Achromat / Achromatic.
Usually, a two element telescope lens, or eyepiece, designed to reduce the false colour effects caused by chromatic aberration.
Airy Disk
This is the central spot of the image of a star, surrounded by diffraction rings, formed when viewed in a telescope. Because of the large distances of stars, it is impossible to resolve a star into a disk, no matter what the magnification. This is due to diffraction. The disk formed by the diffraction of stellar images is known as an Airy Disk, after the English Astronomer Royal, George Airy.
Altazimuth Mount
This telescope mount allows movement in two directions: azimuth ( horizontally ), and elevation ( vertically ). This is a common mounting, though tracking objects is more difficult, as it requires moving the telescope in both axes, which can be difficult. Dobsonian Telescopes are mounted this way.
This is the diameter of the objective lens, or primary mirror usually expressed in millimetres, or sometimes in inches. The larger the aperture, the greater the light gathering ability of the telescope.
Apochromat / Apochromatic
Usually, a three element telescope lens, or eyepiece, designed to provide a high degree of correction for false colours caused by chromatic aberration.
This is a defect in a lens, or mirror, which causes light, which is off the central axis of the lens or mirror, to form an ellipse, or straight line, instead of being brought to a point focus. A lens designed to avoid such defects is known as an Anastigmat.
Barlow Lens
An extra lens, used in conjunction with a telescope’s eyepiece to increase the magnification, usually by a factor of two or three times, by increasing the effective focal length of the telescope. Named after the English physicist Peter Barlow.
A reflecting telescope comprising a primary mirror with a central hole through which the light from the primary mirror is reflected to an eyepiece at the focus, the cassegrain focus, beyond the primary mirror. The design is often used in compact and portable telescopes.
A series of telescopes comprising a mirror with a full aperture lens ahead of it, which is used to correct for spherical aberration in the primary mirror.
Chromatic Aberration
This is the name given to false colour fringes, or coloured halos, around objects, and is caused by unequal refraction of light of different wavelengths being brought to a focus at different distances from the lens. This is a problem commonly associated with refracting telescopes. The use of achromatic or apochromatic lenses greatly alleviates the problem, but does not completely eliminate it.
The process of aligning the optical system of a telescope so that the light gathered is brought to a focus at the correct position.
A defect in an optical system which gives rise to a blurred, pear shaped, comet-like image at the edge of the field of view.
Good image contrast is desirable for viewing low contrast objects such as the lunar surface and planets. Newtonian and catadioptric telescopes have secondary (or diagonal) mirrors that obstruct a small percentage of light from the primary mirror. Light scattering and diffraction from such obstructions can cause a reduction in image contrast. It is commonly believed that image contrast is severely reduced with Newtonians or catadioptrics because of this obstruction, but this is not the case. This would only be true if more than 25% of the primary mirror’s surface area was obstructed by the secondary.

Seeing conditions (or air turbulence) is the single most important factor that affects image contrast when seeking planetary detail through a telescope. Instrument problems that can also adversely affect contrast in order of decreasing importance are: collimation, baffling and a small increase in central obstruction. Note that the increase in central obstruction is rated as the lowest contributor adversely affecting contrast.
This is an set of auxiliary mirrors so arranged as to always direct light to a fixed position, usually the polar axis, irrespective of the actual position of the telescope. It is used with instruments, such as spectrographs, which are too large to mount on the telescope itself.
A system for measuring the altitude of a celestial object, expressed as degrees north, or south, of the celestial equator. Angles are positive if a point is North of the celestial equator, and negative if South. It is used, in conjunction with Right Ascension, to locate celestial objects.
The spreading out of light as it passes over a sharp edge, or through a narrow (in terms of wavelength) slit. The obstructions in the tube of a telescope can give rise to such effects, thereby reducing image contrast and giving rise to rings and ‘spikes’ around stellar images, which should be purely point-like.
The means of automatically compensating a telescope for the Earth’s rotation so that it always points to the same place in the sky.
