Field-Of-View

9008-062

9008-062a

In the analysis for Optical SETI it was generally assumed that only one photodetector was used, and that the signal from the alien planet could not be resolved from that of the planckian light from its star. However, we know that 10 meter diameter telescopes outside planetary atmospheres can easily resolve stellar diameters at tens of light years distance, and thus can even more easily resolve their planetary systems. This applies both to ETI transmitters and planetary reflection. However, in the case of the latter, the exceeding dimness of such planets makes it very difficult to "see" them.

If an array of very small area photodetectors is used, just as Charge Coupled Detectors (CCD) are presently employed for very sensitive electronic imaging purposes, use can be made of the array to not only provide parallel signal processing to reduce the SETI "search" time as previously indicated, but in the area of light sciences to resolve planetary systems. Schematic 9008-060 shows the image field of a telescope being focused on to a 2 x 2 array of PIN photodetectors. If the size of the image field, detector array size and pixel element size are properly matched, then each photodetector's diameter is equivalent to a pixel element and is approximately equal to the half-power beamwidth or the beamwidth defined between the first zeroes. The local oscillator beam is not shown here, but as illustrated in schematic 9006-028, it illuminates each photodetector element with a plane wavefront.

9008-062b

If the field of view of the telescope is 0.5^o, then the image formed on the photodetector array covers 0.5^o of the sky. If the half-power beamwidth of the telescope is 0.014", then the number of pixels within 0.5^o is given by 0.5 x 3600/0.014, i.e., 128,571. If we take the array length L to be approximately equal to the diameter of image field, and if the image field is 20 cm in diameter, then each photodetector element occupies 0.2/128,571 m, i.e., 1.6 µm. The two dimensional array would contain 1.65 x 10¹⁰ elements! If we used a beamwidth definition out to the first zeroes, the number of elements required would fall by a factor of 5.6.

Anyway we look at this, this is a tremendous number of photodetector elements and channels to analyze. What we would more likely do is to use a much smaller instantaneous field of view and a smaller array, without increasing the array element diameters. If we used a modest 2 cm x 2 cm array, the number of photodetectors falls to 165 million. Without necessarily moving the telescope, we could use a small mirror to move the 20 cm diameter image field over the 2 cm x 2 cm array. The search would take longer but the hardware is considerably simplified.