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Optical SETI Survey - Problems of Targeting Tight Beams (Part C)Radobs 7CVersion 1.0
Preface: Most of the questions in this section relate to the problem of
aiming a very narrow beam over tens and hundreds of light years.
What we think will be the ability of advanced technical
civilizations (ATCs) to do this, is perhaps the most important
assumption to be made when comparing different interstellar
communication techniques. The benefits of the optical approach
really hinge on this ability, so consider your response very
carefully. Read questions 2, 3 and 4 together before answering,
so that you are aware of the differences.
1. Just to give a flavor of what humans can do today, you should note that
the specifications for the Hubble Space Telescope call for it being
held on target to an accuracy of 0.012 arcseconds, which is pretty
impressive as far as our own crude technologies go. Would you expect
that mankind would be able to do even better in time?
YES
DON'T KNOW
NO
2. It is thought by some to be relatively easy for ATCs to precisely aim
(control) beams which have widths smaller than 1 second of arc. Do you
agree? If so, indicate what you think would be the narrowest beam that
an ATC could bring to bear on a desired target. This basic question
relates only to the ability of actually being able to steer a tight
beam onto a small target with high accuracy, and overcome any inherent
problems associated with laser beam wander. It does not relate to the
problem of actually knowing where the target is because of the lack of
visibility, relative motions and the light distance of the target.
These problems are addressed in the two following questions.
(a) < 0.1 arcsecond
(b) < 0.01 arcsecond
(c) < 0.001 arcsecond
YES
DON'T KNOW
NO
3. It is a part of this Optical SETI rationale that it should be possible
for ATCs to transmit optical beams with beamwidth << 1 second of arc
and hit targeted nearby stars (less than a few hundred light years
away) with forward predictive targeting, even though the beam diameter
at the target will be a smaller than 1 astronomical unit. The problem
here relates to the fact that ATCs must have a very accurate idea about
the relative motion of the targeted star with respect to the
transmitter if they are to "hit the bull's eye". If the ATC's
knowledge about the position of the target is still limited by the
speed of light, then the problem amounts to being able to land a beam
centrally on a star, which will have moved (transversely) from its last
known position (when the photons left the transmitter) by the relative
distance travelled by the star in twice the light time distance away.
Allowances would also have to be made for the aberration of light, and
perhaps the gravitational bending of light by other stars close to the
line-of-sight. Considering these difficulties, do you think that ATCs
could land such a narrow beam on a star?
YES
DON'T KNOW
NO
4. It is also part of the rationale that it should be possible for ATCs to
transmit optical beams with beamwidth << 1 second of arc and hit
targeted planets in nearby star systems (less than a few hundred light
years away) with forward predictive targeting, even though the beam
diameter at the target will be smaller than 1 astronomical unit. This
is an even more stupendous feat for ATCs than just hitting a star with
its beam. They could have information about our planet, perhaps having
actually visited here at some point in our history, or they may have
sent out probes which have transmitted (by photons?) back information
about us, or their knowledge may come via the more basic technique of
actually being able to directly image the planets in the Solar System
with their large space-based telescopes. Simply put, if they have the
optical telescope technology to transmit to us effectively, they also
have the capability to "see" our planets.
They might, of course, also know our position and motion by detection
of radio frequency leakage, which would also tell them that there was
intelligent life on this planet. Just because we may have been
detected by radio waves does not necessarily mean that they would use
the same technology to signal us. Do you agree that they could know
about the positions of planets in nearby star systems and thus ensure
that when the photons arrived at the targeted star, they passed through
the star-system in a region of space occupied by the targeted planet?
YES
DON'T KNOW
NO
5. Transmitted beams might be shaped with phased array elements to produce
a fan-shape. If the target planetary's plane of ecliptic is known to
the aliens, and the line-of-sight is more or less along the plane of
the ecliptic, then the energy density can be maintained to a
significantly higher level by a fan-shaped beam, than if a circular-
profile beam is employed which is as large as the entire planetary
system. In this way, the targeting problem is substantially reduced
(one doesn't need to know the position of the planets in their orbits),
and energy is not wasted outside the plane of the ecliptic. For
instance, the beamwidth might be 1 X 0.014 seconds of arc, such that at
a range of 10 light years, the beam would have dimensions
3.6 X 0.05 A.U. This would decrease the SNR by about 18.5 dB, but make
it much easier to simultaneously illuminate the entire biosphere of
nearby stars. Under such beaming techniques, a symmetrical 10 meter
diameter visible light system has an SNR only about 5 dB less than is
available from a symmetrical 300 meter diameter 1.5 GHz microwave
system. Is a fan-shaped beam a good approach for overcoming the
targeting problem if the line-of-sight is near the target's planetary
plane of the ecliptic?
