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Explanation of terms used in this website. If a word is written in italics in a description this means that this word is also explained on this page.


Aberration - A design-inherent distortion or other image error of an optical system like a telescope or eyepiece. The aberration is therefore not a defect, but rather a result of the optical design itself. Compare chromatic aberration, coma

Achromat - A telescope or lens that corrects large amounts of the chromatic aberration that appears when light passes through the lens. Some false colour remains however. Compare apochromat

Alignment - See GoTo alignment or polar alignment. The term is also sometimes used as a synonym for collimation of a telescope.

Alt-Azimuth mount - A mount for a telescope, spotting scope or binocular that moves in horizontal and vertical direction (azimuth and altitude). Primarily used for nature observing. Compare equatorial mount

Altitude - The angle above the ground of an object as seen from an observer. For example an object halfway up has an altitude of 45°. An object straight above the observer in the zenith is at an altitude of 90°. Movement in altitude means the vertical movement of an alt-azimuth mount, i.e. an up-down movement.

Aperture - The diameter of the lens or the primary mirror of a telescope.

APO - See apochromat

Apochromat - A telescope or lens with superior colour correction than an achromat. Often abbreviated as APO. Compare ED

Apparent Field of View - The size of the area that you can see through a telescope or binoculars as it appears to you. For more information see our article about eyepieces.

Arc minute - A unit of measurement of an angle. Equal to a sixtieth of a degree.

Arc second - A unit of measurement of an angle. Equal to a sixtieth of an arc minute or 1/3600th of a degree.

Autoguider - A camera used to correct the tracking error of an equatorial mount for astronomical imaging. See our article about astro imaging.

Azimuth - The horizontal direction of an object as seen from an observer. Measured in degrees from the north. For example an object precisely in the east has an azimuth of 90°, an object in the south 180° etc. Movement in azimuth means the horizontal movement of an alt-azimuth mount, i.e. a left-right movement.


Ball head - A mechanical device used to hold a camera on a tripod and move it freely in any axis of movement. Compare panhead

Bahtinov mask - A tool that helps during focusing, especially for astronomical imaging.

Barlow lens - A lens that extends the focal length of a telescope. Equivalent to a teleconverter for photography.

Bayer matrix - A colour filter matrix used to create a colour image in single-shot colour cameras. Each pixel of the sensor has a red, green or blue filter placed in front of it that will filter out a part of the spectrum. The matrix makes it possible for the camera to distinguish between different colours.

Binocular viewer - An accessory for telescopes that allows you to observe with both eyes.

Binoviewer - see Binocular viewer


C-Mount - A thread commonly used on cameras and accessories for photography. Formally UNC 1"-32. In addition to the thread dimensions the term C-mount also defines the back focus of a camera to be 17.526mm.

Carbon fiber - A very strong and very light material sometimes used for telescopes or tripods.

Catadioptric telescope - A telescope that uses both lenses and mirrors to collect and focus light. Catadioptric telescopes include Schmidt-Cassegrains and Maksutovs. See our article about telescopes.

CCD camera - A digital camera that uses a CCD sensor (Charge-coupled device). Many CCD cameras are designed specifically for astronomical imaging. See our article about CCD cameras.

Cheshire - A particular type of collimation tool for Newtonian telescopes.

Chromatic aberration - An aberration of an optical system like a refractor or an eyepiece where different colours are not focused into the same point. Chromatic aberration appears as a coloured fringe along dark/light contrasts, e.g. around bright stars. This fringe is usually referred to as false colour.

Collimation - The process of adjusting the optical components of a telescope so that they are positioned precisely on the optical axis. Collimation needs to be done at regular intervals on Newtonian telescopes. Refractors usually do not need to be collimated. See here

Colour correction - This term describes how well an optical system corrects the chromatic aberration. See also achromat, apochromat, false colour.

