The Great Telescope ABC

Glossary of all important terms.

Glossary of important technical terms


Non-chromatic, non-coloured; Name for a lens combination, which can adjust the most important colour defects. With high magnification one can recognize so-called secondary colour defects in achromatic objectives.

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Amici prism

(Diagonal prism) An Amici prism causes an upright and side corrected image reflection in an astronomical telescope. Pre-supposing, no further optical system (inverter lens or zenith reflector) is used in the telescope at the same time.

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Opening (of a telescope).

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The point in the orbit of a planetary object, furthest in distance from the sun.

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Bezeichnung für eine Linsenkombination, die alle Farbfehler, incl. des sekundären Farbfehlers, ausgleichen kann und somit absolut farbrein in ihrer Wiedergabe ist.

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Equatorial mount

(Parallax mount) Adjustment of an astronomical instrument onto the celestial pole (Right Ascension axis is parallel to the Earth’s axis).

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Not spherical, not shaped like a sphere.

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One of many thousand small celestial bodies, which circle the sun. Often called a small planet.

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Astronomical unit

(AU) 1.49 x1011 metre, corresponds to the average distance from the earth to the sun.

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Separation ability either in the sense of better recognition of detail (angular resolution) or particularly with CCD astronomy in the sense of the separation of different stages (dynamic dissolution).


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Dying-down time

Time interval, which is required for an instrument to settle down again after its mount has been heavily hit.


Gerät, das die Nachführkontrolle und Korrektur während der Belichtung eines Astrofotos automatisch vornimmt, ohne daß ein Bediener zugegen sein muß.

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Test and adjustment procedure, with which a ray of light is passed twice through the optical system, so that errors appear to be twice as large than they are in reality.

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Altazimuthal mount

Adjustment of an astronomical mount in the horizontal/vertical direction (“Right Ascension “- axis is parallel to the direction of the force of gravity and points to the zenith).

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Barlow lens

Increases the focal length of the objective by a certain factor and increases the magnification accordingly. (the factor 2x or 3x is usual)

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Image field rotation

Rotation of the picture in the telescope, when the equatorial set up is not carried out precisely (in particular with the azimuth set up).

Image field de-rotator

Instrument, which compensates for image field rotation, by precisely the image field in reverse.


Type of optical glass with special properties (transmission, refractive index) for optical applications. 

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Arc minute

This is what one uses in the sky, in order to find objects more easily. It is a sixtieth of an angular degree. (e.g. a ten-cent piece seen from a distance of 68 meters) Symbol: '

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Arc second

This is an angular measurement. It is a sixtieth of an arc minute. If one turns once in a circle, that is 360°. Thus 1° corresponds to 60 arc minutes (60 '), which are divided again into 60 arc seconds (60 "). To give an example - one arc second corresponds to the distance between two headlights seen from the distance London to Birmingham.

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Focal length

Distance objective lens or main reflector to the focal point. Magnification is calculated from the focal length of the telescope and the eyepiece using the formula focal length of the telescope in mm/focal length of the eyepiece in mm.


Mineral (kein Glas), das glasähnliche Eigenschaften aufweist, aber nur mehr selten für Linsenobjektive Verwendung findet.

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Chromatic aberration

Colour defect of a lens.

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CS (Clear Skies)

Gebräuchlicher Gruß in Foren und Unterhaltungen unter Astronomen: Clear Skies. Man wünscht sich einen klaren Himmel, um astronomische Beobachtungen durchführen zu können.

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DC servo motor

 Direct current motor with position control and favourable characteristics for computer control.

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short for „declination“.

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Deep sky

All celestial objects, outside our solar system are called deep sky objects (galaxies, star clusters, nebula,…).

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Declination (DEC)

One of the celestial coordinates. It describes the angular distance of a celestial object north (+) or south (-) the celestial equator. The declination corresponds to the geographical latitude, if one were to project the Earth’s coordinate grid onto the sky.

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German mount

Equatorial (parallax) mounting with the German coordinate system. This type of setup is becoming extremely popular world-wide. It was introduced for the first time many years ago in Germany.

Double pass laser autocollimator

Double run laser autocollimator, see „autocollimation“ .

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Dark adaption

Ability of the eye to adjust itself to darkness whereby its sensitivity is increased. Dark adaptation requires about 20-30 minutes rest without bright sources of light and is disturbed by white light. Therefore red light is used, which disturbs the dark adaptation only slightly during night-time astronomy.

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Extra-Low-Dispersion-Glas, Glas mit besonders niedriger Dispersion, einer optischen Eigenschaft, die die Schaffung kompakter, leichter und nur zweilinsiger apochromatischer Objektive ermöglicht.

