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Glossary of Telescope and Binoculars terms


Any optical defect and/or design error which causes any of the processed light to split before the focal point, therefore reducing the quality of the image.
Absolute Magnitude
How bright an object would appear if placed at a distance of 10 parsecs from the earth.
Achromatic Lens
A refractor lens, usually made of two separate lenses, which has the effect of bringing red and blue light into focus, but the human eye is most sensitive to green light and this causes chromatic aberration.
Alt-azimuth Mount
A simple mount that allows movement in altitude (up and down) and in azimuth (side to side).
Anti-reflection Coating
A thin film applied to an optical surface to increase light transmission.
An open that allows light to pass through.
Aperture Size
The diameter of the primary mirror or lens.
Apochromatic Lens
A refractor lens, usually made of three or more separate lenses and/ or special glass, this allows at least three colours to have the same focal point, minimising chromatic aberration. Apochromatic is often shortened to “APO”.
Apparent Magnitude
How bright an object appears form earth.
A group of stars that appear to make an easily recognized shape and is smaller than a constellation, such as the ”Saucepan” or the ”Pointers”.
A relatively small, inactive, rocky body orbiting the Sun. They may also be referred to as minor planets or planetoids.


Barlow Lens
A lens which, increases the focal length and therefore magnification but decreases the field for everything after it.
Black Hole
A black hole is a region in space where gravity is so strong that light is not able to escape. The strong gravity occurs because matter has been pressed into a tiny space. This can take place at the end of a star’s life.


Created using a concave primary mirror, and a convex secondary mirror, which places the focal point out the rear of the telescope. The inclusion of a convex mirror greatly increases the focal length while keeping the telescope compact.
Catadioptric System
A system using both lens and mirror components, often the lens corrects for the distortion caused by the mirrors. Maksutov and Schmidt telescopes are catadioptric.
Celestial Sphere
An imaginary ball with the earth at its centre. All astronomical bodies, are assigned a stationary position and coordinates (Right Ascension and Declination) on the surface of this ball.
Chromatic Aberration
The tendency of a lens to bend light of different colours by unequal amounts. It can produce nasty halos around bright objects. A well-made apochromatic lens system minimises this problem.
Coated Optics
In lenses this is an anti-reflection coating. In mirrors, a coating is applied to increase reflectivity.
The process of aligning all the elements of an optical system. Collimation is routinely needed in reflectors, but seldom in Catadioptric systems and refractors.
This causes stars and other point objects to appear to have a cone radiating from it.


Declination (Dec)
Similar to Latitude on the Earth’s surface, it is the distance in degrees North or South of the Celestial Equator (the projection of the Earth’s Equator onto the Celestial Sphere). The degrees can be sub-divided into minutes and seconds.
Deep Sky
A name given by amateur astronomers to objects beyond our Sun and its planets.
Dew Shield
A tube extending forward from the front lens of a telescope. It prevents dew from forming on the lens as it cools down, and acts as a sunshade to reduce reflections during the day.
A mirror or prism system which changes the angle and orientation of the light rays coming from the telescope to the eyepiece.
A reflecting telescope on a floor/ tabletop cradle (Dobsonian mount) is referred to as a Dobsonian telescope or Dob for short.
Double Star
Two stars that appear very close to each other. True double stars are in orbit about one another, while optical doubles simply seem close from our point of view.


The blocking of one astronomical body by another as seen from the earth. The most common of these events are Solar and Lunar eclipses.
Elliptical Galaxy
A galaxy whose structure shaped like an oval, is smooth, and lacks features such as spiral arms.
Equatorial Mount
A telescope mount with an axis parallel to the axis of the earth. This provides easy tracking of sky objects and is very helpful for photography when combined with a clock or motor drive.
Erecting Eyepiece or Prism
These turn the image seen through the telescope the right way up, and the right way around.
A small tube that containing lenses needed to bring a telescope’s image into focus. Telescopes usually come with at least two eyepieces (See Magnification).
Eye Relief
The distance between the eyepiece and the eye required to see through the telescope. People who wear glasses while observing, will appreciate the benefits of longer eye relief.
Exit Pupil
The diameter of the beam of light exiting the eyepiece (in mm). It is determined by dividing eyepiece length (in mm) by the focal ratio. 2mm is best for lunar and planetary viewing; 3-4mm for general viewing in areas with some light pollution; and 5-7mm for dark sky sites.