Coma - An aberration in Newtonian telescopes and other optical designs that results in a distorted image. A star that is not located in the center of the field of view will appear to have a "tail" like a comet.

Coma corrector - A corrector lens that is designed to reduce or eliminate the coma of a Newtonian telescope. Mainly used for astronomical imaging.

Crayford - A particular type of focuser used on many telescopes. Crayfords tend to move very smoothly, but have a limited load capacity for heavy equipment.


Dark current - A small current that flows through the sensor of a camera even when no light reaches the sensor. Every camera sensor has a certain amount of dark current. Dark current is one cause of noise on cameras.

Deep Sky Object - An astronomical object outside our own solar system. Deep Sky Objects include galaxies, globular clusters, open clusters, gas nebulae, planetary nebulae and multiple stars.

DEC - See declination

Declination - One of the two axes in the equatorial coordinate system. The declination axis of an equatorial mount is the axis that will move the telescope in declination. Declination is the distance from the equator in degrees. Compare Right Ascension

Dielectric coating - A special coating that consists of multiple layers of dielectric material. Dielectric coatings allow for precise control of reflectivity in different wavelengths. Used on high quality filters and as high reflectivity coatings on mirrors.

Digiscoping - Photography with a digital camera through a spotting scope or binoculars to use the scope's light gathering power and magnification.

Dobsonian telescope - A Newtonian telescope on a simple alt-azimuth mount called the rocker-box. The rocker-box is often made of wood and usually has a very simple design that allows for easy and cheap production. The idea is to minimize the amount of money needed for the mount so that the money is spent for a larger and better telescope instead. Dobsonian telescopes

Dynamic range - The luminance range of a camera, i.e. the ratio between the faintest and the brightest details that can be discerned on a photograph with that camera.


ED - Extra-low Dispersion glass. A special glass used in refractors and other lenses for improved colour correction. See apochromat, chromatic aberration.

Equatorial coordinate system - The main coordinate system used for astronomy. Roughly similar to the latitude/longitude system used on Earth. Instead of northern and southern latitude the EQ coordinate system used Declination. Instead of longitude it uses the Right Ascension. The dividing line is the celestial equator which divides the northern and southern celestial hemisphere.

Equatorial mount - A type of mount for astronomical telescope that allows for easy tracking of the movement of objects on the sky. One axis is set up parallel to the Earth's axis of rotation which allows you to compensate the Earth's rotation by turning the mount at constant speed in the opposite direction.

Erecting prism - A prism that provides an upright and non-reversed image and a comfortable viewing position for nature observing with a telescope. Compare star diagonal. See our article about diagonals and prisms.

Exit pupil - The diameter of the "bundle" of light that exits the eyepiece of a telescope or binoculars. If the exit pupil is larger than 7mm the bundle of light will no longer fit into the pupil of the human eye, meaning that light is lost on the iris. See our articles about eyepieces and binoculars.

Eyepiece - The lens you look through when observing with a telescope. The eyepiece determines among others the magnification you get with the telescope. See our article about eyepieces.

Eyepiece projection - A technique used in astrophotography where an eyepiece is used between the telescope and the camera. The eyepiece changes the effective focal length of the telescope and thus changes the magnification. Compare prime focus

Eye relief - The distance you can keep with your eye when looking through an eyepiece where you can still see the full field of view. See our article about eyepieces.


False colour - see chromatic aberration

Field curvature - Field curvature describes the fact that the focal plane of a telescope is not flat, but rather curved. The focal plane is the plane in which you can a sharp, focused image. The human eye can compensate for field curvature to some extent, but for photography the result of field curvature is that the image is only focused in the center whereas the edges of the field are out of focus. To correct field curvature and get a flat field you can use a field flattener.

Field flattener - An image corrector used for astrophotography that corrects the field curvature of the telescope. Used on refractors and Ritchey-Chretien telescopes.

Field of view - The size of the area that you can see through a telescope. There are two different aspects of field of view: Apparent field of view and true field of view. For more information see our article about eyepieces.