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The ecliptic corresponds more or less to the circular path, on which the Earth moves around the sun. This also determines the sun’s passage across the sky. In addition the planets of our solar system follow a path relatively close to the ecliptic. The term ecliptic (from the Greek) means blackout. The reason lies in the fact that the moon and sun become dark only if the full or new moon are on the on the ecliptic.

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Cross hair eyepiece

Eyepiece with cross hairs in the centre. Modern designs have an etched glass plate, on which double cross hairs are etched. The star being observed is not covered by the square created in the centre.

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Foucalt test

Optical test for determining the accuracy of a reflector.

Focal point photography

Photography in the focus of the telescope without the eyepiece.


Bedeutet Scharfstellen.

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Siehe Calcium-Fluorit.

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Frequency converter

Equipment, which is needed with telescope drives with synchronous motor for the influence of the engine speed.


Function for the automatic localization and positioning of a celestial object with hand computers for telescopes.


Global Positioning System: This system serves to determine theobservation location, the date and the time. This American system usessatellites in the Earth’s orbit.


Angular unit (e.g. a ten-cent piece seen from a distance of 1.13 meters). Abbreviation: ..°

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The brightness of a star visible in a telescope or with the naked eye.

Main reflector focusing

Focusing by moving the main reflector back and forth in the tube, as opposed to focusing by means of the eyepiece. It has the advantage that there are no moveable mechanical parts on the outside of the telescope and focusing path for many accessories remains short.

Heavy Duty

Particularly solid and stable construction.


HEXAFOC Optiken zeichnen sich durch einen wesentlich wertigeren Okularauszug aus: Hexagonales Design gefertigt aus Aluminium. Dadurch ist der OAZ wesentlich steifer und tragfähiger als die Vorgängerversion. D.h es kann schweres Zubehör wie Kameras und Okulare uneingeschränkt benutzt werden. Größerer 2.5" (78mm) Durchmesser des Auszugsrohres, wodurch sich eine größere freie Lichtwegöffnung von 64mm und weniger Vignettierung des CCD Kamarasensors ergibt. Spielfreie Führung des Auszugsrohres, dadurch kein Shifting beim Fokussieren.

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Distance of an object above the horizon, measured in degrees, minutes and seconds . Positive values show that the object stands over the horizon, negative that the object lies below the horizon.

Intersection function

Exact positioning method with the help of two or three reference stars, which can be observed in the proximity of the object.


Anfängliche "Eichung" eines Teleskops.

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Interference filter

Filter, which consists of several, individual layers of a special material with a thickness of only 1/4 wavelength, which determine the permeability of light, in a narrow spectral (colour) range.

Kellner eyepiece

Tripple lens eyepiece with good image according to Kellner


Time that two objects are positioned next to each other.

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Small objects from the cloud of debris at the edge of the solar system, which often circle the sun on very strong elliptical paths.

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Relationship of the light intensities of two neighbouring areas.

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Light collecting ability

Ability to collect and combine light from an entire surface at the focal point. With reflector telescopes with a secondary mirror in the light ray path, its surface must be chosen for its ability to collect light.


Bedeutet Lichtdurchlässigkeit.

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A reflecting telescope with particularly good image properties when little contrast is present.


Verbessertes Kellner-Okular mit größerem Eigengesichtsfeld und besserer Farbkorrektur.

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Messier objects

110 nebula objects (deep sky objects), compiled in the 18th Century by the French astronomer Charles Messier (1730 to 1817)..

Micro slewing

Micro-fine procedure for a telescope drive for precise positioning.

Tracking motor

The tracking motor can only be used with equatorial mounts. It is fitted to the Right ascension axis and compensates for the movement of the earth. A tracking motor is absolutely essential for astrophotography.

Night display

Back-lit, red display of the hand control box (important for the preservation of the dark adaptation).


The point in the sky, opposite to the zenith, therefore directly beneath the observer.

Null test

Optical test, with which the quality of the overall system is judged on the basis of producing a smooth, even surface. This is gauged as “optical zeroing“.

Object library

List of celestial objects, which is stored in the electronics of the telescope.




Durchmesser eines Objektives (Spiegel oder Linse).

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Verhältnis von Öffnung zu Brennweite des Objektives.

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Verhältnis von Brennweite zu Öffnung eines Objektives.

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A type of magnifying glass for magnifying the image produced by an objective.

OAZ (Okularauszug)

Gebräuchliche Abkürzung unter Astronomen. Siehe Okularauszug.

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Der Länge nach verstellbare Einrichtung an Teleskopen, um das Teleskop zu fokussieren.

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Orthoskopisches Okular

Vierlinsiges Okular mit besserer Farbkorrektur als z.B. Kellner-Okulare.

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The apparent change in the position of an object in the sky throughout the year. Due to the different positions of the Earth on its orbit around the sun close objects appear to wobble. The same effect can be achieved if an observer looks at an object close to and first closes the left eye and then the right.


