck at the rear of the property, but it is hoped to relocate the dome to the roof of the detached garage by next summer.
If use of the Superwedge (equatorial mount) is required, alignment with the Pole star is possible at this location, though the motor housing of the satellite dish adjacent to the dome, slightly obscures Polaris as seen from the SCT. The equatorial mount is more useful when observing stars near the zenith, since the enclosure for the photon-counter strikes the base of the fork mount at high angles of elevation. At low declinations, the altazimuth mode of operation is more convenient.
The telescope is controlled by a 486 DX2 66 MHz PC, running Windows 3.1. The astronomy program, Epoch 2000J, produced by Farpoint Research, is used to control the telescope and display the relevant star map. Time and frequency domain signal processing boards will be added later for processing the photon count. An audible alarm system will be activated by the control computer if energetic bursts of photons are suddenly detected above the set ETI discriminator threshold.
A CCD TV camera telescope may be employed with an optical filter and image intensifier to show the star-field, despite severe city light pollution. This will allow for NTSC images of the star and star-field under observation to be compared to the sky map on the control computer screen, and can also be recorded onto video tape. The combination of image intensifier and filter allows the effects of city light pollution to be suppressed. Not shown in the diagram is an additional CCD tracking telescope which can be used to remove the residual guidance error. This will ensure that the Targeted Star remains in the center of the FOV throughout the observation period, and allows smaller spatial filters to be used to cut down sky background levels.
An Optical Filter Wheel, probably containing the standard UBVIR filters will be used to divide the optical spectrum into several broad bands in the desire to improve the contrast ratio between the would-be ETI beacons and the stellar background. Depending on the part of the spectrum under investigation, a Photomultipler Tube (PMT), Avalanche Photodetector (APD) or Vacuum Photodetector will be employed to detect the photons.
Hamamatsu has donated their H4730-01 photon-counting head which contains a PMT. Although this is not as fast as the author would like, and will be replaced later, it is capable of detecting pulse widths of several nanoseconds, and is linear up to a count rate of about 3 Mcps. Its internal pulse-stretcher produces pulses of 30 ns width with a pulse-pair resolution of 50 ns.
As previously mentioned, initially the photon-counting head will not be used in the normal manner; rather, the gain will be reduced and the discriminator threshold set high to eliminate almost all the stellar and sky background photons. This implies a desensitization of the receiver, but it keeps the data rate and storage requirements very modest. More about the data rate problem shortly.
For recent shakedown measurements, a stand-alone B&K Universal Counter was used to obtain preliminary stellar and background data. Eventually, the observatory will employ a universal counter on a card that plugs into the control PC.
Separate to an electronic counter is a simple visual display and audio system that allows the count rate to be monitored visually and audibly. When the output of the photon counter head is applied to a video monitor, what we see is something very much like normal TV "snow", particularly if the count rate is very high and of the order of 1 Mcps. By applying the same signal to a 555 Timer chip in a monostable mode, the duration of each pulse can be extended sufficiently, i.e., from 30 ns to 1 ms, so that it can be reproduced by an audio system. This is particularly useful when the count rate is very low - the ear being a "sensitive" detector of periodic signals.
Multichannel Pulse Height Analyzer
Perhaps the ideal type of photon counter for this application would be adapted nuclear radiation instrumentation like the EG&G Turbo-MCS High Performance Multichannel Scaler, Model T914. This top-of-the-line system can sample for periods as short as 5 ns, though with only a 1 bit amplitude resolution.
Data Rate & Data Storage
From Figure 3 it can be seen that if every detected photon is counted from a solar-type star at a range of 100 light years, that a 25.4-cm telescope will produce a photon-count of approximately 60,000 counts per second (cps). This is well within the linear region of fast PMT-type photon-counters. However, this count rate is rather high for transmission via modem over normal phone lines, and the data storage requirements for a six-hour AMOSETI session would amount to over 1 Gbyte. This assumes that each photon or pulse detected is only allocated one byte of information.
Thus, the observatory will, not so much dump the excess data, as to never actually collect it. As previously indicated, this will be done by setting a high discriminator threshold on the photon-counter, so that only unusually intense pulses are detected and required to be stored. The data rate is then modest, and so are the nightly storage requirements.
