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EJASA - Part 7
Page 42
AMATEUR OPTICAL SETI
Working on the assumption that highly advanced ETI technology could
appear to late Twentieth Century humanity like "magic", it is imagined
that ETIs will be using much larger transmitting telescopes or arrays
and transmitter powers far greater than 1 kW. [56] In practice, the
signal is likely to be pulsed, and, depending on the duty cycle, even
less detectable by normal integrating detectors, i.e., the unaided
eye, photographic plates, or standard CCDs. Optical SETI is a branch
of science to which the enthusiastic amateur astronomer may be able to
make a useful contribution. In so doing, this may increase public
and scientific interest in all forms of SETI so that this field of
scientific endeavor will at last get the financial support and effort
it richly deserves.
Optional I
Intensifier ----------<---------
-- ------ |
Signal Pr | | | | |
-----------> | | -----> | | -----> ----- PIN Photodetector,
-----------> | | -----> | | -----> / \ APD or Photomultiplier
-----------> | | -----> | | -----> -----
Background | | | | | -----
Pb -- ------ | | |
Narrow-Band --->---| |----->
Optical Bandpass Filter | |
(or Monochromator) Bo -----
Low-Pass Electrical Filter
Be
Figure 7 -
Incoherent (direct) detection optical receiver. The image or photon
intensifier is only required if a zero-gain PIN photodetector is
employed. The narrow-band optical filter (Bo < 0.1 nm) is ideally
a tunable device like a scanning grating monochromator. The photo-
detector current I is proportional to the received signal Pr.
Figure 7 is a basic schematic of an incoherent photon-counting
receiver for an Amateur Optical SETI Observatory. The high cost and
technical difficulties of optical heterodyne detection in the visible
and near-infrared spectrum means that the amateur's receiver will most
likely have to use photon-counting, a little cooling, and a mono-
chromator. Unlike coherent receivers, incoherent receivers do not
have the ability to reject Planckian starlight and daylight background
noise if the signal is weak.
Figure 8 results use slightly more conservative assumptions than
employed to derive Table 2 (Equ. 15, Page 78). It is assumed that the
amateur telescope has a diameter of thirty centimeters (twelve inches),
uses a low-resolution scanning grating monochromator bandwidth of
100 GHz (0.143 nm) at 656 nm, and employs a receiver consisting of a
Page 43
single perfect photon-counter. For a received flux density of
10^-12 W/m^2, the SNR is about 39 dB re 1 Hz (Equ. 31, Page 87). In
the region of the graph where the SNR is reduced due to Planckian
starlight, daylight background further reduces the SNR by a few dB.
In the Microwave Cosmic Haystack, the flux densities of interest
lie in the range of 10^-27 to 10^-20 W/m^2. It is suggested that the
corresponding flux levels in the Optical Cosmic Haystack would be in
the range of 10^-20 to 10^-10 W/m^2. As indicated in Figure 8, an
EIRP = 10^23 W at a range of ten light years produces a received signal
intensity Ir = 10^-12 W/m^2, with an apparent visual magnitude of
eleven. This would not be visible to the unaided eye even if it was
not completely outshone by the second magnitude star.
This 39 dB Signal-To-Noise Ratio represents an SNR penalty
compared to the performance of a 10-meter heterodyning array receiving
telescope of about 34 dB. This 34 dB SNR penalty figure should not be
confused with the 34 dB CNR that was established in Table 2 (Page 22)
for a 1 kW transmitter. Starlight and daylight sky backgrounds only
slightly affect the SNR for this range, intensity, and optical
bandwidth. The effect of the 10 to 20 dB Fraunhofer Planckian
suppression factor has not been included in the graph of Figure 8;
allowance for which would improve the night sky performance for weaker
signals and/or larger optical bandwidths.
If a powerful ETI signal is detected, given an adequate SNR, it
might even be possible for an amateur observer to demodulate a signal
of moderate bandwidth, not just detect the presence of an excess
number of photons arriving in a given time! A photodetector bandwidth
of about 1 MHz would probably be desirable, and well as a spectrum
analyzer covering a similar frequency range.
As can be seen from Figure 8, the SNR is degraded by Planckian
starlight at low signal intensities and larger optical bandwidths. In
this regime, if the signal flux drops by 20 dB, the SNR falls by 40 dB
because the receiver is no longer signal quantum noise limited.
