Glossary of Common Astronomical Terms

Achromatic 'Without colour'.

Achromatic lens A lens that is designed to produce a view or image that is free of the fringe of rainbow colours that surrounds the images produced by simpler lenses. An achromatic lens produces good colour correction because it is made of two lenses pressed together - a convex lens and a concave lens. The colour distortion produced by one of these components is cancelled-out by the opposite distortions produced by the other component. Since an achromatic lens consists of two components, it is often called a 'doublet'. You can identify a doublet by looking at the edge of the lens. The join between the two components is clearly visible.

Air-spaced This refers to the type of achromatic lens where the two components are just pressed together, not glued ('cemented') together. The air space between the two components of the lens can be so thin that the two components are touching.

Altazimuth A mounting for a telescope or camera that has two, mutually perpendicular axes of rotation - one horizontal, the other vertical. This permits the telescope or camera to be slewed vertically and horizontally. A camera tripod is an example of an altazimuth mount.

Amici roof prism Invented by Giovanni Battista Amici (1786-1863). This type of reflecting prism is used to divert the light through 90 degrees and at the same time it completely inverts the image. It is used in a telescope to produce a view that is the right way up and the right way round.

Anti-reflection coated See 'Coated'

Apochromatic lens A lens that consists of three components. This enables lenses with short focal lengths to enjoy the same freedom from colour distortions as the longer focal length achromatic lenses.

Apparent field of view The apparent field of view of any eyepiece is the angle through which the observer would need to move his eye in order to see the entire width of the view. Good quality eyepieces, such as Plossls, have an apparent field of view greater than fifty degrees, while Erfle eyepieces can have an apparent field of view up to eighty degrees.

Astronomical telescope A telescope whose optical construction is designed to minimise light loss within the optical path. Such telescopes often produce an inverted image.

Baffles Baffles are thin metal plates with a central hole that are fitted inside the main tube and draw tube of a telescope. The size of the hole is just large enough to allow the eyepiece to view the whole diameter of the objective lens. There are usually several baffles along the length of the telescope. Their purpose is to prevent stray light, reflected from the inside surfaces of the telescope, entering the eyepiece and thus reducing the quality of the image.

Balsam This is more correctly called Canada balsam. It is a natural, colourless resin that is used to glue optical components, such as achromatic lenses, together.

Bloomed See 'Coated'.

Cemented The components of most small achromatic lenses are glued together ('cemented' together) with a transparent adhesive. Canada balsam was the traditional adhesive, but it has been somewhat replaced by modern, UV-curing adhesives.

Chromatic aberration This is the phenomenon of rainbow colours surrounding a view seen through a telescope. It is most noticeable when the object being viewed is in sharp contrast to its surroundings. Chromatic aberration is caused by the fact that light of differing colour is brought to a focus at slightly differing distances from the lens. Achromatic and apochromatic lenses attempt to correct this defect.

Coated Some lenses have a thin, colourless coating evaporated on to their surfaces to minimise the reflection of light from the glass surface. This enables more of the light to pass through the lens. Such coatings often appear blue, purple or green when seen in reflected light. The coloured sheen seen on the lenses of binoculars is due to this coating. The lenses are said to be 'coated' or 'bloomed'.

Diagonal See 'Star diagonal'.

Dial-sight eyepiece See 'Plossl eyepiece'.

Doublet A lens made of two, closely-fitting lenses. The front component of the pair is made of crown glass and is convex on both surfaces, like a magnifying glass, while the rear component is made of flint glass and is concave, so that it is a perfect fit against the front element. Doublets are thicker than simple lenses and the join between the two layers of glass is clearly visible on the edge of the lens.

Drawtube The sliding tube of a telescope that permits the telescope eyepiece to be moved to such a position that the telescope is in focus.

Element Each of the components of a lens or other optical system is called an element of that system. Achromatic lenses, for example, consist of two elements - a convex lens and a concave lens.

Equatorial A mounting for a telescope that has two, mutually perpendicular axes of rotation, one of which can be pointed to the north celestial pole. This type of mount permits the telescope to follow the movement of a star as it travels across the sky.

Erfle eyepiece Invented by Heinrich Valentin Erfle (1884-1923). This is a type of eyepiece that consists of five or even six elements. It can easily be identified by the fact that its field lens is invariably concave. It has the widest field of view of all the eyepieces and gives a breath-taking view. It suffers from a rather short eye-relief and is always expensive.

Eye-lens This is the lens within an eyepiece that is nearest to the observer's eye.

Eyepiece A lens, or more usually a combination of lenses, whose function is to magnify the image produced by the primary lens or mirror of a telescope. There are many patterns of eyepiece, ranging from two-element designs such as the Ramsden and the Huyghenian, to four element (the Plossl) and six element (the Erfle) eyepieces.

Eye-relief This is the distance from the surface of the eyepiece lens to the place where the observer's eye sees the greatest field of view. If the eye-relief is too short the eyepiece is uncomfortable to use, because the observer has to press his eye against the eyepiece. Also, spectacle wearers would have to remove their spectacles in order to see through such an eyepiece. The more complex eyepieces tend to have longer eye-reliefs.

False colour See 'Chromatic aberration'.

