Optical SETI Observations during the Day - Discussion

Radobs 04

The following text, and the separate theory document is an edited version of
about half a dozen recent messages to Bob Dixon:

I have suggested that the Perkins Observatory might be used for Optical
SETI.  Certainly it would be a unique location for both Microwave and
Optical SETI activities.  Because Optical SETI will use very narrow band
incoherent (Scanning Fabry-Perot Spectrum Analyzer) or coherent
(heterodyning) receivers, light pollution should not be a problem.  Indeed,
my calculations indicate that we might be able to operate in daylight
without signal degradation from a clear blue sky!  Of course, there is the
question of whether we should first use an infrared telescope for the
"magic" Carbon Dioxide wavelength of 10,600 nm.

One doesn't normally consider the possibility of doing optical astronomy
during daylight hours, but the narrow optical bandpass demanded by Optical
SETI appears to make this possible.  If I have done my calculations
correctly, it would appear that for a 10 meter diameter diffraction limited
telescope pointing at 45 degrees to the zenith under a broad daylight clear
blue sky, sky noise does not degrade SNR until the optical bandwidth at
656 nm becomes greater than about 10 MHz.  Surprisingly, for CO2 at 10,600
nm, the SNR begins to degrade when the optical bandwidth is greater than
about 400 Hz.  Note that with a bandwidth of 10 MHz, substantial degradation
in SNR will occur for small and non-adaptive ground-based telescopes that
cannot resolve nearby alien stars from their planets at visible wavelengths.

This is due to Planckian radiation, and is on top of any signal degradation
due to scattered daylight.

I note that Bernard Oliver, near the bottom of the left hand column on
page 51 of the 1973 revise edition of the Cyclops Report, also indicates
that his modeled optical and infrared systems could be used in daylight,
and that even his 3 MHz photon-counting system (optical system B) was hardly
affected by daylight.

Note that my calculations are based on a situation where the diameter of the
active region of each photodetector in the array corresponds to the
diffraction limited spot or pixel size.  In other words, the amount of
skylight detected by each pixel in the focal image plane is many orders of
magnitude less than the total skylight power in the image plane.  Except for
scatter within the telescope, each pixel sees photons from a very small part
of the sky - an area approximately corresponding to that occupied by the
image of a star.  If 1 million pixels and photodetectors cover the field of
view, then there is approximately a 10^6 factor in total detected skylight
at the telescope.  It we have twice the aperture diameter (all other things
being equal) we will have four times the collected light, but we will also
have four times as many pixels, so the background per pixel remains
constant.

Thus, there is hope that with a collection of high-Q electronically-tunable
Fabry-Perot interferometer bandpass filters and a photon-counting array, we
could do some real "poor-man's" Optical SETI even in daylight.  This would
probably be a lot easier than optical heterodyne detection, especially as we
will want to tune over a large part of the spectrum.  I expect that we would
try to ramp the Fabry-Perot (spectrum analyzer) to compensate as much as
possible for our local Doppler chirp (drift).  We would also need to use a
photon-counting photodetector array, if we are to be efficient in our search
strategy.  Note that it would be advisable to specify special telescope
optics that not only function throughout the visible and near-infrared but
also at 10.6 microns.

Whatever one wants to say about daytime Optical SETI, we clearly need not be
concerned about night time Optical SETI in clear visibility conditions.  I
don't even believe (this needs to be checked) that the strong spectral lines
produced by gas discharge lighting scattered from the sky will be a problem,
because these lines are relatively wideband, and there should be
insignificant energy in a 10 MHz optical bandwidth.  Interestingly, Optical
SETI may be less affected by man-made artificial sources of light that
Microwave SETI is by man-made artificial sources of R.F.!  Think about that!

This comes about because the optical regime is much noisier by nature than
the microwave regime, so we have to try much harder to really muck things
up.  We can't do much about the number of cloudy days in Columbus.  It's a
pity that OSU doesn't operate a telescope in Hawaii - that's my favorite
part of the world!

Of course, we can't put a 10 meter diameter telescope in the Perkins
Observatory.  However, since my Optical SETI rationale also includes the
view that Optical ETI transmissions will be quite strong, we can still hope
to detect signals with a substantially smaller diameter.  Remember, I
estimated a CNR of 34 dB re 1 Hz for a 10 meter symmetrical visible system
over 10 light years, with only 1 kW of transmitted power.  If nearby aliens
are putting out megawatts of gigawatts, we have some margin on receiving
mirror size.

