Ask Dr. SETI ®
by Marlin (Ben) Schuetz
Boquete Optical Observatory, Panama
Is amateur optical SETI a viable option? After a few years of research, developing hardware and considering this issue, a fair case can be made in the affirmative. But before getting into the basis for the “yes” answer, let’s consider some of the rudiments of Optical SETI. It has been shown by many that an appropriately scaled pulsed laser system can outshine a parent star by several orders of magnitude out to more than 1000 light years. That statement seems straight forward, but requires some explanation. During the brief period of a laser pulse (~10^-8 to 10^-9 seconds), many more laser photons can pass through a distant unit of area than photons from a parent star. For example, if the rate of detected stellar photons is 10^6 per second and a laser pulse length is 5 nanoseconds, then on average during that very brief pulse interval one could expect only about 0.005 stellar photons, but there could be tens or even thousands of photons from a laser pulse and these would be sufficient for detection. On this brief time scale, a specialized detector can ignore nearly all of the stellar background flux.
The amateur may well hold the best cards for detecting ETI laser signals. Large institutional telescopes are few in number and fewer still have allocated significant blocks of time to Optical SETI searches. Moreover, for signals originating from up to ~400 light years, small telescopes have many advantages over their larger cousins.
Recently, for another paper, I was tasked with comparing the sensitivity of the Boquete Observatory’s system to laser signals with that of other Optical SETI projects. Prior to this, I was aware of most of those comparisons, but when the whole picture was laid out a new appreciation was formed for the potential contributions amateurs can make. The Boquete telescope has only a 0.5 meter (20”) aperture compared with institutional optical SETI instruments ranging at present from 0.9 meters to 1.8 meters. How can the Boquete system compare favorably with these? Other SETI laser signal detection schemes typically employ two or more photo-detectors (photomultipliers or avalanche photodiodes) whose outputs are compared to reveal photoelectron pulses that are coincident in time (piled up). This method requires fussy design and precise construction using beam splitters to divert portions of the incoming starlight to the photo-detectors. It is expensive, somewhat complicated and not so simple to calibrate and maintain. Also, to minimize false positives, the detectors’ sensitivities are usually further attenuated.
The Boquete photometer uses only a single photomultiplier in combination with a number of unique steps that result in a simplified detection method having improved sensitivity and without concerns for false positive detections. The later is exemplified with the observation of nearly 4000 stars during which only a single (presumed) false positive event was experienced. In addition to coincident pulse detection, the photometer is also sensitivity to non-coincident pulsed signals, (i.e. photoelectron pulses spread out in time to 25 nanoseconds or more). The single detector photometer is described in some detail at the Boquete Observatory’s website.
The range of stellar magnitudes is yet another aspect that favors small telescopes for optical SETI. For example, candidate stars most often include magnitude extremes of ~2 to 14. The 0.5 meter telescope/detector is limited to stars no brighter than about magnitude 6 without a method of light attenuation; larger telescopes are even more constrained. Thus, considering stellar flux limitations a small telescope with a single detector can have a sensitivity advantage over a larger one with multiple detectors. Furthermore, telescopes with apertures as small as 10”- 14” can be applied to SETI searches of the several thousand stars within ~100 light years.
We now know that exoplanetary systems are ubiquitous in our galaxy and beyond, but there is still the very great uncertainty regarding the proclivity for intelligent life beyond earth. Some have guessed one planet in a million – give or take a bunch. Needed now are many small optical SETI observatories to survey the hundreds of thousands of nearby stars. Hopefully it won’t be necessary to expand the search beyond that.
This all too brief discussion may be a useful starting point for others to dig deeper and perhaps become involved in the very challenging effort to detect signals from other worlds. At present, and to my knowledge, The Boquete Optical SETI Observatory is the only facility that is full time functional and dedicated to this project. Bruce Howard’s Owl Observatory in Michigan is also functional and dedicated to OSETI, but it awaits his imminent retirement to begin full time operation. With the exception of the Harvard All-Sky Survey (2006), institutional OSETI programs general function only intermittently. It is not clear if the Harvard program is still active. However, if there are others, we’ll be glad to know about them and endeavor to foster good working relationships. METI International is spearheading a campaign to develop a Global Network of Optical SETI Observatories.
There is much to do. Please join us.
Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.
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this page last updated 7 January 2017
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