The distance from the earth that an object must have to have parallax of a one arc second. Corresponds to approximately 3.26 lightyears.

Periodischer Schneckenfehler

Geringfügige Abweichungen (Pendelausschlag) eines Teleskopantriebes während der Nachführung.

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The point in the orbit of an object, when it is next to the sun.


One of the large, well-known celestial bodies, which circle the sun. Descends from the Greek word for Wanderer.

Planetary nebula

Aplanetary nebula is formed, if a sun, which has a max. of 1.4 times the mass of our sun dies. The outer layers are rejected and usually form ring-shaped nebulae. The term “planetary nebula” is derived from the fact that in small telescopes they look like planets i.e. diskshaped.

Plössl eyepiece

Refined orthoscopic eyepiece with large visual field and improved edge clarity and colour correction.

Polarisation filter

Neutral filter, which deals with the weakening of light by polarization.

Polar wedge

Mechanism, which tilts the drive of a telescope by an angle from the horizontal corresponding to the geographical latitude of the place where the instrument is located, so that the RA axis is parallel to the Earth’s axis and allows for compensation of the rotation of the Earth by means of only one axis.


Schalttafel des Teleskopantriebes.

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A very slow oscillating motion of the Earth’s axis, which is triggered by the gravitation forces of the moon.

Projection photography

Photography by means of an intermediate eyepiece, whereby the effective magnification increases.


Special mode, with which the reticle of an eyepiece receives tension for short instances, so that fainter stars can be followed, which would otherwise receive too much illumination due to the brightness of the reticle.


The accurate frequency is given by a quartz, which is necessary for adjusting to the star speed.

RA (axis)

Short for „Right Ascension“.

Reference stars

For the initialization of a telescope with computer control, a reference star list of all bright stars and also stars further away, can be used.


Reflector telescope (light reflection by reflector). See the diagram on page 45.


Lens telescope (Light refraction = refraction of light through the lens). See the diagram on page 45

Right Ascension (RA)

Corresponds to the geographical longitude, if one were to project the earth’s coordinate grid on to the sky. The Right Ascension axis of a parallactic or equatorial mount is pointed to the celestial pole and is aligned parallel to the Earth’s axis. It can be driven by means of an axle or by a tracking motor. With this adjustment the Earth’s rotation is compensated for and, once adjusted, the star remains in the field of vision of the telescope. The RA axis is also known as the polar axis.

Ronchi test

Optical test for the examination of the accuracy of a reflector’s surface.


Interface to and from personnel computers for communication with external devices (e.g. telescope) or communicating amongst themselves.

Schmidt (corrector) plate

Correction lens with both sides having an aspherical polish in a Schmidt Cassegrain telescope.

Schmidt-Newton telescope

Combination of a Newton telescope with the idea of a Schmidt camera. Combination of the advantages of both systems whilst avoiding their individual disadvantages.


Conditions of visibility.

Siderische Geschwindigkeit

Siehe Sterngeschwindigkeit.

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Brennweitenreduzierende Linse, die dadurch die Beobachtung und Aufnahme eines größeren Himmelsgebietes ermöglicht.

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Sky tour

A tour of the sky limited at the wishes of the user, where one object after the other is automatically observed.


Reconciliation of the periodic worm conveyer error (English: PEC).

Spotting Scope

Telescope for terrestrial applications.

Star speed

Speed of the stars in the telescope, if this is not adjusted. Produced by the earth ‘s rotation. Scattered light screens prevent scattered light in telescopes and optimize thereby the contrast.


Siehe Rektaszension.

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Viewfinder / Spotter scope

Smaller telescope, which serves for manually observing objects.


The day on which the sun can be seen exactly for half a day. There is a spring equalise on March 21st and an autumn equalise on September 23rd respectively. On this day the sun goes through one of the intersections between the ecliptic and the celestial equator, spring or Autumn point.


Adapter ring between amirror reflex camera and the universal T-thread (M42 x 0,. 5mm), with focal and projection adapters as well as the off Axis Guider end.

Extension area

Range from the maximum to the minimum useful magnification of a telescope; should be equipped ideally with 5 to 6 eyepieces and distributed as evenly as possible.


Treating of a lens or a mirror, by which the transmission and/or the reflection of the light and the resistance to cleaning are increased at the same time.

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White dwarf

A white dwarf is the collapsed core of a sun, which had a maximum of 1.4 times the solar mass. The outer layers were rejected and form a so-called planetary nebula. The White Dwarf is about as large as the Earth, but weighs about as much as our sun.

Zenith / diagonal prism

Reflector with 90°-deflection, which facilitates viewing objects close to the zenith.

Zenith / diagonal mirror

Mirror with 90°-deflection, which facilitates the viewing objects close to the zenith.


Ein Zoll = 1" = 25,4 mm.

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