Field of View
The view angle or width of view at a set distance (eg. 100m). Wide angle eyepieces give apparent Field of View. To determine the true Field of View (or actual Field of View), divide the given angle by the magnification.
A piece of coloured glass or film that sits in front of the telescope eyepiece or objective. It only allows transmission of certain wavelengths of light. (Solar filters must always be placed in front of the objective).
Finder Scope
A low power telescope or sight attached to the main tube which helps to locate objects for viewing.
Focal Length
The distance of the light path from the objective to the point of convergence. The convergent spot is called the Focus or Focal Point.
Focal Ratio
The relationship between the aperture and the focal length. It can be calculated by dividing the Focal Length (in mm) by Aperture (in mm). Sometimes referred to as the ”speed” it affects the exposure time required for photography. f/4 to f/6 is considered fast and ideal for astrophotography.
A device which helps to resolve an image from the light in a system. Common designs include rack and pinion, friction (Crayford) and helical (a screw thread).


A large group of stars that is often elliptical in shape, but can also be spiral shaped like Andromeda; a barred spiral like the milky way or, many other shapes including rings.
Galilean Moons
This term applies to Jupiter’s 4 largest moons Callisto, Europa, Ganymede, and Io. They are easily visible with a 130/650 telescope using a 10mm eyepiece. Globular Cluster
A very old, dense cluster of stars, bound by gravity. Many form spherical clouds around galaxies. Our galaxy is surrounded by at least 130 globular clusters.


Kuiper Belt
A ring of icy, objects beyond Neptune, they are believed to be remnants of Solar System formation.


A transparent optical element consisting of one or more pieces of glass. A lens has curved surfaces to bring distant light into focus.


The amount by which a system increases the apparent size an object. Magnification is determined by dividing the Focal Length of the telescope by the Focal Length of the eyepiece.

A meteoroid that has entered the atmosphere but has not yet reached the ground and is burning up. If it reaches the ground, is then called a meteorite.
A small object in space, they can be rocky or metallic in structure. They are smaller than an Asteroid
In a telescope, it is a highly polished surface made to reflect light. Primary mirrors are usually made spherical, or paraboloidal (parabolic) to focus the light rays.
Also known as a natural satellite, this term applies to any natural object orbiting a planet. If it is particularly small it may be called a moonlet.


A cloudy object composed of gas and dust which glows with its own light is called an emission nebula while one illuminated by the starlight of nearby bright stars is a reflection nebula. A cloud of dust which blocks light from star fields or bright nebulae beyond it is a dark nebula.
Newtonian Telescope
A Reflecting Telescope with a flat secondary mirror.


The primary or largest element in an optical system; sometimes called the “fixed optics”.
Open Cluster
A group of stars, normally resolvable, which are bound together by gravity. They are usually about the same age, having come into existence together from a collapsing nebula.
Optical Tube Assembly
The housing and optical train of a telescope; not including the mount, diagonal, eyepiece or accessories. It is often shortened to “OTA”.


Parabolic Mirror
A parabolic or more accurately a ”paraboloidal” mirror, is ground to a shape which brings all incoming light rays to a perfect focus, on axis.
A unit of distance equal to 3.26 light years or 31 trillion kilometres.
Planetary Nebula
A circular or oblong region of gas that has been thrown off by a central star. Its name comes from its apparent similarity to the disk of a planet seen in a very small telescope.
Polar Axis
A telescope mount’s axis that is parallel with the earth’s axis. With a drive motor, the motion of stars due to the earth’s movement can be counteracted so that they remain in the field.
See Magnification.
Prime Focus
The focal point of the objective mirror or lens. Important for photography.


Reflecting Telescope
A telescope that uses mirrors to collect light from a large area and focus it, so that with the aid of an eyepiece, an image is formed. Also known as a “Reflector”.
Refracting Telescope
A design that uses only lenses to focus light and form an image at the eyepiece. The better refractors use Apochromatic lenses.
Resolving Power
The ability of a telescope to separate closely positioned points.
Right Ascension (R.A)
Similar to as Longitude on the Earth’s surface. It is at a right angle to the celestial equator, and is divided into 24 one-hour units. Just like time the hours can be sub-divided into minutes and seconds.
Ritchey-Chr├ętien Telescope
A Cassegrain that has two hyperbolic mirrors which causes a reduction in the amount of coma. These are excellent for photography.


Setting Circles
Circular scales attached to the telescope. They are marked off in degrees of Declination and hours of Right Ascension. Together, the circles allow the position of a known object to be found by setting the dials to the equatorial coordinates.
Solar System
This term is used to describe the Sun and all the objects in orbit, that are bound by the gravity of the sun. Extrasolar systems are sun and planet systems other than our own.
Spherical Aberration
A blurring of the image caused by the inability of a spherical mirror to focus all light from infinity to one focal point. Light rays from the edge of the spherical mirror focus to different points than those from the centre.
Star Cluster
A group of stars that travel together through space. See Globular Cluster and Open Cluster.


True Field
How much sky, in angular measure, is available at the eyepiece. It is contrasted with Apparent Field, which measures the field of the eyepiece alone.


Wide-field Eyepiece
An eyepiece with an Apparent field of more than 50 degrees.


The point in the sky that is directly overhead.
Zoom Lens
An eyepiece which provides a variable focal length and therefore variable magnification.