Glossary of Terms and Astronomy Acronyms

The Glossary lists the description of some of the technical terms you hear or see in this hobby, as well as a list of Astronomy Acronyms and modern constellation abbreviations.

Hopefully it will help you understand what these words and abbreviations mean and how they are being used. Use your browsers search function "Ctrl-F" to find words or acronyms on this page.

If you have any suggestions of words to add, or notice any errors in the glossary, please contact me and let me know.

Many thanks to IceInSpace Forum members ballaratdragons and Geoff White for their contributions to this list.

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· Astronomy Acronyms
· Modern Constellation Abbreviations


Achromatic lens
A lens with two or more elements designed to produce an image substantially free from false colour.

Airy Disk
The Airy disk is the brightest spot formed by a star image as seen through a telescope.  It is surrounded by alternating rings of light and dark (these are due to diffraction - any light passing through an aperture is diffracted, and the effect is inversely proportional to the size of the aperture.)  An optical system of good quality increases the relative brightness of the central Airy disk compared to the surrounding diffraction rings.  (Defocusing a star image will accentuate the diffraction rings and is sometimes useful for assessing a telescope’s optics.).

The reflective property of a non-luminous object. A perfect mirror would have an albedo of 100% while a black hole would have an albedo of 0%.

The angular distance of an object above the horizon.

The process of coating a telescope mirror with a thin layer of vaporised aluminium.

The size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number represents a smaller opening while a lower number represents a larger opening.

See Apochromatic.

A lens system that brings all three primary colours to the same point of focus, and is thus considered free of chromatic aberration.   (Some consider that a true apochromatic lens ought to eliminate spherical aberration also.).

Apparent Field of View (AFOV)
The angular size of the field as seen through a particular eyepiece without a telescope.  (Varies with different designs, and is often specified on the eyepiece.  Roughly, Orthoscopics 40 degrees; Plossls 50 degrees; Superwides 60-70 degrees; Ultrawides 70+ degrees.).

Apparent Magnitude
The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude.

A small planetary body in orbit around the Sun, larger than a meteoroid but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets.

The distortion of an image resulting from a failure of light rays from different parts of the same zone to focus in the same plane.

Astronomical Unit (AU)
A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.

A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases.

A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth's northern hemisphere and the Aurora A

Aurora Australis
Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Borealis in the northern hemisphere and Aurora Australis in the Earth's Southern Hemisphere.

Aurora Borealis
Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field. Known as the Aurora Australis in the southern hemisphere.

Also known as the poles, this is an imaginary line through the center of rotation of an object.

The angular distance of an object around or parallel to the horizon from a predefined zero point.

The name given to a telescope mount that is movable in Altitude and Azimuth.



Barlow lens
An intermediate concave lens place in front of the eyepiece of a telescope to increase magnifaction.

A system of two stars that revolve around a common centre of gravity.

Black Hole
The collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The gravitational pull is so strong that not even light can escape.


A telescope that uses a system incorporating both lenses and mirrors to form an image. Common types: Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (MCT, mak) telescopes.

Celestial Equator
An imaginary line that divides the celestial sphere into a northern and southern hemisphere.

Celestial Poles
The North and South poles of the celestial sphere.

Celestial Sphere
An imaginary sphere around the Earth on which the stars and planets appear to be positioned.

Cepheid Variable
This is a variable star whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter cepheids will have a longer period.

A distinctive area of broken terrain.

The part of the Sun's atmosphere just above the surface.

The term used for adjusting a telescope to gain maximum optical performance by means of alignment of the optical parts.

An area of dust or gas surrounding the nucleus of a comet.

A gigantic ball of ice and rock that orbit the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun's rays.

An event that occurs when two or more celestial objects appear close together in the sky.

A grouping of stars that make an imaginary picture in the sky.

The outer part of the Sun's atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen in most solar eclipse photos.

Cosmic Ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.

