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Fraunhofer Spectra and Some Initial Thoughts on the

Optical Search Strategy

Radobs 19

 
To Dr. Walter Mitchell
Department Of Astronomy
OSU
Dear Walt,
Your Solar Flux Atlas spectra, which covers the wavelength range 296 nm to
1300 nm, arrived here on Wednesday in the post.  Thank you very much.  What
I am doing here is just a piece of detective work to narrow down our sights,
though I am cognizant of the fact that it may be an exercise in futility.  I
have a number of questions and observations:
1.   Is there data available for longer wavelengths into the infrared?
2.   Do you also have such (infrared) data for outside the atmosphere,
     because we mustn't rule out the possibility that one or more of these
     standard intragalactic frequencies are in the near infrared, and
     absorbed by our atmosphere?
There are some really deep troughs, i.e., > 20 dB, in the near-infrared
spectra you sent me, but I suspect they are due solely to atmospheric
absorption.  I do know that within the atmosphere, there are major
absorption bands at 940 nm, 1130 nm, 1380 nm, 1900 nm, 2700 nm, 4300 nm,
6000 nm, and 15000 nm.  I would like to determine if any of the Nd:YAG/YLF
lines directly, or those of semiconductor lasers, coincide with a real
Fraunhofer line.
3.   For instance, is there a Fraunhofer line at or near 1319 nm or 1321 nm?
These wavelengths are just off the scale of the Solar Flux Atlas spectra.
The spectra indicate some interesting Fraunhofer lines where the atmosphere
is transparent and where semiconductor lasers operate.  I am referring to
849.8 nm, 854.2 nm and 866.2 nm (CaII lines).  While these lines are not
deep, they could be said to indicate the positions of preferred frequencies,
and this is right on the peak of the quantum efficiency curve for silicon
photodetectors.
When we visited Perkins in December, you demonstrated to me that the "depth"
of the H-alpha line was more like -20 dB, i.e., about 1%.  However, the data
you sent me, indicated a depth of slightly less than 20%, which is not
even -10 dB.  On the surface, if we were operating a visible laser
transmitter at 656.028 nm, it wouldn't make much difference to the
detectability of our signals.  Still, perhaps the frequency might be an
intragalactic standard just because it is a major Fraunhofer line and is
close to a fundamental or frequency-doubled laser wavelength.  The super-
wide Calcium lines at 393.4 nm and 396.9 nm look pretty dramatic, though
even they only go a little below 10% in relative intensity.
The Full Width Half Maximum (FWHM) maximum for the 656 nm line looks about
0.2 nm, about half the "effective linewidth" which is quoted in my
astrophysical data book, and which I used in RADOBS.18.  The so-called
effective linewidth will be somewhat larger than the FWHM because of the
Fraunhofer line-shape, which appears to somewhat Lorentzian.  I would
estimate that the spectrographic resolution used to obtain the data from the
Solar Flux Atlas was about 0.01 nm.
4.   Can you confirm this resolution?
The H-alpha line looks very clean, both within it and adjacent.  There
doesn't appear to be much fine line structure.  From this data, it looks so
clean that one is tempted to think that it is a natural place for ETIs to
gather!  However, I would assume that if we used improved resolution we
would discover even more structure.  It could be that the fact that the
Fraunhofer minimum is about 20% of the continuum, signifies that there is a
lot of very fine line structure that cannot be resolved.
5.   Do you have data on the super-fine line structures with H-alpha and
     other Fraunhofer lines?
I seem to recall that the spectrographic plots you showed me at Perkins had
greater resolution than in the data you have just sent.
Even at this early phase of trying to identify the "magic-frequencies", I
would say that is very unlikely that I would propose to Dr. John Billingham,
the optical equivalent of the "All Sky Survey", even for selected wavelength
regions, because the Optical Cosmic Haystack is too large and "most of space
is empty".  I am much more likely to suggest that the optical part of what I
have provisionally named MOOP, i.e., the Microwave and Optical Observation
Project, would be directed only at a targeted search.  You never know, we
may get lucky very early on.  This will probably not satisfy the SETI
Institute, who will likely maintain (the sands shift again) that if one
cannot do an "All Sky Survey" with a reasonable amount of hardware in a