For stars as close as 10 light years, the data storage requirements per night for larger PROSETI telescopes could exceed several Terrabytes! Today, optical disk storage of that quantity of information would cost over $500,000 per night! Also, it should go without saying that it is not really possible to store that quantity of information and then find the time to later search it for "unusual" events. It has been suggested to the author by Guillermo Lemarchand that perhaps the cost of mass storage is not yet low enough for viable PROSETI.16,20
The initial data rate reduction technique allows the data to be stored on standard VHS video cassettes. It is intended to store an American NTSC version of the VGA control computer display on VHS tape, along with the data encoded onto the linear mono and the hi-fi audio tracks. Between the visual and audio means of reproducing the signal we have an excellent low-cost way of monitoring the count rate. Indeed, for some amateur observatories, this may be all that is affordable. For moderate count rates, the human eye can readily spot a repetitious pulse on a video monitor, and the white dots may appear to produce diagonal lines across the screen. The human ear is also fairly good in pulling out a repetitious clicks from the background "white noise".
The intended photon-counter data rate is also compatible with normal modem communications so that the signal can be made available via the ETI Photonics BBS. This low data rate means that the data storage requirements for a six-hour session, excluding the video signals, will be about 100 Mbytes. A simple adapter can be used to hook the audio output into a computer's serial port to allow simultaneous play back of the data onto any personal computer while viewing the control computer's screen.
The use of standard video tape for data storage purposes has several advantages. It is the cheapest bulk storage medium. Most people have VCRs. It can be easily copied, and converted to PAL or SECAM if required (the author has the means to do digital TV standards conversion). Should an ETI signal be detected, it can make the dissemination of that data and verification, as per requirements of The SETI Protocols, that much easier.11
The control computer is a 486 DX2 66 MHz PC with 16 Mbyte of RAM. The control computer runs under Windows 3.1. The astronomy program Epoch 2000J is used to control the Meade LX200. Figure 5 shows a typical display from the control computer's screen. Jill Tarter's original list of 800 stars has been entered into the program's database, so it is just a matter of clicking on the star's image in the displayed sky map for the telescope to move to the vicinity of the star.
The Columbus Optical SETI Observatory has a new 240 sq. ft. Control/Conference Room. The room is located a few meters from the present location of the dome. As part of a planned educational OSETI operation, it is intended to conduct show-and-tell demonstrations to small groups, including school parties. It is intended that periodic, professional seminars on SETI will be held here.
As previously mentioned, the stars that will be observed for signs of ETI laser beacons are the same stars visible in the northern hemisphere that are the subject of the new privatized version of NASA's former High Resolution Microwave Survey (HRMS) Targeted Search. This program is now known as Project Phoenix. The Targeted Optical Search will formally begin next year.
In order to be certain of the magnitude of the stellar and sky background counts for the Meade LX200, some measurements were undertaken with the Hamamatsu H4730-01 Photon-Counting Head. The stellar background count was checked against a number of known stars and the sky background measured at the zenith for different fields-of-view (FOV). The stellar and background data so obtained was used to fine-tune Figure 3.
The Hamamatsu PMT was set up as per manufacturer's instructions with a +1,000 V and a -1.000 V discriminator voltage. These are the optimum conditions for normal photon-counting. At 25 oC and 1,000 V, the PMT has a dark current count of 44 cps.
The theoretical resolution of the 10" f.10 Meade LX200 SCT is 0.45 arcsec. The image scale may be obtained by dividing 57.3 by the focal length. Since the focal length of the telescope is 2,500 mm, the corresponding image scale is 0.57o/inch. This image scale is equivalent to 81 arcsec/mm. The FOV for the PMT was set by the use of a small aperture in the focal plane, just in front of the PMT. As mentioned earlier, an aperture (pinhole) diameter of 1.2 mm was employed. This has a corresponding FOV of about 100 arcsec (7,854 arcsec2 in area).
Figure 6 shows a close-up view of the SCT taken in 1993 before it was mounted in the new dome. Finally, Figure 7 is a photograph of the new 10 ft. diameter, 6 ft. high astronomical dome in its temporary location in the backyard of the author's home and place of business. Electrical power, signal and control lines for the telescope are obtained from interface boxes at the foot of the satellite dish, which is located just behind and to the north of the dome.
Back to Top
The author's Optical SETI strategy is mainly driven by the professional approach to the subject. However, there is a dual nature aspect to this research activity, in that it is the desire to make OSETI affordable for amateur optical astronomy groups and societies.