Cosmic String
A tubelike configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.

The study of celestial systems, including the solar system, stars, galaxies, and galactic clusters.

A branch of science that deals with studying the origin, structure, and nature of the universe.

A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.


Dark Matter
A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

The angular distance of an object in the sky from the celestial equator.

The amount of matter contained within a given volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The density of water is 1.0, iron is 7.9, and lead is 11.3.

The surface of the Sun or other celestial body projected against the sky.

Dobsonian mounted reflector. A simple alt/az mounted scope that is easy to use and offers very good value for money.

Double Star
A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.


The total or partial blocking of one celestial body by another.

An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.

ED (Extra-low Dispersion)
ED glass is a type of glass that reduces the dispersion of light into its different wavelengths, allowing for better chromatic aberration correction in refractor.

Electromagnetic Radiation
Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.

Electromagnetic Spectrum
The full range of frequencies, from radio waves to gamma waves, that characterizes light.

An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.

Elliptical Galaxy
A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.

The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen in the evening sky and a planet at greatest western elongation will be seen in the morning sky.

A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects.

An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years

Equatorial Mount (EQ)
A telescope mount with one axis parallel to the earth’s polar axis. This provides easy tracking of celestial objects, and is preferred by many for astrophotography.

The two points at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21 and September 22.  The equinoxes signal the start of the Spring and Autumn seasons.

Event Horizon
The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light.

Evolved Star
A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in the form of stellar wind.

The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere.

A term that means outside of or beyond our own galaxy.

A term used to describe anything that does not originate on Earth.

The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument.  Eyepieces are available in different types and powers, yielding differing amounts of magnification and field of view.

Telescope eyepieces

The following eyepiece chart created by Ted Kurkowski



An early 20th-century, 5-element eyepiece design that performs reasonably well at low-power focal lengths.  Although Panoptics and Naglers are more highly regarded,  an Erfle can provide a decent wide-field view at a much lower price.




A 3-element eyepiece design that can perform reasonably well at low-power (wide-view) focal lengths.  Although this is a very old eyepiece design it is often sold at reasonable prices, under the “modernized” name Super Modified Achromat (SMA).




A 5-element eyepiece design that provides a wider FOV than orthoscopic eyepieces, without sacrificing very much contrast.  The highly regarded Takahashi and Celestron Ultima eyepieces use this design.




A premium eyepiece invented by Al Nagler (an extremely well-regarded optical designer formerly with NASA) and sold by his company TeleVue.  Nagler eyepieces deliver the widest FOV of any eyepiece sold, and provide a wonderful field of view of DSOs (at a high price).  However, at high-power focal lengths the 6 lens elements of this eyepiece design will tend to reduce contrast, so other designs with fewer lens elements tend to be preferred by planetary observers.




Orthoscopic.  A 4-element eyepiece design, sometimes referred to as an Abbe design after its inventor, the great German optical designer Ernst Abbe who created the design in 1880 (and subsequently became a partner in the Carl Zeiss Optical Works).  In high-power focal lengths an ortho provides an excellent view for planetary observing, at a reasonable price.




Panoptic.  An eyepiece design invented by the extremely well-regarded optical designer Al Nagler and sold by his company TeleVue.  Panoptics have a somewhat lesser FOV than do Nagler eyepieces, but still provide a very wide FOV.  They provide a terrific view of DSOs, at a lower price than comparable Naglers and are regarded as eyepieces that have a very high value to price ratio for wide-field observing.  But they still are expensive eyepieces – fortunately you don’t need very many of them to enjoy observing DSOs.




A manufacturer of very high quality (and expensive) eyepieces whose design works particularly well for medium-power to medium-high-power views.  In that range, personally I absolutely love these eyepieces, but note that eyepiece preference is a personal thing that varies depending on the idiosyncrasies of any particular person’s eyesight.




