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 0.5o, then the image formed on the
photodetector array covers 0.5o of the sky. If the half-power beamwidth of the
telescope is 0.014", then the number of pixels within 0.5o 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.