Dobsonian Telescope
Named after the American astronomer, John Dobson, this is a Newtonian telescope mounted onto a form of altazimuth mount such that the mount is at the mirror end of the telescope. This design allows large aperture telescopes to be made for relatively low cost, when compared to other mounts.
A lens system comprising two elements, used to reduce chromatic aberration.
Equatorial mount
A method of mounting a telescope whereby one axis is parallel to the Earth”s axis, and points to the celestial north pole. This is known as the polar axis. The other axis is mounted at right angles to the polar axis, and is therefore parallel to the plane of the Earth’s equator. This is known as the declination axis. Such a mounting system allows an object to be kept in the centre of field of view by moving the polar axis only. If the polar axis is driven at the sidereal rate, it will compensate for the rotation of the Earth, and keep the object centred in the eyepiece.
Exit pupil
The width of the beam of light exiting the eyepiece. It is equal to the aperture divided by the magnification. If it is larger than the size of your pupil in the dark, around 5 or 6mm, you will not be using all the available light effectively.
Sometimes known as an ocular, the eyepiece is a system of lenses closest to the eye. Its purpose is to magnify the image at the focus of the telescope. The magnification of an eyepiece can be found by dividing its focal length into that of the telescope, provided the units of measurement are the same in each case.
There are several types of eyepiece designs.

The most popular are:
  • Kellner: These are low cost and the most popular for cheap telescopes. They have short eye relief and narrow fields of view. Best avoided if you can afford better ones.
  • Orthoscopic: These have good all round price / performance.
  • Erfle: These have a wide field of view, and are expensive.
  • Plossl: These are a good compromise and, perhaps, offer the best all-around price / performance.
A small telescope, with a wide field of view, mounted on the main telescope tube to enable an observer to easily locate celestial objects, and place them within the field of view of the main telescope.
Focal Length
The distance between the objective lens, or primary mirror, and its focus, or focal plane.
Focal Ratio
Focal ratio, or f number, is the focal length of a lens, or mirror, divided by its diameter. A focal ratio of 8 would be written as f/8. An f/8 telescope is “slower” than an f/4. Fast telescopes give wider, brighter images with a given eyepiece than slower ones. In general, the slower the telescope, the more forgiving it is of defects in the objective / mirror and eyepiece.
The mechanism which holds the eyepiece and allows adjustment for focussing the image.
German Mount
A variant of the equatorial mount.
Reference marks, or measuring scale, placed at the focal plane of a telescope to aid object centring, or to make measurements.
An optical component of telescopes, binoculars and cameras which is made of a transparent material, and is so shaped as to bend light in such a way as to aid the formation of an image.
Light Bucket
A slang term given to large aperture telescopes such as dobsonians.
Limiting Magnitude
The faintest object that can just be detected by a telescope.
This is the degree to which the image of an object is enlarged by a particular telescope / eyepiece combination. The magnification can be calculated by dividing the focal length of the telescope by the focal length of the eyepiece in use.
Nagier Eyepiece
An eyepiece having a very wide field of view, typically greater than 80 degrees. Particularly suitable for comet hunting.
Usually refers to a type of reflecting telescope based on a parabolic mirror. First invented by Sir Isaac Newton in 1668. Newtonian telescopes are the most common types used for amateur astronomy.
The main light gathering element in a telescope or binoculars. In a refractor, this is the large lens at the front, sometimes called an object glass. In a reflector, it is a mirror.
Primary Mirror
The principal light gathering mirror in a reflecting telescope.
A telescope in which the main light gathering element is a mirror. The most common type for amateur astronomy is the Newtonian, but there are also Cassegrain, Schmidt and Maksutov types found in common use.
A telescope in which the main light gathering element is a lens, known as the objective, or object lens.
A wide field reflecting telescope which uses a sperical mirror and correcting plate instead of a parabolic mirror. Mainly used for photographic sky surveys.
A device used to attach a fork mounted telescope to a tripod.