Optical SETI System Description
We have built a system to detect a pulsed laser beacon signal, using a
relatively simple high-speed optical detector at Harvard's 61" optical
telescope. This telescope, which is located at the same site as the 84'
radiotelescope used for The Planetary Society's BETA search, is currently
engaged in a continuing radial velocity survey of 2500 nearby solar-type
stars, conducted by David Latham and Robert Stefanik. The optical SETI
piggybacks on their experiment compatibly, using the ~25% of light that
cannot be used by their echelle spectrograph.
The block diagram shows how it works. We extract a beam containing
approximately 25% of the full focused beam intensity from the 61"
telescope. The design of their instrument allows us to simply deflect our
subsidiary beam out to our attached detector. Following Dan Werthimer's
experience at Berkeley, we use a beamsplitter and a pair of fast
photodetectors (Hamamatsu "hybrid avalanche photodiode") in coincidence to
reject occasional "hot" events in contemporary fast photodetectors. (These
arise from internal pathologies such as positive ion "afterpulses," glass
scintillation from both radioactive potassium decay and electron impacts,
and cosmic-ray-muon induced flashes.)
A real laser pulse, if delivering at least a hundred photons per pulse,
will always generate a coincidence. Each detected coincidence will trigger
a measurement of optical pulse width and intensity, which, along with
absolute time and target coordinates, is logged to a PC. The observers at
the telescope can watch the data in real time; our current 56 kbps line to
Cambridge lets us monitor these events also, probably on the next working
day (as we do with BETA). We will let web visitors view the observing
results, somehow; tell us your suggestions!
We have built the electronics shown here in block diagram form, and we
have built a "camera" that includes the detectors, etc -- see the photos.
(You can view also the separate components of the optical detector system.)
We generate simulated alien laser pulses with a fast LED, and
watch the system report the resulting coincident pulse. The photodetectors
are able to respond to light pulses on a time scale of a few nanoseconds
(billionths of a second), and they can tell the difference between a flash
that has only a single photon (particle of light), and one that has
several. Technically inclined readers may enjoy looking at the
pulse-height display, taken as an oscillographic time exposure.
We put the system on-line for its "first light" shakedown on 19 Oct 1998.