A 4-element eyepiece design invented by the Austrian optician Simon Plössl in the 19th century, that provides a somewhat wider FOV than does an ortho.  Unlike orthos, the design of Plössls has been modified and improved by manufacturers to the point that it now provides excellent views at both long and short focal lengths.  The design provides good value for its price and has thus become the most popular general-eyepiece design among folks new to observing, or observers on a more limited budget.   Nevertheless at any specific short, medium, or long focal length there are better specific eyepiece designs available (albeit at generally higher prices) so in the long run experienced observers usually end up not using many Plössls (in much the same way that experienced mechanics have acquired specific wrenches for specific purposes and end up tending not to use adjustable wrenches very often.




Super Modified Achromat - a fancy term for an eyepiece using the Kellner optical design.



Eye relief
The distance the eye must be placed from the eyepiece to obtain sharpest vision.


Field of View (FOV)
The area visible through an optical device (the angular separation between opposite edges of the visual field).

A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun.

A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location.

An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom.


A large grouping of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth.

Galilean Moons
The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.

The highest energy, shortest wavelength form of electromagnetic radiation.

Geosynchronous Orbit
An orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet's surface.

German Equatorial Mount (GEM)
A common type of equatorial mount.  As opposed to an English equatorial mount, here the OTA is offset from the centre of the mount and is balanced by means of counterweights on the other side and an interconnecting shaft.  The shaft rotates at 90 degrees to the mount’s polar axis (i.e. the shaft points constantly at 0 degrees declination but can move through all hours of right ascension), and the OTA can rotate about the shaft’s axis so as to point at any angle of declination..

Giant Molecular Cloud (GMC)
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation.

Globular Cluster
A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found around the central regions of a galaxy.

GRS (Great Red Spot)
A large high pressure storm system on Jupiter akin to a cyclone on earth. It is almost three times the diameter of the earth, produces winds of up to 400km/h and has been raging for at least 300 years. It transits the face of Jupiter every 8 hours.

GSO (Guan Sheng Optical)
Maker of telescopes in Taiwan, many of which are re-branded as "Bintel", "Southern Cross", "SkyWatcher", etc, particularly the dobsonians.

A mutual physical force of nature that causes two bodies to attract each other.


Hubble's Law
The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.


A term used to describe water or a number of gases such as methane or ammonia when in a solid state.

A measure of the tilt of a planet's orbital plane in relation to that of the Earth.

Inferior Conjunction
A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.

Inferior Planet
A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.

Interstellar Medium
The gas and dust that exists in open space between the stars.

Irregular Galaxy
A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.


A temperature scale used in sciences such as astronomy to measure extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Kepler's First Law
A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's Second Law
A ray directed from the Sun to a planet sweeps out equal areas in equal times.

Kepler's Third Law
The square of the period of a planet's orbit is proportional to the cube of that planet's semimajor axis; the constant of proportionality is the same for all planets.

Kuiper Belt
A large ring of icy, primitive objects beyond the orbit of Neptune. Kupier Belt objects are believed to be remnants of the original material that formed the solar system. Some astronomers believe Pluto and Charon are Kuiper Belt objects.


Lenticular Galaxy
A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.

Light Year
An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.

The outer edge or border of a planet or other celestial body.

Local Group
A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.

The amount of light emitted by a star.

Lunar Eclipse
A phenomenon that occurs when the Moon passes into the shadow of the Earth.  A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow.  In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.

Lunar Month
The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.

The interval of a complete lunar cycle, between one new Moon and the next.  A lunation is equal to 29 days, 12 hours, and 44 minutes.


Magellanic Clouds
Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic clouds are visible in the skies of the southern hemisphere.

Magnetic Field
A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

Magnetic Pole
Either of two limited regions in a magnet at which the magnet's field is most intense.

A measurable increase in the apparent size of an object.  Magnification = (focal length of a telescope) / (focal length of the eyepiece).