Filter - An optical accessory used on telescopes and cameras that filters away a part of the spectrum. Designed to improve contrast and visibility of details of certain objects. See our articles about filters here

Finderscope - See here

Flip mirror - An optical accessory that allows you to connect two eyepieces or an eyepiece and a camera to a telescope. One of the two accessories then sees the image provided by the telescope and you can quickly switch between the two, but both accessories cannot see the light from the telescope at the same time.

Focal length - The distance from a telescope's or binoculars main optics (lens, mirror) to the focus point. The focal length has an influence on the magnification you get with a given eyepiece or when doing imaging with the telescope. For further details please see our article about telescopes.

Focal ratio - The ratio between the focal length of a telescope and its aperture.

Focal reducer - See reducer

Focuser - The mechanical device on a telescope that allows you move the eyepiece or camera until you have a focused, sharp image.


Galaxy - A massive system of millions or billions of stars, each orbiting the galaxy's own center of mass. The Milky Way is the galaxy in which our own sun is located. The nearest large galaxy is the Andromeda Galaxy.

Gas nebula - An interstellar of gas, e.g. hydrogen and helium. Different types of gas nebulae include H-II regions such as the Pelican Nebula and planetary nebulae such as the Ring Nebula M57.

Globular cluster - A spherical collection of stars that orbits a galaxy. In contrast to Open Clusters, globular clusters often have a high stellar density near the center.

GoTo - Computerized. A GoTo mount or telescope comes with a controller with integrated database. You can select any object from the database and the mount will point the telescope at the object so that you do not need to find the object yourself.

GoTo alignment - A procedure that you need to do when starting up a GoTo mount so that the mount then knows its current position on the sky. Alignment is typically done by pointing the telescope at one or more known stars.

Guidescope - A second, often smaller telescope that is mounted on top of the main telescope or side-by-side with it. See here.


H-Alpha - One of the main emission lines of celestial objects. Objects will emit a significant portion of their light in this particular wavelength. By using filters it is possible to let only this wavelength pass through the telescope and block the rest of the spectrum, which will result in increased contrast. Special H-Alpha filters are also used as solar filters. Some unique details of the sun are visible in this wavelength, e.g. the sun's prominences. Important note: H-Alpha filters for nighttime use are technically very different from H-Alpha filters for solar observing. Never use a regular H-Alpha filter for solar observing!

H-Beta - One of the main emission lines of celestial objects. Objects will emit a significant portion of their light in this particular wavelength. By using filters it is possible to let only this wavelength pass through the telescope and block the rest of the spectrum, which will result in increased contrast.


Image processing - The process of changing a raw image taken with a camera electronically to work out details, change contrast and colour, remove artifacts and distortion etc.

Interpupillary distance - The distance between both eyes, measured from the middle of one pupil to the other.




Light gathering power - The amount of light that a telescope or binoculars can collect and focus into the observer's eyes.

Light pollution - Artificial light originating from street lights, interior lighting, cars etc. which illuminates the sky and reduces the number of stars visible. Simultaneously it will also reduce contrast on deep sky objects and significantly reduce visibility of details. Light pollution is the main reason why amateur astronomers usually will travel far out from the civilization centres out to the countryside for astronomical observations and astro-imaging.

Limiting magnitude - The magnitude of the faintest star that can be seen with a particular telescope.

LRGB - A method of getting a colour photograph with a monochrome camera by taking four (or more) separate images and then combining them during image processing. LRGB stands for the four channels Luminance, Red, Green and Blue.


Magnitude - Measure of the brightness of an astronomical object. The higher the magnitude, the fainter the object. For example the sun has -27mag, the moon -13mag, the star Vega 0mag and the faintest stars visible without a telescope or binocular have about 6mag.

Maksutov-Cassegrain - A particular type of catadioptric telescope. Maksutov-Cassegrains are comparatively compact and well suited for observing the planets.