ETI Photonics BBS
Two years ago, ETI Photonics started a computer bulletin board mainly dedicated to promoting Amateur Optical SETI activities [Tel: (614) 258-1710]. It is intended that this will be a means by which world-wide AMOSETI observations of selected targeted stars can be coordinated. It is hoped by early 1995 to connect this BBS to the Internet, at least for email and newsgroups. In addition, through a Doorway on the BBS, users will be able to "view" the screen and data when The Columbus Optical SETI Observatory is on-line. This will be done using one of the commercial remote communication software packages, such as Close-Up.
What Size Telescope?
One of the more interesting questions one can ask about Visible Optical SETI is "What size of telescope is required for this activity?". Figures 2 and 3 suggest that any automated telescope of reasonable aperture will do the job. Indeed, on a fixed budget, it is probably counter-productive to spend too much on the large aperture telescope - the money would be better spent on improving the data collecting system and increasing data storage capabilities.
To better illustrate this point, let us look at the cost of equipping an OSETI Observatory with the 10" Meade LX200 SCT and its new top-of-the-line 16" counterpart. The former SCT (with Super Wedge) costs $3,291, while the latter costs $14,950. Other than paying for a more robust SCT, the additional $11,659 would arguably be better spent on improving the signal detection and processing system, than providing two-and-a-half times as many photons in any given period of time! For as Figure 3 indicates, increased light grasp is not required when the stellar background is the dominant noise source and several signal photons are capable of being detected during each beacon pulse. To increase system sensitivity, and with it the data-rate, the photon-counter head gain should be increased and the discriminator level lowered so that much more of the background stellar flux level is detected.
Of course, if the aim is not only to detect the expected strong ETI beacon pulse but also the weaker ETI data-stream, then it would indeed be desirable to employ as large an aperture as possible.
We will take a moment here to summarize the differences between Professional Optical SETI (PROSETI) and Amateur Optical SETI (AMOSETI). The basic construction of The Columbus Optical SETI Observatory is compatible with PROSETI and AMOSETI.
Professional Optical SETI (PROSETI)
PROSETI is an expensive affair with high efficiency and sensitivity. The important features of PROSETI are:
Amateur Optical SETI (AMOSETI)
AMOSETI has the benefit of being low budget, but at the expense of losses in efficiency and sensitivity. The important features of AMOSETI are:
We have stated that if ETIs wish to use lasers for interstellar SETI-type communications, their pulsed lasers would have sufficient intensity to be "seen" across thousands of light years. Indeed, by using short pulses, they would be able to outshine their stars. In addition, they would have the technical expertise to aim and "point-ahead target" their very high gain transmitting telescope arrays.
Furthermore, based on conjectured ETI transmitter EIRPs in excess of 1033 W, it has been proposed that small, amateur-sized telescopes can be usefully employed to undertake both Professional and Amateur Optical SETI. The fact that such pulses have not been detected accidentally by astrophysicists conducting stellar photometry does not mean that such signals do not exist. Rather, it indicates that such signals are rare, and cannot be detected by the normal slow speed devices employed in stellar photometry.
We have described the design and construction of the first Visible Optical SETI Observatory in North America, only one of two (or three) such facilities on this planet today. This facility is a prototype for both Professional and Amateur Optical SETI observatories. The basic infrastructure of The Columbus Optical SETI Observatory is now installed, though hopefully, within a year the dome can be moved from its temporary ground-based location to the roof of the author's garage.
Up to now this OSETI activity has been self-funded. Private funding is presently being sought to finance the relocation of the dome, and to underwrite the acquisition, construction and installation of much-needed specialized equipment. The latter will turn what is presently little more than a very sophisticated amateur optical astronomy facility into a one-of-a-kind Professional Optical SETI Observatory.
I wish to acknowledge donation of the photon-counting head by Hamamatsu Corporation and the Epoch 2000J astronomy program produced by Farpoint Research. I also acknowledge my association with Dr. Robert Dixon, who directs the Ohio State SETI Group at the "Big Ear" Radio Observatory. I wish to thank Dr. John Billingham for his early readiness to provide a platform for an OSETI challenge to long-standing conventional MSETI wisdom, in a 1991 seminar at NASA-Ames. I am especially appreciative of the strong encouragement given by Dr. Arthur C. Clarke in numerous communications exchanged between us over the past three years. Also, I thank Arthur for his video and phone participation in the 1993 First International Conference on Optical SETI.