reasonable time, then no "sensible" alien would use the optical spectrum.
On philosophical grounds, I would suggest that since the targeted microwave
search was O.K. with conventional SETI people for several decades, it should
be alright now for the optical regime.  An optical version of MOP need not
be identical to MOP.  I am thus likely to propose a targeted optical search
program only, at least for the next few decades.  It could be, that just as
has occurred with the microwave search, major developments in technology
will produce orders of magnitude improvements in our ability to do an all
sky optical search in a reasonable amount of time, i.e., in years rather
than tens of thousands of years!  But we must learn to walk before we can
run (where have I heard that before?), so a targeted search will probably be
both the first priority and the only option available at this time.  The
combination of a targeted search, say 1,000 stars, and selected wavelength
bands, would allow for relatively quick searches to be done.
When one uses incredibly high telescope directivities so that something like
10^15 beams or directions are available, what one finds is that most of the
beams or pixels are pointed at empty space, even if we describe a
communications (celestial) sphere 1,000 to 10,000 light years in diameter,
and included all the pixels or beams defining stellar biospheres.  It really
doesn't make a lot of sense to waste the vast majority, e.g., 99.9999% of
the search time sampling empty space!  This may be systematic, but it is
really dumb!
I have not yet decided whether it is better for the focal plane array to be
two dimensional, or whether a line array with a mirror scan or scanning by
telescope slew would be preferable.  I could envisage a system where there
were 1024 (or more) parallel Multi-Channel Spectrum Analyzers (MCSAs), each
10 GHz in bandwidth and each containing one hundred thousand 100 kHz bins. 
These could be hooked up to a linear array of photodetectors, and the imaged
scanned across the array with a mirror or by very slowly, and ever so gently
moving the telescope.
As you know, just as with the MOP, I envisage that MOOP would also have
"light science" spin-offs.  Consider both the visible and infrared spectrum,
and the spectrum available to both ground-based and space-based optical
telescopes.
6.   As an astronomer, can you suggest to me the benefits of the super high
     spectral resolution capabilities of Optical SETI systems?
7.   In particular, what astrophysical bodies and phenomena would you and
     your colleagues like to investigate?
On a more down-to-earth topic, I had hoped to have been able to upload some
documents giving more details of the optical search strategy this weekend,
but I got rather behind in my work this week.  Understandably, along with
tens of millions of others, I spent too much time last week watching TV! 
Having a satellite dish that allows me to watch three international
satellites carrying British news broadcasts, and raw news feeds from Israel
(CNN & NBC) and Saudi Arabia, does little to discourage TV addiction.  CNN
and NBC have dedicated links from Jerusalem and Tel Aviv respectively, which
are on 24 hours a day.  When the sirens sound in Tel Aviv, Jerusalem or
Dharan, I know about it immediately.  This morning I watched General
Schwartzkopf getting ready for his interviews with the major networks.  I
write this as I have one eye (and two ears) on a Sunday evening news
bulletin from ITN in London.  Oh, what has Arthur C. Clarke wrought!  This
surely must be the first "Looking-Glass War"  What we have seen in recent
days in terms of U.S. guidance technology is pretty stupendous, but tame
compared to what Advanced Technical Civilizations (ATCs) could do in aiming
photons.  Of course, photons are not like cruise missiles - they don't
contain stellar maps or guidance systems, and once they leave the
transmitter their course is set, except perhaps when grazing a star.
You see I am very appreciative of microwave communications technology, its
just that I feel that ETIs will generally use optical communications in
preference to microwaves.
Regards
               Stuart
January 20, 1991
RADOBS.19
BBOARD No. 332
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* Dr. Stuart A. Kingsley                       Copyright (c), 1991        *
* AMIEE, SMIEEE                                                           *
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