Clearly, if ETIs want their signals to be detected by relatively small
incoherent receivers, it pays to use pulses with low duty-cycle in
preference to C.W. signals. High peak EIRPs can override all external
and internal noise sources and thus make their signals as detectable
as possible for a given mean EIRP.
In Table 2 we showed that the 1 kW signal at a range of ten light
years produces a received intensity of 2.04 X 10^-17 W/m^2. If this
was received by a one-meter diameter incoherent adaptive ground-based
telescope, the normalized SNR in a 100 GHz (0.143 nm) optical bandwidth
(not allowing for Planckian dark line continuum suppression) would be
about -42 dB re 1 Hz. In this situation it would indeed help to
operate the transmitter within a Fraunhofer line. The SNR would be
increased to -32 dB re 1 Hz for a 10 dB Fraunhofer line contrast
factor. Either way, the presence of the signal would not be detectable
without considerable integration. However, if the ETI transmitter mean
power was increased to 1 GW, leading to a received intensity of
2.04 X 10^-11 W/m^2, the SNR would increase dramatically to about
Page 44
Postdetection Normalized SNR, dB re 1 Hz
|
|
80 |
|
| Ir = 10^-10 W/m^2 EIRP = 1.1 X 10^25 W (6th Magnitude)
60 |* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
|
| Ir = 10^-12 W/m^2 EIRP = 1.1 X 10^23 W (11th Magnitude)
40 |* * * * * * * * * * * * * * * * * * * * * * * * * * * *
| *
| Ir = 10^-14 W/m^2 EIRP = 1.1 X 10^21 W (16th Magnitude)
20 |* * * * * * * * * * * * * * * * * * * * * *
| *
| Ir = 10^-16 W/m^2 *
0 |.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*....................*.....
| * *
| Ir = 10^-18 W/m^2 *
-20 |* * * * * * * * * * * * * *
| * *
| Ir = 10^-20 W/m^2 * *
-40 |* * * * * * * * *
| * *
| * *
-60 | * *
| * *
| * *
-80 | Night Sky * *
| *
| *
-100 --------------------------------------------------------------
10^0 10^2 10^4 10^6 10^8 10^10 ^ 10^12
|
Optical Bandwidth, Hz |
100 GHz (0.143 nm)
Figure 8 -
Signal-to-noise ratio versus optical bandwidth for (perfect) Photon-
counting 656 nm receivers. Range = 10 light years, diameter = 30 cm,
antenna efficiency = 0.7, spectrometer efficiency = 0.5, quantum
efficiency = 0.5, excess avalanche gain noise factor = 0, dark
current = 0. EIRP of a solar-type star = 3.9 X 10^26 W. A
diffraction limited 10-meter diameter 1 GW transmitter produces an
EIRP = 2.3 X 10^24 W, and appears to be 0.6 percent of the brightness
of a second magnitude solar-type star.
63 dB re 1 Hz; essentially independent of Planckian background. This
signal would stick out like a proverbial sore thumb. In the case of
Professional Heterodyning Optical SETI, we were dealing with stronger
detected signals and a local-oscillator produced shot noise floor.
Because we are dealing here with smaller, incoherent receivers that use
an avalanche photodetector, the precise analysis for the CNR or BER is
Page 45
extremely complex when the received signal power is very small and/or
larger post-detection bandwidths are employed. The reader is
cautioned, that the above results may be somewhat optimistic.
Figure 8 forces us to consider whether such easily detectable
signals could have been missed by professional optical astronomers?
Perhaps, because there are so many stars and frequencies to search,
and with the limitations of conventional spectrographic equipment, we
can hope that these signals have been missed or overlooked. Again, if
the signals have low duty-cycle, the mean signal powers detected by
integrating detectors would be considerably less.
Scanning grating monochromators/spectrometers are available with
ten times the resolution previously quoted, i.e., 10 GHz (0.0143 nm)
optical bandwidths. High-Q Fabry-Perot spectrometers with bandwidths
as small as 1 MHz are perhaps less useful here because of their free-
spectral range and multiple response characteristics, requiring
additional broadband filtering. However, the tandem combination of a
scanning grating monochromator and a Fabry-Perot would form a very
powerful optical filtering and spectral analysis system, comparable in
many respects to what could be achieved with a heterodyne system.
