Select the first letter of the word from the list above to jump to appropriate section
of the glossary. If the term you are looking for starts with a digit or symbol,
choose the '#' link.
Recent advances in deformable mirror technology and laser guide stars allows most of
the distortion produced by the atmosphere to be removed. This results in near
diffraction-limited performance of ground-based receiving telescopes, giving image quality
similar to that produced by the Hubble Space Telescope.
Avalanche Photodetector. The solid-state equivalent of a PMT, usually restricted
to operation in the red, near-infrared and infrared part of the spectrum. For
photon-counting applications, they are operated in the so-called "Geiger Mode".
The device is noisier than a PMT but has greater quantum efficiency. Maximum quantum
efficiencies are about 80%.
Advanced Technical Civilization. A civilization capable of communicating across
the galaxy or even greater feats of technology.
Most SETI research is concerned with the detection of an attention-getting
"beacon" or acquisition signal, not a wideband data channel.
A bin has a bandpass filter response with certain frequency bandwidth. A spectral
analysis requires the signal to be divided into a large number of bins. The signal
power spectral density at each bin frequency is thereby measured.
Continuous Wave (CW) transmission is when there is an electromagnetic carrier wave
present and the message signal, be it voice, video or data, is impressed upon the carrier
by modulating its amplitude, phase or frequency. It is the system employed for
terrestrial radio and TV transmissions. When the wave is unmodulated the signal is
highly monochromatic. The average power in the modulated wave is approximately that
in the unmodulated carrier component.
Daylight Optical SETI
Optical SETI observations can be done during the day under a clear blue sky with large
professional telescopes. See also Symbiotic Optical SETI.
Communication engineers like to express large power ratios in logarithmic decibels
(dB). Every factor of 10 in magnitude is a ratio of 10 dB. Thus, an antenna
gain of 1015 has a corresponding logarithmic gain of 150 dB. The gain is
the product of 10 and the logarithm (to base 10) of the ratio.
This is decibels with respect to 1 watt of power.
This is decibels with respect to 1 mW of power. +30 dBm = 0 dBW.
Electromagnetic waves diffract around the edges of opaque objects or on passing through
or reflecting off a finite aperture, like a dish, lens or mirror. Even if such a
wave is perfectly collimated, so that the beam emitted is parallel, it will eventually
spread out. The larger the aperture in relation to the wavelength, the smaller is
this beam spread. An electromagnetic beam is said to be "diffraction
limited", when it is so collimated that it cannot be made more narrow in the
near-field. Receivers are also said to be "diffraction limited" when the
energy is focused into the smallest possible spot size at the focal plane. An
optical receiver that is not "diffraction limited" may be said to be a
"light bucket" - it collects photons, but cannot concentrate them into a very
small tightly focused spot. See "light bucket" below.
The Effective Isotropic Radiated Power (EIRP) of a transmitter (uplink) is the power
that the transmitter appears to have if the transmitter was an isotropic radiator, i.e.,
if it radiated equally in all directions. By virtue of the Gain of a radio antenna,
dish, radio telescope or optical telescope, a beam is formed that preferentially transmits
the energy in one direction. The EIRP is given by the product of the Gain and the
- Fraunhofer was the inventor of the diffraction grating. He assigned letters to the
black absorption lines in the Solar Spectrum. These resonance lines arise from
energy absorption by elements in the outer atmospheres of stars that are cooler than the
stellar gases below.
A measure of the increase in forward transmitter power of an antenna or telescope with
respect to that of an isotropic radiator. An antenna is said to have a
"directivity" since it directs more energy in one direction. It is
proportional to the aperture area of the telescope and inversely proportional to the
square of the wavelength. Even small optical telescopes can have very high gain,
since the wavelength is so small. Typically a 10 meter diameter visible telescope
has a gain of about 1015 (150 dB), while a 100 meter diameter microwave dish
operating at 10 GHz has a gain of only 108 (80 dB). This is a difference of a
factor of 107 (70 dB), i.e., 10 million times. No wonder then, that
optical uplinks are vastly superior to their microwave counterparts!
An interferometer in which the two signals differ in frequency and cause a beat
frequency to be produced. Usually, one signal is a powerful local oscillator.
A system containing two or more signals that are combined in phase to cause
The velocity of propagation of electromagnetic waves depends upon frequency.
Thus, unless the wave is highly monochromatic, the energy in the wave arrives at the
receiver at differing times. This can severely limit the modulation bandwidth, since
the process of modulating data onto an electromagnetic carrier wave broadens its
Electromagnetic waves as they propagate though the interstellar media suffer
frequency-selective fading. The resulting corrupted signal is said to undergo
scintillation effects. The higher the modulation bandwidth the worse the effect.
- A basic unit of energy. A 1 Watt transmitter radiates 1 Joule of energy every
Light Amplification by the Stimulated Emission of Radiation. Lasers can operate
in the infrared, visible and ultraviolet regions of the optical spectrum.
A phenomenon (defined here for the first time) similar to radio interference but
unlikely to become a significant problem for a very long time. Presently, there is a
rush to undertake as much Microwave SETI observations as possible before the sky becomes
too polluted with orbiting satellites that will interfere with the reception of any radio
signals of extraterrestrial origin. Because of the immaturity of terrestrial
free-space laser communications and the high directivity of laser transmitters and
receivers, optical interference from lasers of terrestrial origin with Optical SETI
observations is not likely to be a problem for the foreseeable future.
