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Glossary of Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z #

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.

- A -

Adaptive Optics

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.


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- B -


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.


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- C -

Continuous Wave

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.

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- D -

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.


Diffraction Limited

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.


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- E -


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 transmitter power.


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- F -

Fraunhofer Lines

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.
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- G -


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!

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- H -

Heterodyne Detection

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.

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- I -


A system containing two or more signals that are combined in phase to cause interference.


Interstellar Dispersion

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 bandwidth.


Interstellar Scintillation

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.


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- J -


A basic unit of energy.  A 1 Watt transmitter radiates 1 Joule of energy every second.
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- K -

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- L -


Light Amplification by the Stimulated Emission of Radiation.  Lasers can operate in the infrared, visible and ultraviolet regions of the optical spectrum.


Laser Pollution

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.


Light Bucket

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.


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- M -

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.



Microwave SETI


Multichannel Scaler

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.


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- N -

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- O -


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

See "Photonics".



Optical SETI. Includes, far-infrared, infrared, near-infrared, visible and ultra-violet parts of the electromagnetic spectrum.


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- P -

Phased-Array Receiver

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 "interferometer".


Phased-Array Transmitter

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%.


Pulse Modulation

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 bit positions.


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- Q -

Quantum Efficiency

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 a PMT.

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- R -

       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.


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- S -


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.


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- T -

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- V -

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- W -

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 per second.

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