For the thirty-centimeter diameter telescope system, ETI signal
detectability will not be substantially degraded for peak signal
strengths higher than about 10^-14 W/m^2 (sixteenth magnitude) if the
spectral resolution < 0.01 nm. If the EIRP was about 10^25 W, the
received signal flux would be at the threshold of unaided eye
visibility of about 10^-10 W/m^2, and yield an SNR of 60 dB re 1 Hz.
This would give an SNR = 30 dB in a 1 kHz post-detection bandwidth, or
a just detectable 0 dB in a 1 MHz bandwidth.
It would appear that as long as we can construct efficient photon-
counting receivers, that the sensitivity of small incoherent receiving
telescopes will not be unduly affected by the relatively large optical
bandwidths of such receivers, though their sensitivity will be degraded
if operated in daylight.
There was no particular reason in choosing the 656.2808 nm
(457.1214 THz) H_alpha line for the purposes of modelling the visible
system. While it could be considered a "magic wavelength", it does
not coincide with a known laser transition. It has an effective band-
width of about 280 GHz, though its half-power bandwidth is somewhat
smaller (Table 4, Page 30). A less expensive way of undertaking
Amateur Optical SETI observations at this single wavelength, instead of
using the more flexible scanning grating monochromator, would be to
employ a standard narrow-band H_alpha solar filter. To further reduce
costs, a photomultiplier could be used in place of the state-of-the-art
cooled avalanche (geiger-mode) photodetector.
It may be possible for amateur astronomy groups to "steal a march"
on NASA as far as the low-sensitivity search for ETI in the visible
and near-infrared spectrum is concerned. For Amateur Optical SETI
to be a sensible pursuit for the astronomical and space enthusiast
requires the belief that ETI technology would appear to emerging
Page 46
technical civilizations comparable to ourselves to be like "magic".
The demands placed on assumed ETI technical prowess are even greater
than when considering the practicality of Professional Optical SETI.
The onus would be on ETIs to make their signals easily detectable.
Since peak EIRPs > 10^23 W are thought possible, which lead to peak
intensities at a range of ten light years greater than 10^-12 W/m^2
(eleventh magnitude), the detectability of such signals with amateur
equipment is imaginable. Telescopes with apertures greater than about
one meter diameter are only slightly affected by daylight when observing
nearby stars, indicating that Daylight Professional/Semi-Professional
Optical SETI may be feasible for larger telescopes with incoherent
receivers. It should be realized that even during the day, the sky is
essentially black when viewed with artificial narrow bandwidth eyes!
It is not yet clear whether the 81-cm (32-inch) Perkins Telescope
in Delaware, could be upgraded with a precision-drive system that would
allow for satisfactory image-tracking during the night and day. Image-
tracking difficulties at night might be mitigated by using a photon-
counting array or image intensifier (or microchannel plate) instead of
a single photodetector. There are also some concerns, regarding the
effects on conventional astronomical nighttime observations, of thermal
currents caused by the observatory dome being open during the day.
Because optical bandwidths of these incoherent Amateur Optical SETI
receivers will be much wider than the effective optical bandwidths in
coherent Professional Optical SETI receivers, there is no concern for
anticipating or removing local line-of-sight Doppler chirps (drifts).
These chirps can be as high as 50 kHz/s (Table 2 and Equ. 40). Such
drifts are insignificant for optical bandwidths of the order of 100 GHz
in any reasonable amount of observation (dwell) time. Allowance should
be made for Doppler shifts of the ETI transmitter and Fraunhofer lines
when making a detailed search of specific frequencies, since these
shifts can be comparable to the width of a Fraunhofer line (Table 2
and Equ. 39). For specific laser frequencies not coinciding with
Fraunhofer lines, this requires knowledge of our line-of-sight velocity
relative to the star being observed. However, for transmissions and
observations within Fraunhofer lines, the receiver could simply be
tuned for minimum Planckian starlight noise. As before, it is assumed
that ETIs will remove their local line-of-sight transmitter Doppler
shift (and chirp) with respect to their star.