The degree of brightness of a star or other object in the sky according to a scale on which the brightest star has a magnitude -1.4 and the faintest visible star has magnitude 6. Sometimes referred to as apparent magnitude. In this scale, each number is 2.5 times the brightness of the previous number. Thus a star with a magnitude of 1 is 100 times brighter than on with a visual magnitude of 6.

Main Belt
The area between Mars and Jupiter where most of the asteroids in our solar system are found.

Major Planet
A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major planets.

A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.

A word used to describe anything that contains mass.

An imaginary circle drawn through the North and South poles of the celestial equator.

A small particle of rock or dust that burns away in the Earth's atmosphere.  Meteors are also referred to as shooting stars.

Meteor Shower
An event where a large number of meteors enter the Earth's atmosphere from the same direction in space at nearly the same time.  Most meteor showers take place when the Earth passes through the debris left behind by a comet.

An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth's surface. Meteors become meteorites if they reach the ground.

Minor Planet
Another name used to describe a large asteroid.


A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of.

Neutron Star
A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars.

A star that flares up to several times its original brightness for some time before returning to its original state.


An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.

Oort Cloud
A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. The oort cloud was named after the Dutch astronomer who first proposed it.

Open Cluster
A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.

The position of a planet when it is exactly opposite the Sun as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.

OTA (Optical Tube Assembly)
The tube that holds your mirrors and/or lenses.

The path of a celestial body as it moves through space.


A large distance often used in astronomy.  A parsec is equal to 3.26 light years.

The point in the orbit of the Moon or other satellite at which it is closest to the Earth.

The point in the orbit of a planet or other body where it is closest to the Sun.

The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.

The bright visible surface of the Sun.

A very large body in orbit around a star. Planets can be composed mainly of rock or of dense gases.

Planetary Nebula
A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colours and shapes.

An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.

Dense regions of molecular clouds where stars are forming.

A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates.

A Trade Name for a glass having a low coefficient of expansion due to high silica content.


A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.

An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active galaxies.


Red Giant
A stage in the evolution of a star when the fuel begins to exhaust and the star expands to about fifty times its normal size. The temperature cools, which gives the star a reddish appearance.

Reflector (reflecting telescope)
A telescope that uses a concave mirror to gather light and form an image at a focal plane.

Refractor (refracting telescope)
A telescope that uses a transparent objective lens to refract, or bend, light in order to form an image at the focal plane.

Retrograde Motion
The phenomenon where a celestial body appears to slow down, stop, them move in the opposite direction. This motion is caused when the Earth overtakes the body in its orbit.

Right Ascension
The amount of time that passes between the rising of Aries and another celestial object. Right ascension is one unit of measure for locating an object in the sky.

Ring Galaxy
A galaxy that has a ring-like appearance. The ring usually contains luminous blue stars. Ring galaxies are believed to have been formed by collisions with other galaxies.

The spin of a body about its axis.


Saber's Beads
Lunar phenomenon seen on extremely young and old crescents. The striking resemblance to 2nd and 3rd contacts during a total solar eclipse was first noted by American amateur astronomer Stephen Saber.

A natural or artificial body in orbit around a planet.

Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.

Sidereal Month
The average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

Sidereal Period
The period of revolution of a planet around the Sun or a satellite around its primary.

The name given to our sun.

Solar Cycle
The approximately 11-year quasi-periodic variation in frequency or number of solar active events.

Solar Eclipse
A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is close enough to completely block the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is not able to completely block the light. This results in a ring of light around the Moon.

Solar Flare
A bright eruption of hot gas in the Sun's photosphere.  Solar prominences are usually only detectable by specialized instruments but can be visible during a total solar eclipse.

Solar Wind
A flow of charged particles that travels from the Sun out into the solar system.

The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the Summer and Winter seasons.

The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.

Grass-like patterns of gas seen in the atmosphere of the Sun.

Spiral Galaxy
A galaxy that contains a prominent central bulge and luminous arms of gas , dust, and young stars that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.