Monochrome - Black-and-white, i.e. different shades of grey but no colour. CCD cameras often have monochrome sensors. You can still take colour images with LRGB filters, though. Visual deep-sky observing is also typically monochrome since colours from the objects are often so faint that they cannot be seen by the human eye.

Multicoating - A lens coating consisting of multiple thin layers. The purpose of this coating is to reduce the amount of light reflected on the lens surfaces and thus increasing the amount of light transmitted through the lens.


Narrowband - A type of filter that allows only a very narrow part of the spectrum to pass. This usually results in a very significant increase in contrast but also makes the image somewhat dimmer.

Near focus - The nearest distance of an object you can still focus with an optical instrument like a telescope, spotting scope, binoculars or camera lens. You can get a focused image for any object that is further away than the near focus distance, but not for any object closer than this distance.

Newtonian telescope - A type of telescope invented by Isaac Newton. The telescope uses a parabolical primary mirror and a flat secondary mirror at 45° angle that reflects the light collected by the telescope out to the side where you can then view it.

Noise - Random variation of brightness or colour information in images taken with a digital camera or CCD camera. Noise is the direct result of how a camera sensor works and thus cannot be avoided. With longer exposure times noise becomes a major factor in the quality of the final image. In order to reduce noise astronomical CCD cameras are cooled. Compare dark current.


O-III - One of the main emission lines of celestial objects. Objects will emit a significant portion of their light in this particular wavelength. By using filters it is possible to let only this wavelength pass through the telescope and block the rest of the spectrum, which will result in increased contrast.

OAG - see Off-Axis Guider

Off-Axis Guider - See here.

Open Cluster - A group of typically a few thousand stars that were formed from the same gas nebula. Compared with the average density of stars on the sky you can typically see a distinct concentration in the cluster, although there are loose clusters in the Milky Way that look hardly different from the stars in the background.

Optical Tube Assembly (OTA) - The "actual" telescope consisting of the telescope's lenses or mirrors, the focuser and the mechanical tube keeping these parts together, as well as the basic accessories belonging to it. In our website OTA usually (but not always) encompasses the telescope's tube, tube rings, a finderscope and sometimes a dovetail plate.

OTA - see optical tube assembly


Panhead - A type of tripod head used on camera tripods that allows you to move the camera (or binocular) in azimuth and altitude. A 3-D panhead also allows you to tilt the camera 90° sideways. Compare ball head

Planetary nebula - A gas nebulae formed from the remnants of a star.

Planisphere - A star chart that allows you to see which constellations and stars are above the horizon at any given date and time. Used for initial orientation on the night sky and to learn how to recognize stars and constellations to begin with.

Polar alignment - The process of setting up an equatorial mount parallel to the Earth's axis of rotation.

Polar scope - An accessory used for polar alignment. The polar alignment scope shows a magnified image of a portion of the sky and contains a reticle that shows the position where Polaris (the North Star) needs to be placed.

Porro prism - One of two common prism designs used on binoculars. Compare roof prism. See our article about binoculars.

Primary mirror - The main mirror of a reflector. Purpose of the mirror is to collect and focus light that can then be viewed by the observer. Equivalent in function to the lens on a refractor.

Prime focus - The focal point or focal plane of a telescope. The term is usually used to refer to prime focus photography where the camera is placed directly in the focal plane without an eyepiece between the camera and the telescope. Compare eyepiece projection.

Prominence - Large gaseous structure extending outward from the sun's surface. Prominences can be anything from a few thousand to several hundred thousand kilometres in size. Can be seen with H-alpha solar filters.


QE - see quantum efficiency

Quantum efficiency - The ratio at which photons that reach a CCD sensor are converted into electrons. An ideal camera would convert 100% of photons into electrons, meaning that it could detect each photon. Real sensors can never reach 100%, but can have a quantum efficiency of up to 90% and sometimes higher.