It should be noted for the record that thermoelectrically-cooled
CCD (Charged Coupled Device) cameras are now available to the amateur
which allow the sixteenth magnitude to be reached in under one minute
of integration, with negligible threshold effects. Even the fastest
photographic film has such low quantum efficiency that only a few
percent of the photons are converted to exposed film grains. The dark
current count for the photon-counter should ideally be kept below about
five hundred counts per second if the SNR of a potential ETI signal is
not to be excessively degraded. It may be reasonable to suggest that
eliciting the help of thousands of enthusiastic amateur optical
astronomers might considerably aid the low-sensitivity Targeted Search
of the entire Northern and Southern Hemisphere skies.
Page 47
HOW TO BUILD YOUR OWN AMATEUR OPTICAL SETI OBSERVATORY
How easy and cheap will it be for amateur astronomy organizations
to combine the efforts and resources of their members to participate in
this activity? The answer to this is that there is no hard figure. It
depends very much on how sophisticated and sensitive one is prepared to
be. There will always be tradeoffs between sensitivity and cost.
Figure 9 shows a basic Amateur Optical SETI system based on the use of
twenty-centimeter (eight-inch) or larger telescopes. While smaller
telescopes (reflectors or refractors) may be used, the potential
detectability of ETI signals will be degraded.
However much the reader may be excited by the statements made
herein, the reality of the situation is that SETI, be it conducted in
the microwave or optical spectrums, can become a rather monotonous
endeavor. It is an activity well-suited for automation. Hence, the
system to be described makes extensive use of computer-driven hardware.
The same computer can be used to analyze the spectral (optical and
electrical) data obtained with various signal processing algorithms to
see if there is a weak ETI signal hidden within the noise.
Particularly for an optical receiver with a wide tuning range,
i.e., one that uses a grating monochromator, the mass of the
additional equipment required to be attached would be excessive for a
small telescope. Hence, the preferred way to couple the SETI receiver
to the telescope would be via several meters of a single strand of
low-loss multimode optical fiber. The output face of the fiber-optic
umbilical replaces the slit normally found in a monochromator/
spectrograph. This approach is additionally useful if cryogenic
cooling techniques have to be employed at the optical front-end.
The optical fiber is positioned to be centrally placed in the focal
plane and the fiber input arranged by suitable imaging, i.e., SELFOC
lens (GRIN rod), to match to the telescope's diffraction limited spot
size (Airy disk). In practice, if daylight SETI is not attempted, the
optical fiber's aperture and FOV may be increased to accommodate image
wander caused by typical atmospheric turbulence conditions. The
diagram shows a beamsplitter sharing the image with the CCD, though the
CCD might make use of off-axis guiding to avoid light loss, i.e., for
locking onto a guide star. The graded-index lens also serves to
convert the focal ratio of the telescope to one that matches the fiber
for maximum throughput, this operation being equivalent to matching
numerical apertures. Some mode scrambling may be required to ensure
that the output numerical aperture (N.A.) of the fiber is fully
illuminated at all times, whatever the light launching conditions.
This ensures that amplitude fluctuations do not occur in the slitless
monochromator or spectrometer as the image of the star and transmitter
dances around the entrance (input end) of the fiber.
Multimode optical fiber essentially depolarizes light, so that any
polarization analysis equipment must be situated at the input, focal
plane end of the fiber. There will be an inherent throughput loss of
about 50 percent in the monochromator because high resolution
diffraction gratings have a tendency to polarize light.