A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.

Star Cluster
A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.

Stellar Wind
The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A star's stellar wind is strongest near the end of its life when it has consumed most of its fuel.

Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field.

The stage in a star's evolution where the core contracts and the star swells to about five hundreds times its original size. The star's temperature drops, giving it a red color.

Superior Conjunction
A conjunction that occurs when a superior planet passes behind the Sun and is on the opposite side of the Sun from the Earth.

Superior Planet
A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and Venus. These two planets are inferior planets.

A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life.  Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions.

Supernova Remnant
An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble "bubbles" in space.


An instrument used to collect large amounts of light from far away objects and increase their visibility to the naked eye. Telescopes can also enlarge objects that are relatively close to the Earth or on the Earth.

The boundary between the light side and the dark side of a planet or other body.

A term used to describe anything originating on the planet Earth.

Terrestrial Planet
A name given to a planet composed mainly of rock and iron, similar to that of Earth.

Tidal Force
The differential gravitational pull exerted on any extended body within the gravitational field of another body.

The passage of a celestial body across an observer's meridian; also the passage of a celestial body across the disk of a larger one.

An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points. Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans.

True Field of View (TFOV)
The angular size (in degrees) of the actual area of the sky that you can view through a particular telescope with a particular eyepiece.  (The TFOV depends on both, and is calculated by dividing the AFOV by the Magnification.)


The area of total darkness in the shadow caused by an eclipse.

Universal Time (UT)
Also known as Greenwich Mean Time, this is local time on the Greenwich meridian. Universal time is used by astronomers as a standard measure of time.


Van Allen Belts
Radiation zones of charged particles that surround the Earth. The shape of the Van Allen belts is determined by the Earth's magnetic field.

Variable Star
A star that fluctuates in brightness. These include eclipsing binaries.

In photography and optics, vignetting refers to a reduction in image brightness in the image periphery compared to the image center.

Visible Light
Wavelengths of electromagnetic radiation that are visible to the human eye.

Virgo Cluster
A gigantic cluster of over 2000 galaxies that is located mainly within the constellation of Virgo. This cluster is located about 60 million light years from Earth.

Visual Magnitude
A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the visible light from the object. On this scale, bright objects have a lower number than dim objects.


The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.

White Dwarf
A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its center.


Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays.

X-ray Astronomy
The field of astronomy that studies celestial objects by the x-rays the emit.

X-ray Star
A bright celestial object that gives off x-rays as a major portion of its radiation.


Yellow Dwarf
An ordinary star such as the Sun at a stable point in its evolution.

Yoke mounting
A form of equatorial mounting in which the polar axis consists of a yoke in which the telescope tube is mounted.


A point directly overhead from an observer.

An imaginary belt across the sky in which the Sun, Moon, and all of the planets can always be found.

Zodiacal Light
A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise.  Zodiacal light is caused by sunlight reflecting off small particles of material in the plane of the solar system.

A definite area on the mirror, measured from the centre.


