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Optical SETI Survey -Facts (Part E)

Radobs 7E

Version 1.0
1.   Once adaptive telescope technology is perfected, say within five years,
     night time Optical SETI observations will be able to be done at visible
     wavelengths with large ground-based adaptive telescopes, even with a
     dark sky suffering substantially from light pollution effects.  Do you
                                                                     DON'T KNOW

2.   Furthermore, daytime Optical SETI will then also be able to be done at
     visible wavelengths with large ground-based adaptive telescopes under a
     clear blue sky!  Optical SETI at the far-infrared CO2 wavelength of
     10,600 nm is more problematic, even though there is an atmospheric
     window, because of the approximate 300 K sky temperature, day or night. 
     This severely limits the optical detection bandwidth that can be
     employed without degradation in signal-to-noise ratio.  Do you think
     that visible SETI can be done in daylight?
                                                                     DON'T KNOW

3.   The more than 100,000 times increase in Doppler shifts (fixed frequency
     offsets) at visible wavelengths with respect to that at microwaves, is
     not a problem since this only represents a very small uncertainty on
     top of a much larger uncertainty in not knowing the "magic
     frequencies".  Thus, assuming the availability of local-oscillator
     receiver lasers that can be tuned across the entire visible spectrum,
     the extra frequency uncertainty doesn't really make it any the more
     difficult to acquire a signal for the first time.  However, the effort
     to acquire the signal again would be increased if its frequency had
     changed in between observations.  Would you agree with this sentiment,
     which seeks to indicate that the actual Doppler shifts are not that
     important?  What is more important is the Doppler drifts or chirps
     which are discussed in the following question.
                                                                     DON'T KNOW

4.   The more than 100,000 times increase in Doppler chirps (drifts) at
     visible wavelengths with respect to that at microwaves, is not a
     problem, because we would expect the aliens to de-chirp their signals
     at the transmitter, and we could de-chirp the receiver to take out our
     local chirp.  Thus, most of the induced chirp could be compensated for
     with relatively simple electronics.  Once a signal was acquired
     (found), conventional automatic frequency control techniques would
     allow our receivers to track the received frequency.  Do you think that
     advanced technical civilizations (ATCs), aware that the problem of
     spectral spreading caused by rapidly drifting frequencies would reduce
     the detectability of their signal, would de-chip their transmissions?
                                                                     DON'T KNOW

5.   Single mode laser linewidths can be obtained which are less than 1 kHz. 
     Do you agree?
                                                                     DON'T KNOW

6.   Interstellar dispersion effects (spectral spreading) at visible and
     infrared wavelengths over distances less than 10,000 light years is
     negligible.  Do you agree?
                                                                     DON'T KNOW

7.   Interstellar absorption in the galactic plane at visible wavelengths is
     negligible up to a thousand light years.  Do you agree?
                                                                     DON'T KNOW

8.   Interstellar absorption outside the galactic plane (> +/- 20 deg.) at
     visible wavelengths is negligible over thousands of light years.  Do
     you agree?
                                                                     DON'T KNOW

9.   Interstellar absorption in the galactic plane at infrared wavelengths
     is generally negligible across the entire galaxy.  Do you agree?
                                                                     DON'T KNOW

10.  Laser powers available to ATCs are likely to be huge, and at least as
     powerful as the microwave power available.  Do you agree?
                                                                     DON'T KNOW

11.  Just because huge microwave systems have the ability to transmit across
     the entire galaxy, and beyond, does not imply that they would also be
     used for relatively short ranged interstellar communications.  Do you
     agree with this statement?
                                                                     DON'T KNOW

12.  ATCs are likely to put their transmitters in orbit about their stars,
     and have them nuclear-pumped or directly pumped by radiation from their
     stars.  A separate stellar orbit would minimize Doppler chirp and
     reduce any danger caused by the high power beam densities in the near-
     field.  Do you agree that this is plausible.
                                                                     DON'T KNOW

13.  Because the efficiency of optical transmitters in getting signal energy
     to a particular target is much higher than with large microwave
     systems, we can expect the signal bandwidths and signal-to-noise ratios
     to be much higher than is presently predicted for the microwave
     spectrum, even considering the various noise penalties.  Indeed, the
     requirement for high bandwidths may well be the overriding
     consideration for preferring optics to microwaves.  In the optical
     regime, larger bandwidths are also more compatible with the effects
     mentioned above, such as Doppler chirp and finite laser linewidths.  Do
     you agree that information bandwidths are likely to be significantly
                                                                     DON'T KNOW

14.  For the same signal strength and information rate reasons, the number
     of frequencies to search in the optical spectrum is not likely to be
     100,000 times or more greater than the number of frequencies in the
     microwave spectrum.  Each channel will have a bandwidth much greater
     than 1 Hz or a few tens of Hz, and hence the optical spectrum need not
     be subdivided into 1 Hz bins as part of the search strategy. 
     Obviously, this considerably eases the magnitude of the "search"
     problem.  Perhaps each bin should be 10 kHz to 1 MHz wide, or even
     greater.  Does this wider bin width assumption seem plausible for the
     search strategy?  The aim of this question is to show that while the
     "optical search" is much more difficult than the microwave search, in
     terms of time and effort, it is by no means 100,000 times the time and
     effort.  It may only require about ten times the effort, even less if
     we concentrate only on those regions of the optical spectrum that are
     at and adjacent to, future identified "magic frequencies".
                                                                     DON'T KNOW

Score out of 14: YES        =
                 DON'T KNOW =
                 NO         =

December 31, 1990
BBOARD No. 298

* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley                       Copyright (c), 1990        *
* AMIEE, SMIEEE                                                           *
* Consultant                            "Where No Photon Has Gone Before" *
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* CompuServe: 72376,3545                                                  *
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