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Any optical defect and/or design error which causes any of the processed light to deviate from reaching the focal point, therefore reducing the quality of the image.

Absolute Magnitude
The apparent brightness a star would have if placed at a distance of 10 parsecs from the earth.

Achromatic Lens
A refractor lens, made of two or sometimes three separate lenses, which has the effect of bringing most of the viewed colours to a sharp focus, thus reducing chromatic aberration.

A simple mount that allows movement in altitude (up and down) and in azimuth (side to side).

Antireflection Coating
A thin layer of film applied to an optical surface that reduces the loss of transmission of light.

The diameter of the primary mirror or lens.

A group of stars that appear to make an easily recognized shape, such as the "Big Dipper" or the "Coathanger".



Barlow Lens
A "negative" lens which, when placed in front of the eyepiece, increases the focal length and magnification and decreases the field.


Catadioptric System
A system using both lens and mirror components to produce an image, allowing these telescopes to be more compact than other designs.

Celestial Sphere
An imaginary ball with the earth at its centre. All astronomical bodies, disregarding their true distance, are assigned a two-dimensional location 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 haloes around bright objects. A well-made achromatic lens reduces this problem.

Coated Optics
In lenses this is an antireflection coating. In mirrors a coating is applied that preserves the aluminum mirror surface.

The process of aligning all the elements of an optical system. Collimation is routinely needed in reflectors, often in Catadioptric systems but seldom in refractors.



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 Cap
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.

Double Star
Two or more stars that appear very close in position. 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.

Equatorial Mount
A telescope mount with an axis parallel to the axis of the earth. This provides easy tracking of sky objects and for photography when combined with a clock drive.

Also called an ocular. This is a small tube that contains the lenses needed to bring a telescope's focus to a final image in the eye. Telescopes usually come with at least two eyepieces: one for low power and a second for a higher power view.

Eye Relief
The distance between the eyepiece lens and the position in which the eye must be placed to see through the telescope. Telescope users who wear eyeglasses while observing, appreciate the benefits of longer eye relief.

Exit Pupil
This is the diameter of the beam of light from the eyepiece which reaches the pupil of the eye. It is usually expressed in mm, and determined by dividing the diameter of the primary (in mm) by the Magnification. Knowing this value and the diameter of your dilated pupil allows you to choose the eyepieces which will work best for you with a specific telescope.



Field of View
The maximum view angle of an optical instrument. The number, in degrees, supplied by the manufacturer is the Apparent Field of View. To find the True Field of View (also known as the Actual Field of View), divide the Apparent Field of View by the Magnification.

This is usually a disk of coloured glass or film that sits in front of the telescope eyepiece or objective. It transmits only certain wavelengths of light while rejecting others. (It is important to remember that a Solar filter must always be placed in front of the objective.)

A low power telescope attached parallel to the main instrument which provides easy object locating and telescope aiming.

Focal Length
The distance of the light path from the objective (primary lens or mirror) to the convergence of the beam. The convergent spot is called the Focus or Focal Point.

Focal Ratio
This is found by dividing an optical system's Focal Length by its Aperture. The resulting value is sometimes called the system's "speed".

A device which brings the light rays in a telescope to a precise focus. Common designs include geared (rack-and-pinion), gearless (Crayford-style) and helical.




Galactic Coordinates
A system of latitude and longitude defined by the plane of our galaxy rather than the equatorial system (RA and DEC) based on the celestial equator. Coordinates can also be specified locally, for example by Altitude and Azimuth.

Globular Cluster
A very old, large, dense cluster of stars, bound by gravity. Many form spherical clouds around galaxies. Our galaxy is surrounded by at least 130 globular clusters.


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


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

The bright flash of light seen when a piece of material from space (a meteoroid) burns up in the earth's atmosphere. A piece of this material which reaches the ground, is called a meteorite.

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.



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.


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 gravitationally. They are usually about the same age, having being born together from a collapsing nebula.

Optical Tube Assembly
The housing and optical train of a telescope; not including the mount, diagonal, eyepiece or accessories.



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.

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 Magnifying Power.

Prime Focus
The focal point of the objective mirror or lens.



The ability of an optical system to reveal details.

Resolving Power
The ability of a telescope to separate closely positioned points.

Right Ascension
Similar to but not the same as Latitude on the Earth's surface. It is the position eastwards from the Vernal Equinox, in 24 one-hour units. The hours can be sub-divided into minutes and seconds.



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.

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.


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


The point in the sky directly overhead.

Zoom Lens
An optical system which provides a variable focal length.


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