YES
DON'T KNOW
NO
6. Dr. John Rather, of NASA Headquarters, has suggested that phased
optical arrays could be constructed that produce annular rings of
energy to match the biospheres of planetary systems. In this way, the
targeting problem is substantially reduced (one doesn't need to know
the position of the planets in their orbits), and energy is not wasted
in the center of the beam. However, this only buys a few extra dB in
signal strength and may not be worth the complication. One would need
to know the plane of the ecliptic. It would also be possible to
rapidly dither a small circular-profile beam to trace out an annular
ring. Do you think this annular ring beam is a reasonable idea?
YES
DON'T KNOW
NO
7. If Optical SETI is restricted to far-infrared wavelengths, e.g., the
CO2 wavelength of 10,600 nm, the diffraction limited beamwidth of a
10 meter diameter transmitting telescope is 0.22 arcsecond, only
slightly less than the approximate 1 arcsecond aiming limitation that
has been suggested by some researchers. Carbon Dioxide lasers are one
of the most efficient continuous-wave lasers and most coherent lasers
known to man, and CO2 is thought to be very common in planetary
atmospheres. The Carrier-To-Noise Ratio (CNR) from a symmetrical
10 meter diameter telescope system at this wavelength, is only about
12 dB less than from a similar size visible telescope system. Also,
the atmosphere is reasonably transparent at this wavelength, which may
allow for ground-based observations. Do you think that 10,600 nm
should therefore be classified as a "magic wavelength"?
YES
DON'T KNOW
NO
8. Even if the above targeting problem is thought to be too difficult for
ATCs, it does not make sense to use too small a transmitting telescope
to produce a relatively large beamwidth. Rather, use the largest
telescope that ATC technology can produce at a cost that is affordable,
and defocus (decollimate) the beam for nearby stars only. In this way,
long range (greater than a few hundred light years) performance is not
degraded. The defocusing aspect would be under computer control during
spatial/time-multiplexing of many targeted star-systems (see following
question). Do you agree with these sentiments?
YES
DON'T KNOW
NO
9. Laser transmitting telescope technology makes it relatively easy (for
ATCs) to rapidly spatially time multiplex their transmitted signal
amongst many target receivers and star systems. Do you agree?
YES
DON'T KNOW
NO
10. Optical spatial/time-multiplexed communications may be a technique in
day-to-day use by ATCs in order to keep in touch with their space
probes, spaceships, space colonies, and perhaps other inhabited planets
in their star system. It is not unreasonable to suggest that the same
technology would be used to signal emerging technical civilizations
(ETCs). Do you think that it is a strong possibility that ATCs will
"spin-off" their conventional communications technology in an attempt
to contact ETCs?
YES
DON'T KNOW
NO
11. For an alien, if its worth sending your Cultural Encyclopedia or the
Encyclopedia Galactica to an emerging technical civilization (us), then
it is worth doing it properly at high data-rates, particularly if one
is time-multiplexing many target star systems. Do you agree with this
philosophy?
YES
DON'T KNOW
NO
12. If there is a Galactic Communications Network, it may rely on ETI
relays spread out through the galaxy, for the Milky Way Galaxy to be
linked in its entirety. Each civilization would add to the message its
own cultural history. Thus, it would not be necessary to suppose very
powerful transmitters that could directly send a strong signal over a
distance equivalent to the diameter of the galaxy, i.e., about 100,000
light years. A relay system would be the most efficient way of linking
the galaxy. Just as we use repeaters in undersea communications, and
terrestrial fiber-optic systems, so repeaters could be employed in
interstellar communication systems. Do you think intragalactic relays
would have merit if intelligent life is relatively common throughout
the galaxy?
YES
DON'T KNOW
NO
13. The 1971/1972 study on Project Cyclops probably has much to do with the
SETI lore that the optical approach is useless. That study compared
the performance of a 6.4 km diameter array consisting of 900 microwave
dishes, each 100 meters in diameter, with an (asymmetric) near-infrared
(1.06 micron Nd:YAG) system employing a transmitting telescope only
22.5 cm in diameter - a telescope size smaller than what many
terrestrial amateurs own. The study did not consider space-based
telescopes or adaptive telescope technology, which severely constrained
the size of the optical telescopes. Allowing for heat dissipation
limitations for small transmitter apertures, the Effective Isotropic
Radiated Power (EIRP) is proportional to the telescope diameter raised
to the fourth power. This has a tremendous effect on the signal-to-
noise ratio (SNR), as an order of magnitude increase in diameter will
increase the SNR by 40 dB. On the receiver side, there could be an
additional increase in SNR of between 20 to 40 dB when the receiving
telescope size is increased by an order of magnitude, depending upon
whether the SNR was previously severely limited by Planckian starlight.
Do you think it was unfair to suppose that ATCs would be so limited in
their optical technology?
YES
DON'T KNOW
NO
Score out of 13: YES =
DON'T KNOW =
NO =
December 27, 1990
RADOBS.7C
BBOARD No. 290
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* Dr. Stuart A. Kingsley Copyright (c), 1990 *
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