Astronomy Acronyms


A/D Analogue to digital
ACF Advanced Coma Free (type of Meade telescope optics)
ADU Analogue to Digital Unit
AFOV Apparent field of view
AIP Astronomical image processing
ALT/AZ Altitude-azimuth (type of telescope mount)
AO Adaptive optics - explanation
APO Apochromatic
APOD Astronomy Picture of the Day (NASA website)
ASCOM Astronomy Common Object Module (set of common mount interface standards)
ASV Astronomical Society of Victoria
ATM Amateur telescope making (or manufacture)
ATS Advanced Telescope Supplies (in Sydney)
AU Astronomical Unit (measure of interplanetary distances)
AVI Audio video interleaved (file format)
BMP Bitmap (a picture file type)
BNIB Brand New In (unopened) Box
bps bits per second (data transfer rate)
CA Chromatic Abberation
CAT Catadioptric
CCD Charge-coupled device
CDC Cartes du Ciel (skychart software)
CG Centre of gravity
CGE Heavy duty computerized GEM (q.v.) by Celestron
CMOS Complementary metal oxide semiconductor
DEC Declination - explanation
DMK Daten Mono Kamera (catalogue identifier of ImagingSource GmbH)
DOB Dobsonian telescope (not really an acronym)
DRAM Dynamic RAM
DSC Digital Setting Circle (eg Argo Navis)
DSI Deep sky imager (Meade product)
DSLR Digital single lens reflex (type of camera)
DSO Deep sky object
DSS Deep Sky Stacker (image processing software)
DTC Digital Telescope Computer (eg Argo Navis)
ED Extralow dispersion
EFR Emerging flux region (on our sun)
EP Eye piece
EQ Equatorial
EQDIR Equatorial Direct (software from Shoestring Astronomy)
EQMOD GEM (q.v.) control software
ET Ephemeris time
EXIF Exchangeable image file (format)
FAQ Frequently asked questions
FIR Far infra red
FL Focal length
FLI Finger Lakes Instrumentation Co.
FMC Fully multi coated
FOV Field of view
FSQ Flat field super quadruplet (Takahashi 4 element refractors)
FTA Field transition arch (on our sun)
FTP File transfer protocol
FUV Far ultra violet
FWHM Full width half maximum - explanation
GCVS General catalogue of variable stars
GEM German Equatorial Mount
GLP Green laser pointer
GMT Greenwich mean time
GPS Global positioning system
GRB Gamma Ray Burst
GRS Great red spot (on Jupiter)
GSO Guan Sheng Optical
HA (or Ha) Hydrogen alpha (light spectrum)
Hb Hydrogen beta (light spectrum emission line)
HC Hand controller
HD Henry Draper (DSO catalogue)
HDRI High dynamic range imaging (or just HDR)
HF High frequency
HST Hubble space telescope
IC Index Catalogue (of DSOs)
ICNR In camera noise reduction
IR Infra red (light)
IIS IceInSpace
IMO In my opinion
IMHO In my humble opinion (usually not being very humble at the time)
ISS International Space Station
JPG or JPEG Joint Photogaphic Expers Group (very common picture file format)
LB Light Bridge (Meade DOB design)
ISS International Space Station
LAN Local Area Network (of computers)
LBT Large Binocular Telescope
LMC Large Magellanic cloud
LPI Lunar planetary imager (Meade product)
LPR Light pollution reduction
LRGB Luminance Red Green Blue
LSB Low surface brightness (ie, dim!)
LST Local sidereal time
LV Lanthanum Vixen
LVW Lanthanum Vixen Widefield
M Messier (DSO catalogue)
MAS My Astro Shop
MCT Maksutov-Cassegrain telescope
MN or MNT Maksutov-Newtonian (type of telescope)
MPCC Multi purpose coma corrector - explanation
mW milliWatt, = 0.001 W
NELM Naked Eye Limiting Magnitude
NGC New General Catalogue (of DSOs)
NIB New In Box (but opened)
NLV New lanthanum Vixen (eyepiece)
NPB Narrow pass band
NPL New Plossl (Vixen eyepiece)
OAG Off axis guide
OSC One-shot Colour
OIII Oxygen III (or 3) (light spectrum emission line)
OT Off-Topic (post)
OTA Optical tube assembly
PAE Pointing Accuracy Enhancement (feature of GoTo mount)
PE Periodic error
PEC Periodic error correction
PGC Principal Galaxies Catalogue
PHD Push Here Dummy (astrophotography software)
PITA Pain in the Arse
Pn Planetary nebula
PM Private Message (between two forum members)
PME Paramount ME (mount by Software Bisque )
PMX Paramount MX (mount by Software Bisque )
POTH Plain old telescope handset
PST Personal solar telescope (Coronado product)
QE Quantum Efficiency (CCD Camera type)
RA Right ascension
RACI Right angle correct image
RAM Random Access Memory
RAW Not an acronym, it just means raw imaging data
RC Ritchey-Chretien
RCOS RC Optical Systems Co.
RCW Rodgers Campbell Whiteoak Catalogue (of DSOs)
RGB Red green blue
SA Spherical Abberation
SAO Smithsonian Astrophysical Observatory
SB Surface brightness
SBIG Santa Barbara Instrument Group
SCP South Celestial Pole - explanation
SCT Schmidt-Cassegrain telescope
SD Super-low dispersion
SDM Size Does Matter (Australian Telescope Maker Peter Read's motto)
SEB South Equatorial Belt (on Jupiter)
SLR Single lens reflex (camera)
SMA Super modified achromat
SMC Small Magellanic cloud
SN or SNT Schmidt-Newtonian (type of telescope)
SPED Saxon premium ED
SQM Sky Quality Meter (brightness reading)
SSC Smithsonian Stellar Catalogue (not a preferred term)
STV SBIG televison (camera)
SV Super view
SWA Super wide angle
SXV Starlight Xpress something
TAK (not an acronym) short for Takahashi (manufacturer)
TEC Telescope Engineering Company
TFOV True field of view
TLA Three letter acronym (like this)
TMB Thomas M. Back (of TMB Optical Co)
TOA Triplet orthoscopic apochromat (Takahashi design)
TSA Triplet Super Apochromat (Takahashi design)
UHC Ultra high contrast
UO University Optics
USB Universal serial bus (data transport)
UT Universal time
UTA Upper tube assembly (of truss Dobs)
UV Ultra violet (light)
UWA Ultra wide angle
WDS Washington Double-Star Catalogue
WO William Optics
ZLM Zenith Limiting Magnitude


