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The Search for Extraterrestrial Intelligence (SETI) in the

Optical Spectrum


Jim Lesh: As I had mentioned earlier this morning in a comment to Dr. Townes' presentation: Yes, there is a trade-off on how imperfect you make that aperture versus how many spatial directions you become subject to background noise.

Barney Oliver: Right - and noise from all those other directions if there is noise. You still lose something, that's what you do.

Jim Lesh: If there is noise in there. If you have stellar images that are point sources and not many other images in the vicinity, then what you do is you have a wider field of view viewing that noise source; the star in this particular case. But if it is a distributed source, then yes, you will automatically increase the noise based on that ratio of the field of view. If it is a point source, or not very distributed source, then there is a limit of how much additional noise you pick up because of that wider field of view.

Charles Townes: Let me just comment on this question of signal to noise: If you were to cut your signal off, then there is no noise. The way I would define signal-to-noise is on the basis of the strength of the signal compared with the noise that's there when there is no signal. In that case this comes out as normal signal-to-noise just the way you are describing it. It is a matter of language as to what you call "signal-to-noise". I think it would be handy and a little more logical to say "the noise is what is there without the signal" - when there is no signal at all. In that case, the noise can be essentially zero.

Jim Lesh: My caution is; beware of simply taking the ratio of those two parameters, however you define the noise. The actual performance of a detection system depends on the particular operating point as well as the ratio of those two values.

Charles Townes: Well, I would say if there were no noise and there were no photons there at all, and then one photon comes along which is your signal and you detect it, then you have a very high signal- to-noise! If you would define the signal-to-noise the way I would define it, I think there is no more worry about it. It come out right.

Jim Lesh: I think that a lot of people that would agree with you. I think there are people that would likewise disagree with you because of the quantum noise that you typically get through fluctuation when the intensity is large. But my point is that it is not a function that depends on the ratio. It's a function that depends on the individual operating points for both whatever you define as noise as well as what you define as signal.

Charles Townes: I think what you are defining as "noise" I would define instead as "signal fluctuations".

Kent Cullers: It seems to me that the issue here is one of detection versus decoding. I tend to side with Professor Townes here. I want to know my detection signal-to-noise because of my special interests. If on the other hand, I am trying to communicate what a signal-to-noise ratio is, it has to tell me how well in fact I can get my message back. These are often different issues in the microwave region, certainly the statistics of detection and decoding look very different.

Clive Goodall (Floor): I just have a point I am trying to track in this debate here. Unfortunately, a lack of breadth of technical knowledge on my part has caused me to lose track. In your critique of the models underlying the traditional approach, are you producing an argument for enhancement of detection possibilities? I'm not sure if I got the direction of criticism right here. Is it a gloomier outcome you are looking at based on what you are saying, or is it a more optimistic picture? It sounds to me a more optimistic one.

Jim Lesh: In may cases, I think it will turn out to be more optimistic situation. You did state it correctly, it is ways of enhancing the detectability of the pulse that I am trying to refer to. In order to communicate, we typically have to detect those pulses. We then try to extract from that some information through the modulation format. In order to do the best job of communicating and getting that information, we also have to do the best job of detecting. It is from that standpoint that I was bringing up these various models.

Clive Goodall (Floor): I see.

Charles Townes: I'd like to move along now. We'll go another couple of notches, then we'll allow further discussion because some of these same issues may well come up again. OK, would you like to make any comments?

Monte Ross: Well, the only comment I'd like to make since I was speaking of such very short pulses, is that I didn't intend to mean in my presentation that somehow there was something magic about one nanosecond. What I meant to convey was that it is in the arena of short pulses, that you gain this bits-per-pulse very readily, and that you gain this potential discrimination against the star's background. There is nothing magic about one nanosecond just as there is no magic frequency. It is just a convenient parameter when one goes through the numbers.


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