Optical SETI - OSETI In most discussions about the Search for Extraterrestrial Intelligence (SETI), the assumption has been made that another civilisation would use the radio part of the electromagnetic spectrum in their attempts to contact us. Initially this appeared to be the optimum choice as, in the radio part of the spectrum, a star does not radiate significantly. It is thus easy for a civilisation to transmit a radio signal that would greatly exceed the brightness of their central star in a narrow part of the radio spectrum. Whilst this is still true, optical technology - particularly the capability of building very powerful lasers - means that it is now quite sensible to search in the optical part of the spectrum. As a result, several Optical SETI (OSETI) searches are now underway. Laser technology As Professor Paul Horowitz (Harvard University, USA) has said: "Using only current Earth technology, we could now generate a beamed laser pulse that appears 5000 times brighter than our Sun, as seen by a distant civilisation in the direction of its slender beam. In other words, interstellar laser communication is altogether practicable." One could possibly expect that a laser system could be used by some distant civilisation to direct short bursts of beacon signals to up to a million stars a day. Our current optical telescopes could detect such signals across a distance of no less than 1,000 light-years. If they would use a somewhat more powerful laser than we have at present - remember they are likely to be far more advanced than us - then even a small amateur telescope could be used to detect such a beam. An old idea In fact the concept of Optical SETI was born already in 1961 when Charles Townes and Robert Schwartz (the inventors of the laser) first proposed that lasers could be used for interstellar communications. It has, however, taken many years for optical technology to advance to the point that practical OSETI observations could take place. Several programmes are in now progress, the majority looking for very short pulses of green light at a wavelength of 550 nm. Current OSETI searches One of the leaders in this field has been an amateur astronomer, Stuart Kingsley (Ohio, USA), who has used a 10-inch (25-cm) amateur telescope and commercial equipment in his "Columbus Optical SETI" observatory (COSETI). This has spurred much professional interest in OSETI and now groups at both Harvard and Berkeley Universities are setting up OSETI searches. Harvard is even building a new dedicated 1.8-m telescope system! False signals One general problem encountered in this work is that of false detections. The high-speed detectors employed give random false triggers at quite a high rate. To overcome this serious problem, a coincidence system is used where, in general, two detectors have to trigger simultaneously. This still seems to give around one false detection a day, so new systems are being commissioned using three detectors. Statistically, this should reduce the rate to about one per year. A further refinement is to make simultaneous observations with two telescopes.

The 10-inch Meade telescope used for optical pulse searches at the Columbus Optical SETI Observatory.