Searching for Low Frequency Optical SETI Signals Buried in Atmospheric Scintillation

摘要

The possible detection of low frequency optical SETI signals in the presence of atmospheric scintillation (starlight twinkling) is examined. Previous work has shown that modulation as weak as four orders of magnitude below the background starlight level could be detected at frequencies where this scintillation level is small. This paper extends those results to frequencies as low as 0.002Hz in the presence of significant scintillation. Three related techniques for extracting signals from time-series data are compared: 1) threshold detection of individual pulses, 2) autocorrelation to find pulses or other repeating waveforms and 3) searches of power spectra to identify signals most readily visible in the frequency domain. Each technique is limited by noise. Using "realistic" signals inserted into measured time-series star photometry it is shown that each technique has distinct advantages. After analyzing photometry from over 300 stars, many "signals" from airplanes, satellites, meteors and light flashes have been found. Essentially all "signals" found to date have been attributed to one of these terrestrial sources. This achievement rests on having four simultaneous measurements of the light from each target star and its surroundings to guarantee that no terrestrial signal is attributed to the target star itself.