To minimize transmitter chirp without actually dechirping their optical
oscillator, the best place to put a transmitter is in orbit about a star. 
There is the danger for the transmitting race of accidentally passing through
the transmitted beam in regions of space relatively close to the
transmitter, since the near-field would stretch out quite a distance.  This
would be like an SDI "death ray", instantly vaporizing anyone or anything
straying into its path!  This is probably another good reason for putting it
into its own orbit about the alien star, well away from inhabited planets or
space colonies, and where it could be directly pumped by stellar radiation.

I personally think that a space-based Optical SETI program may be ideally
suited for an activity associated with Space Station Freedom (whose
advisability is presently being seriously questioned).  NASA is always
looking for additional ways to justify the cost of the space-station.  It
could be that the reliability issue which has loomed in importance recently
as far as the repair and maintenance of the space station is concerned,
would also be important as far as maintaining a variety of local-oscillator
lasers in the Optical SETI Space Telescope.  Thus, having people around to
fix the lasers if they should need repair, or even replace consumables (such
as laser dyes), is an ideal match for a space-station activity.  Of course,
an Optical SETI Space Telescope in low earth orbit is not the best place for
producing a low level of local Doppler chirp.

If it is indeed true that we can do Optical SETI in daylight without
significant degradation in SNR caused by scatter within the telescope, then
we could establish Optical SETI activities at all the present major and soon
to see first light large optical telescopes, without interfering with
conventional astronomy.  Simply put, Optical SETI would be done during
daylight hours, and conventional astronomy during the night, allowing 24
hour utilization of optical telescopes.  The implication of this realization
is profound.  There would be no good grounds for not instrumenting the
world's biggest telescopes for the optical search.  This time sharing
(multiplexing) of optical telescopes should be received quite well by
conventional astronomers, and would mean that Optical SETI could be done at
minimal cost since we wouldn't need a dedicated large aperture telescope or
telescopes.  A symbiotic relationship could grow between conventional
optical astronomy and Optical SETI.  If this can be done on an adaptive
telescope with deformable mirrors, so much the better.  I'd like to propose
an Optical SETI activity for the Perkins or Columbus Telescopes.  This could
give a tremendous boost and attention to OSU's SETI activities.

Up to now, hyperfine spectral resolution in the visible region has not been
of interest to optical astronomers because of the severe lack of sensitivity
at such high resolution levels - there are so few photons arriving here per
second from distant narrow-linewidth natural phenomena.  This probably
explains the lack of development of optical heterodyne technology in
conventional astronomy for the visible part of the spectrum.

I'd like to propose a fanciful idea -  that the aliens know that
Interstellar Optical Communications, at least the reception part, can be
done during daylight hours within an atmosphere.  They also know that it can
be done by time-sharing great ground-based optical telescopes with
conventional astronomy.  That this knowledge is just another reason why
beamed (directed or targeted) Interstellar Optical Communications to
emerging technical civilizations would be preferred over its microwave
counterpart.  You must admit Bob, that whatever you or Bernard Oliver might
say about my technical approach, I certainly have an imagination - perhaps I
should take up writing science fiction!

Now the angular resolution of a 10 meter diameter telescope outside the
atmosphere is about 0.014 arcseconds, while the performance inside the
atmosphere is more like 0.5 to 1 arcseconds.  The ratio of focal plane areas
formed by these spots is about (1/0.014)^2.  This is equivalent to 37 dB. 
This is the factor by which the Planckian energy density in the focal plane
is reduced because the light is smeared over a greater area.  The margin
between the Planckian starlight level and daylight background is about
(72 - 32) dB, i.e., 40 dB.  The smearing of the image within the atmosphere
would appear to account for most of the difference in energy densities. 
Thus, it would appear that the fact that we cannot see 2nd magnitude stars
in broad daylight, may be largely due to the smearing of stellar images by
the atmosphere.  Clearly, if we are going to use existing large ground-based
telescopes, it is highly desirable to include a deformable mirror system to
clean up the image before reaching the focal plane of the SETI receiver
imaging array.


December 16, 1990
RADOBS.04
BBOARD No. 268


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