A Brief Introduction to Optical SETI Signal Detection

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By Marlin (Ben) Schuetz, Director of Boquete Optical SETI Observatory

Greetings from the Boquete, Panama Optical SETI Observatory.  It is the beginning of February and the dry season is well underway.  The dry season is usually accompanied with gusty winds, especially in the canyons.  This year, however, the winds have been unusually light.  I guess that’s just another reminder climate change is occurring everywhere.   Also, normally at this time here, SETI observations would be in high gear.  Unfortunately, the new telescope has had some teething problems that have taken a lot of time to sort out.  These have mainly come from a polar misalignment that can’t be corrected without a complete disassembly.   Tracking has never been a problem, but rapid access of targets has been elusive.  So, I’ve done some workarounds and software adjustments to help with targeting.  The disassembly and ultimate mechanical fix will have to wait until the beginning of the rainy season sometime in late May.  Otherwise, the systems are working well and I am very pleased with the stability of the paired telescopes and the increase in sensitivity using the larger telescope.

In the last post, I said that there would be some explanations regarding how we look for SETI laser signals.  But first, a comparison with Radio SETI may be useful.  Radio SETI systems are large and extremely complicated.  The needed level of sensitivity requires the “large” and the range of frequencies, narrow bandwidths and simultaneous channel monitoring requires the “complicated”.  A visit to the Allen Telescope Array website’s technical overview at:  www.seti.org/ata will be helpful in that understanding. Of course, radio SETI has the advantage over Optical SETI of being able to detect leakage signals such as radio, TV and radar over large astronomical distances. But considering the scale and complexity of radio SETI, it seemed clear to me that a small privately owned radio SETI system would not be very useful. 

Optical SETI on the other hand relies on the assumptions that a laser signal from another world would be purposely directed and have sufficient intensity to be detectable - perhaps even by a small optical telescope.  We can be confident of this because even though we’ve only had high power laser capabilities for a few decades, an earth station could now launch laser signals detectable by small telescopes at great interstellar distances. 

Laser signals can be powerful, confined and directed to a very high degree of accuracy.  Although serendipitous interception of a laser signal meant for another star is possible, it is much more likely that a laser signal impinging on earth will have been purposely directed toward us. Then, all that remains for that first contact is for us to be looking - but where?  There are a few thousand stars within 100 light years.  Very roughly, multiply that number by 10 for each additional 100 light years distant.  If one does a few back of the envelope calculations it becomes clear that, no matter how optimistic one is regarding the number of civilizations that may exist per thousand stars, a successful search for a laser signal probably requires observing hundreds of thousands of stars, not just once, but many, many times. Thus, the large number of stars available to be observed with a small telescope, i.e., out to 300 light years, is really the good news. But, many, many small observatories are needed to improve the odds and timing.

OK, so how intense are the expected laser signals?  To answer this we rely on the example of our own infant laser technology.  Pulsed laser systems exist that, in combination with a large transmitting/collimating telescope, can outshine our star by several orders of magnitude over very great interstellar distances.  I’m fudging on the numbers here because if one gets specific, then the many other factors of that specificity need to be included as well and that gets too involved for the purposes here.

Wow, that means if other planets are directing their lasers at us, all we need do is look for stars that unexpectedly brighten!  Not so fast. The amount of laser energy needed to be seen that casually is way, way too much.  No, the laser signal would probably be in the form of brief pulses; really brief pulses, e.g. <1 to 50 nanoseconds.   The reasons for this bracketing include: energy consumption, equipment cost, size, complexity and the signal-to-background ratio.  I’m relying on our parochial views regarding the equipment thing, but physics pretty well defines the other arguments.

So, we’re looking for brief pulses of light coming from the vicinity of other stars.  Sure, but they’re still really bright, huh? Well, uh, it gets a little tricky.  We detect light from stars as photons.  A star that you can dimly see with the naked eye streams photons toward us at roughly 25 million photons per second per square meter.  A laser signal may only be expected to have a few tens, hundreds or thousand photons per square meter, but these would occur in a few nanoseconds.

When we compare the number of stellar photons to the number of laser photons in a few nanoseconds, the number of the later can exceed the number of the former by a very large amount.  The design of a functional detector includes the means for high speed detection of individual photons that are piled up or closely spaced in time and the rejection of photons that are much more spread out in time, i.e., stellar background photons.

There are several methods of doing such detection and each has its merits.  For now, we build devices having fairly narrow optical bandwidths (300-600 nm and alternatively NIR) and specific detection criteria.   But with technology advances we will soon see detectors that cover greater optical ranges and that use built in software to detect all sorts of anomalous signals.  These won’t be available for the small observatories (and small budgets) for quite a while though.

Next time, I’ll write a bit about some recent observational challenges. 

At 76, I’m still hopeful to be around when the incredible event happens.  In the mean time, won’t somebody make a good, respectable “Alien Hunter” tee-shirt?

At this time of year, here in Panama, The Southern Cross, Alpha Centauri and The Southern Oracle are coming into view. 

Marlin (Ben) Schuetz