An optical receiving telescope that collects photons but is not diffraction limited.
Its effective field-of-view is larger than a diffraction limited telescope and
cannot discriminate so well from background noise photons. This may not be a problem
if the expected pulsed laser beacon signal is very strong.
Mach-Zehnder Interferometer (MZI)
The most common form of interferometer consisting of a fiber optic or
integrated optic device that contains an optical power splitter and
combiner. There are two distinct optical paths between the power
splitter and combiner. Optical interference takes place at the power
combiner, so that phase modulation introduced into either of the two arms or
paths in the interferometer, produces phase to intensity conversion at the
output of the interferometer.
An instrument with a large number of channels for analyzing a signal in small time
slots. Used for nuclear, particle and photon counting.
Multichannel Spectrum Analyzer (MCSA)
An instrument with a large number of channels for analyzing a signal in small frequency
bins. Used for rapid radio frequency analysis.
That part of the electromagnetic spectrum covering the spectral range from the
far-infrared to the ultra-violet. It is a superset of the visible
and infrared spectral regimes.
Optoelectronics or Optronics
Optical SETI. Includes, far-infrared, infrared, near-infrared, visible and ultra-violet
parts of the electromagnetic spectrum.
A collection of spaced antennas, dishes, mirrors or lenses that when the individuals
signals collected are properly summed in the correct phase, can act in unison with the
diffraction-limited beam performance of a single element as large as the extreme
dimensions of the array. This is also sometimes referred to as an
A collection of spaced antennas, dishes, mirrors or lenses that when properly excited
in the correct phase, can act in unison with the diffraction-limited beam performance of a
single element as large as the extreme dimensions of the array.
A particle with energy equal to a single quanta. The energy of the particle is
proportional to frequency. Electromagnetic energy may sometimes be considered to be
wave-like, and at other times, particle-like. At frequencies above the
millimeter-wave band, the energy is more often considered to be carried by photons, rather
than by waves. The energy in a photon is given by the product h x f, where h is
Planck's constant (6.63 x 10-34 J.s) and f is the frequency. Usually,
radio-frequency energy is not described in terms of photons due to the fact that even for
very low intensities, the number of radio-frequency photons arriving per second is large
and receiver system noise is dominated by the thermal kT component, where k is Boltzmann's
constant (1.38 x 10-23 J/K) and T is the temperature in degrees K. Since
optical frequencies are some five orders of magnitude higher than microwave frequencies,
each optical photon is far more energetic than its microwave counterpart. Optical
receiver noise is often dominated by the quantum hf component.
The electronic technology involving light and photons at all wavelengths between the
far-infrared and the ultra-violet. Also called "Optoelectronics".
Photomultiplier Tube. A low-noise vacuum device requiring high voltages to
accelerate electrons and producing very high gains. Can detect individual photons.
One detected photon on the photo-cathode can produce a very short pulse containing
millions of photoelectrons. PMTs usually operate between the blue and red regions of
the visible spectrum, with greater quantum efficiency in the blue-green region, depending
upon photo-cathode materials. Maximum quantum efficiencies are about 25%.
A method akin to digital modulation where the intensity of a carrier is modulated
between two states; either maximum or zero. If the pulse width is small and the
repetition rate slow, the peak power in the pulse can be vastly above that of the mean
power. For instance, if a pulse of 1 nanosecond (10-9 s) duration is
transmitted every second, then the peak power is 1 billion times or 109 (90 dB)
above the mean power. It is very easy for this "lighthouse" laser beacon
to outshine a star during each brief flash! During each flash, the rate at which
photons arrive at the receiver is much higher than from the stellar or sky backgrounds.
For the example given, this ratio can be as high as 10 million times!
Pulse Position Modulation (PPM)
A means of encoding digital data by varying the position of a bit in a word that has M
The ratio of hole-electron pairs or photoelectrons to the number of photons received by
a photodetector. The quantum efficiency can be as high as 80% for an APD and 25% for
Retrospective Optical SETI
Looking through the historical record of stellar spectrographic plates in an attempt to
find anomalous spectral lines that may signify a laser beacon signal.
The Search for Extraterrestrial Intelligence. This is a "passive"
search in that we are listening for signals. This does not presently involve the
"active" means of transmitting, be it in the microwave or optical regime.
If we transmit such a signal, the activity would then be called "CETI" -
Communications with Extraterrestrial Intelligence. Modern SETI describes the Electromagnetic
Search for Extraterrestrial Intelligence. It should make no preconceived
assumptions about what constitutes the best part of the electromagnetic spectrum for such
means of signaling.
Symbiotic Optical SETI
Optical SETI Observatory facilities that are shared with conventional telescopes.
While conventional astronomy is done at night, Optical SETI may be done both at
night and during the day. See Daylight Optical SETI. An Optical Serendip
approach may also be possible.
Wall Plug Efficiency
The ratio of the transmitter output power, be it microwave or optical, to the
(electrical) power consumed by the transmitter.
A basic unit of power corresponding to the production of energy at the rate of 1 Joule