Fraunhofer Spectra and Some Initial Thoughts on the
Optical Search Strategy
To Dr. Walter Mitchell Department Of Astronomy OSU
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.
January 20, 1991 RADOBS.19 BBOARD No. 332
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