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Optical Detection Bandwidth Rationale




In Microwave SETI it is assumed that the final detection bandwidth will be no smaller than 1 Hz. One of the main reasons for this assumption is the belief that interstellar dispersion effects will broaden monochromatic signals to about this bandwidth. Hence, the there would be no point in transmitting signals with smaller bandwidths. Indeed, so as to not degrade signal quality, modulation bandwidths in coherent systems should be significantly greater than the dispersion. Because a visible wavelength of 656 nm is about 3 x 105 times the frequency of the microwave "water-hole", we might expect that interstellar dispersion (spectral broadening) would limit the minimum bandwidths at visible wavelengths to about 300 kHz. For the Carbon Dioxide wavelength of 10.6 m, the corresponding figure might be about 20 kHz.

In this analysis we compare the three types of SETI systems by normalizing to a 1 Hz bandwidth. This does not imply that it is possible to use infrared or visible systems at this small bandwidth. Indeed, the underlining assumption is that Optical SETI systems will use much larger bandwidths for communications than is presently being assumed for Microwave SETI. This is for reasons that have to do with the efficiency of the communications technique, laser linewidths, doppler chirps, interstellar dispersion, and the number of frequencies to search.



The convenience of normalizing everything to a 1 Hz bandwidth is generally that we can simply subtract 10 dB from the SNR for every factor of ten by which we increase the detection bandwidth (Planckian radiation neglected). It does not mean that we or aliens could build optical systems that could function properly at such bandwidths.

Some may argue that this leads to an unfair comparison. This author would respond that there is nothing "magic" about a 1 Hz bandwidth for communica-tions, and that if it is worth the resources of an advanced technical alien civilization to send us and other star systems their encyclopedia and perhaps the Encyclopedia Galactica, then it is worth doing it properly at a decent data rate, particularly if they are time-sharing their transmitting telescope with many target star systems. A 1 Hz bandwidth may in fact be far too small for the bandwidths aliens employ.

To simplify the comparative analysis, only CW signals are assumed. However, it is easy to modify the analysis to take into account the effects of possible modulation formats. This does not materially affect the validity of the comparative analysis, except for reducing the effect of Planckian radiation even more.


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