Modern Constellation Abbreviations


And Andromeda
Ant Antlia
Aps Apus
Aqr Aquarius
Aql Aquila
Ari Aries
Aur Auriga
Boo Bootes
Cae Caelum 
Cam Camelopardalis 
Cnc Cancer 
CVn Canes Venatici 
Cma Canis Major 
Cmi Canis Minor 
Cap Capricornus 
Car Carina 
Cas Cassiopeia 
Cen Centaurus 
Cep Cepheus 
Cet Cetus 
Cha Chamaeleon 
Cir Circinus 
Col Columba 
Com Coma Berenices 
CrA Corona Australis 
CrB Corona Borealis 
Crv Corvus 
Crt Crater 
Cru Crux 
Cyg Cygnus 
Del Delphinus 
Dor Dorado 
Dra Draco 
Equ Equuleus 
Eri Eridanus 
For Fornax 
Gem Gemini 
Gru Grus 
Her Hercules 
Hor Horologium 
Hya Hydra 
Hyi Hydrus 
Ind Indus 
Lac Lacerta 
Leo Leo 
Lmi Leo Minor 
Lep Lepus 
Lib Libra 
Lup Lupus 
Lyn Lynx 
Lyr Lyra 
Men Mensa 
Mic Microscopium 
Mon Monoceros 
Mus Musca 
Nor Norma 
Oct Octans 
Oph Ophiuchus 
Ori Orion 
Pav Pavo 
Peg Pegasus 
Per Perseus 
Phe Phoenix 
Pic Pictor 
Psc Pisces 
PsA Piscis Austrinus 
Pup Puppis 
Pyx Pyxis 
Ret Reticulum 
Sge Sagitta 
Sgr Sagittarius 
Sco Scorpius 
Scl Sculptor 
Sct Scutum 
Ser Serpens 
Sex Sextans 
Tau Taurus 
Tel Telescopium 
Tri Triangulum 
TrA Triangulum Australe 
Tuc Tucana 
UMa Ursa Major 
UMi Ursa Minor 
Vel Vela 
Vir Virgo 
Vol Volans 
Vul Vulpecula


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