Early optical SETI and the all-sky Harvard system
An argument can be made that we are more likely to find alien signals in the optical spectrum than at either radio frequencies or microwaves. For one thing, it is easier to deal with noise at optical wavelengths. Attempting SETI at radio frequencies means contending not only with interference from terrestrial sources such as radars and radio stations but also with cosmic sources, including the background left over from the Big Bang. And, of course, there is noise intrinsic to the receiver itself. Although the most sensitive detectors are cooled to almost absolute zero to minimize internal noise, this cannot be eliminated entirely. The only significant terrestrial source of interference for optical SETI is lightning, which is at worst a sporadic problem with a very low probability. In the early days, many investigators dismissed optical SETI, believing that the sender’s star would be an overwhelming source of noise. But they did not appreciate that if a short-pulse laser is used instead of a continuous one then it is possible to outshine a star during the time the pulse transmitter is ‘on’ With a short-pulse laser, both spectral and temporal discrimination in the receiver can be readily attained since a laser shines at a single wavelength whereas a star shines in a broad spectrum, which enables the laser receiver to reject much of the spectrum but still have a much wider spectral acceptance than microwave receivers. The laser can deliver very large peak powers for brief intervals. And because direct detection for SETI at optical wavelengths is not obliged to preserve the coherence of the signal at the detector, large collectors can be made more cheaply than those of the same size which must preserve the phase of the signal across the face of the collector in order to produce an image.
KeywordsRadio Frequency Quantum Efficiency Direct Detection False Detection Optical Wavelength
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