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Planetary Detection



Schematic 9008-061 shows a small row of PIN photodetector elements and a superimposed polar response (diffraction pattern), with the image of a planet falling on the central photodiode illustrated. The separation of the star and its planet and their relative size, is not to scale.

The image plane size and size of the photodetectors are arranged so that the image of a star at the range being observed, would fall completely within one pixel element or photodetector. Typical diameters of each pixel would be a couple of micrometers. The side-lobes of the "bright" star would fall onto adjacent pixel elements. Each object in the field of view generates a diffraction pattern similar to the one shown, except displaced sideways along the particular row of pixel elements illustrated.

The irradiance of the dim planet may be more than 85 dB below that of the star. The light received by the central pixel element from the star is severely attenuated by its diffraction response. Further discrimination is provided by the differential Doppler shifts between the planet and its star, and absorption spectra from the planetary atmosphere, if present. Because the spectra is analyzed in optical detection bandwidths of between 1 and 10 GHz, and the planet is very dim, considerable signal integration must be done in order to extract a useful signal for measurement purposes.





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