Page 48
-------------------- Beamsplitter/Off-Axis Guiding CCD Imaging/
| 8" - 14" |-- _ Tracking Camera------------->--------------
| | |_|->- |
| Schmidt-Cassegrain |-- | Optional Polarizing Optics & Multimode |
-------------------- | Fiber-Optic Umbilical in Focal Plane |
| | | |
| | | ----------------- ----- |
--------- | | Scanning | | APD | |
| Drive |<>- ->>| Grating |->-| or |->- |
--------- | | Monochromator | | PM | | |
| ----------------- ----- | |
| ^ | |
| | ----- |
| | | Amp | |
| | | | |
| | ----- |
| | | |
----------- | | ------------- | |
| | | | | Optional | | |
| VDT | | | | Spectrum |<----| |
| | | | | Analyzer | | |
----------- | | ------------- | |
| | | RS-232/IEEE-488 | | |
------------- <>------------<>---------- Baseband Signal | |
| PC |<-------------<----------------------<------------| |
------------- <----- ------- | |
Optional | CCD Video | Low | | |
FFT Spectrum | Audio <---| Pass |<-------| |
Analyzer Card | | Filter| | |
| ------- | |
| ----------- | |
| | Video |<-------------------- |
| | Monitor |<-------------------------|
| | Or TV | CCD Video |
| ----------- |
| |
------------------------------------------------
Figure 9 -
Basic Amateur Optical SETI or Poor Man's Optical SETI. Only a single
photodetector is used, which can be either an avalanche photodiode
(APD) or a photomultiplier (PM). The optical filter can be a computer-
controlled scanning monochromator or a relatively inexpensive fixed
interference filter. Additional focal-plane optical fibers and photo-
detectors may be employed for maintaining star-lock. An electronic
mixer and filter may be included between the photon-counting receiver
and the display/audio devices to beat down the detected spectrum to
lower frequencies. This electrical local-oscillator would likely be
driven by the PC. The TV (video) monitor can be used both to display
the star field via the CCD imaging/tracking camera and the detected
signal, or these could be displayed on the PC. Later, several
telescopes could be slaved together to increase light gathering power,
sensitivity, and SNR of a would-be ETI signal.
Page 49
The output of the fiber is expanded and collimated in the usual way.
However, if a single photodetector is employed, as indicated in
Figure 9, some form of cylindrical output lens will be required to
match the aspect ratio of the beam from the diffraction grating(s) to
the photodetector. For this reason, some investigators may prefer to
use a photomultiplier with a large cathode to collect all the photons.
As this document was nearing completion, the author's attention was
drawn to a recent report by Douglas et al [93] on an astronomical
heterodyned spectrometer. The title of the report is somewhat
misleading as this author feels that the word "homodyned" would have
been more applicable. Unless fringes actually move across a photo-
detector at an interference beat rate, a system cannot be said to
really employ heterodyne techniques. However, the report does describe
a high resolution spectrometer using a fiber-optic umbilical, and in
that respect is relevant to the discussion here.
In Figure 9, the purpose of the conventional CCD is just to display
the star field on a television (TV) or personal computer (PC) monitor
and for precision star tracking. In this preferred design, it does
not detect the ETI signal; that job is performed by a relatively fast
single solid-state Avalanche photodetector (APD) or photomultiplier
(PM). APDs have the advantage of high quantum efficiency but the
disadvantage of higher dark current; the converse being the case for
photomultipliers. With state-of-the-art solid-state photodetectors
like the RCA SPCM-100-PQ Single Photon-Counting Module, the cooling to
reduce dark current noise is applied via Peltier (thermoelectric)
coolers, and their mass is relatively insignificant. Though the
imaging CCD can itself be used as the ETI detector, this approach might
compromise detection sensitivity and bandwidth. It would also require
a very high-quality and expensive CCD array. This would be incompa-
tible with the use of the device for star field imaging and fine
guidance because of the narrow-band optical filtering requirements of
the SETI receiver. The input end of the fiber-optic umbilical might be
dithered in the focal plane to aid guidance, and to ensure fine
dynamic-tracking on a star's image. Indeed, four additional optical
fibers with unfiltered photodetectors might surround the ETI-detecting
fiber and be used for this purpose.
Note that the audio monitor in the schematic is for listening to
the hiss of stellar noise and perhaps audibly detecting the presence of
a strong artificial signal. The Planckian background in a 100 GHz
optical bandwidth for a 2nd Magnitude star, produces a photon-count
rate of about 18,000 s^-1, which should be compared to the dark-current
count rate for a high-quality cooled photodetector or photomultiplier
of less than several hundred counts per second. An essential component
will be a variable threshold detector connected to an alarm system.
The TV or PC monitor could also serve to display a noisy raster and the
presence of any coherent signals. It is unlikely though, that an ETI TV
picture will pop up (in any TV standard), considering the deficiencies
in SNR and bandwidth with amateur receivers! However, if high SNR and
bandwidth can be supported by ETI transmitters and terrene professional
receivers over interstellar distances, a sequentially scanned TV [36]
picture would be the most effective bridge between our two cultures.
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