RA - see right ascension

RC - see Ritchey-Chretien

Reducer - Reduces the focal length of a telescope and thus results in a faster focal ratio. Used mainly for photography in order to get shorter exposure times and a larger photographical field of view.

Reflector - A telescope that uses mirrors in order to collect and focus light. Reflectors include Newtonian telescopes and Ritchey-Chretien telescopes. Compare refractor and catadioptric telescope.

Refractor - A telescope that uses lenses in order to collect and focus light. Refractors include achromats and apochromats. Compare reflector and catadioptric telescope.

Right Ascension - One of the two axes in the equatorial coordinate system. The Right Ascension or RA axis of an equatorial mount is the axis that will move the telescope in Right Ascension. RA is the angular distance in eastwared direction measured from the vernal equinox (the point where the sun is during the equinox on March 21st). The full 360° circle is divided into 24 hours which in turn are divided into minutes and seconds. Example: Instead of 180° we would call it 12 hours. Compare Declination

Ritchey-Chretien - A reflector telescope that uses hyperbolic primary mirror and a hyperbolic secondary mirror. RCs have rather short tubes which makes them very compact. This telescope is usually optimized for astro-imaging and less well suited for visual observing.

Roof prism - One of two common prism designs used on binoculars. Compare porro prism.


S-II - One of the main emission lines of celestial objects. Objects will emit a significant portion of their light in this particular wavelength. By using filters it is possible to let only this wavelength pass through the telescope and block the rest of the spectrum, which will result in increased contrast.

Schmidt-Cassegrain - A particular type of catadioptric telescope. Schmidt-Cassegrains are very compact and universally suited for observing all types of objects and for doing astro-imaging.

SCT - see Schmidt-Cassegrain

SC thread - A thread used on Schmidt-Cassegrain telescopes and on many astronomical accessories. Formally UNC 2"-24, i.e. 2" (50.8mm) diameter and a pitch of 24 turns per inch. Sometimes called 2" thread, but we recommend not to use this term since it is easy to confuse with the 2" filter thread. See our article about adaptors.

Secondary mirror - The second mirror used in different types of reflectors. The secondary mirror will reflect the light collected by the primary mirror out of the telescope so that it can then be viewed with an eyepiece.

Second of arc - see arc second Seeing - The blurring and twinkling of astronomical objects caused when the light of the object passes through the Earth's atmosphere. Seeing limits the amount of details you can see and puts a limit to the maximum magnification you can use. Astronomical seeing conditions vary with different observing locations and from night to night.

Solar filter - A special type of filter used for observing or photographing the sun. Warning: Never observe the sun with any type of optical instrument without use of a proper solar filter!

Spectroscope - A device that splits the spectrum of light into its separate colours.

Spectrum - The range of all possible frequencies of electromagnetic radiation, e.g. the colours of visible light, infrared light and ultraviolet light.

Spotting scope - A compact telescope built for nature observing. Unlike astronomical telescopes a spotting scope uses an integrated erecting prism to provide an upright, non-reversed image.

Star chart - A map showing the position of the stars and of deep-sky objects.

Star cluster - See globular cluster and open cluster.

Star diagonal - An accessory that provides a more comfortable viewing position on some types of telescopes. Provides an upright, but mirror reversed image. Compare erecting prism. See our article about diagonals and prisms.


T2 - A thread very common on cameras and accessories for astronomical imaging. Formally M42x0.75, i.e. 42mm diameter and a pitch of 0.75mm per turn.

Telescope - An optical instrument built for astronomical observations and astronomical imaging. See our article about telescopes.

Tripod - A three-legged mechanical device used as a basis that holds a camera or an astronomical mount.

True Field of View - The size of the portion of the sky that you can see in a telescope. For more information see our article about eyepieces.







Zenith - The highest point in the sky, precisely 90° up from the horizon.

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