Field-lens This is the lens within an eyepiece that gathers the light from the objective lens or mirror. It is the eyepiece lens that is nearest to the objective lens or the primary mirror.

Finder More usually called a star-finder. It is a small, low-magnification telescope, whose eyepiece is fitted with a pointer or cross-hair. This small telescope is mounted on the astronomical telescope and is used to help point the larger telescope at any desired object.

Fluorite This is a solid, transparent chemical substance that is used to make one of the elements in a triplet, apochromatic objective lens. Such a component is very delicate.

Focal length The focal length of a lens is the distance from the lens to the place at which light from a distant object is brought to a focus. The focal length of a lens is decided by the curvature of its glass surfaces - the more curved the surfaces, the shorter the focal length.

Focal plane When a lens brings light to a focus, the image produced by the lens lies on an imaginary plane, which is the same distance from the lens as the focal length.

Focal ratio This is the relationship between the diameter of a lens and its focal length. It describes the 'speed' of the lens. For example, a lens with a focal length of 60cm and a diameter of 4cm has a focal ratio of f/15, because 60cm divided by 4cm is 15.

Graticule A glass disc that is inserted at the focal plane of an eyepiece. The disc is engraved with a cross, circle and dot or a scale, so that the eyepiece can be used to centre upon, or measure, an object seen through the telescope.

Guide scope A high-magnification telescope that is fixed to an astronomical telescope. It is used to keep the astronomical telescope exactly aligned with an astronomical object, such as a star, while the astronomical telescope is being used to photograph the object.

Huyghenian eyepiece Invented by Christian Huygens (1629-1695). This is a low-cost eyepiece, often marked with 'H' or 'HM'. It consists of two simple lenses that are separated in the eyepiece by more than their focal lengths. The effect of this is that the focal plane of the eyepiece lies between the two lenses. The consequence is that the eyepiece cannot be used as a magnifying glass. Huyghenian eyepieces have a short eye-relief and a narrow field of view. They are perfectly acceptable in microscopes, but generally perform poorly in telescopes.

Kellner eyepiece Invented by Carl Kellner (1826-1855). This is another low-cost eyepiece, often marked 'K'. It consists of three elements - an achromatic eye-lens spaced apart from a simple (usually planoconvex) field lens. Kelner eyepieces generally work well in telescopes.

Mount The mechanical device that connects the telescope to its tripod and allows the telescope to move freely. The commonest types of mount are the 'altazimuth' and the 'equatorial'.

Objective See 'Objective lens'

Objective lens This is the large lens at the front of the telescope. It is invariably either achromatic or apochromatic. Simple lenses are never used nowadays as the objective lenses of telescopes. It is often called 'the OG' (objective glass).

Ocular Another name for 'eyepiece'.

Orthoscopic eyepiece Invented by Ernst Abbe (1840-1905). Often marked 'O' or 'Or'. This is a four-element eyepiece, which consists of a triplet field lens and a singlet eye-lens. It performs excellently in telescopes.

Planoconvex This describes the shape of a lens that is flat on one face and convex on the other.

Plossl eyepiece Invented by Georg Simon Plossl (1794-1868). An eyepiece often marked with the letter P. This type of eyepiece consists of four elements, in the form of two achromatic, doublet lenses. The two doublets are disposed with their most-curved surfaces almost touching. The Plossl eyepiece is probably the easiest eyepiece for the home constructor to make. It is characterised by a long eye-relief and a wide, distortion-free field of view. It is also known as the 'dial-sight' or 'symmetrical' eyepiece.

Rack-and-pinion focuser A device that is used to move the drawtube of a telescope so that the eyepiece is correctly focussed. A toothed strip (the rack) is attached to the drawtube and a toothed wheel (the pinion) is meshed with the rack and is rotated by a handwheel on the side of the telescope. The rotation of the pinion causes the rack, and thus the drawtube, to move forwards or backwards.

Ramsden eyepiece Invented by Jesse Ramsden (1735-1800). An eyepiece often marked with the letters 'R' or 'SR'. The eyepiece consists of two elements, in the form of two identical planoconvex singlet lenses. The two lenses are disposed with their curved surfaces facing one another, separated by a distance equal to the focal length of the lens. Ramsden eyepieces perform well but have a short eye-relief.

Refractor A type of telescope in which the light from a distant object is gathered by a large lens at the front of the telescope, which then focuses the light on to the eyepiece.

Reflector A type of telescope in which the light from a distant object is gathered by a large, concave mirror at the rear of the telescope.

Relay lenses A set of lenses inside a telescope, between the objective lens and the eyepiece. The purpose of this set of lenses is to turn the telescope image the right way up and the right way round.

Spherical aberration This is the distortion to the view, seen through a telescope. It is due to the lenses being inappropriate for the design of telescope. 'Barrel distortion' is a curvature of the view, where the top, bottom and sides of a flat object, such as a wall, seem to bulge outward. The opposite type of distortion is called 'pin-cushion' distortion.

Singlet A lens that is made of only one piece of glass. It is a single-element lens.

Star diagonal A device that contains a mirror or right-angle prism whose purpose is to turn the light from the objective lens of the telescope through a right-angle. This makes the telescope more comfortable to use when it is pointed upwards. The star diagonal is an accessory that fits in the rear of the drawtube.

Star-finder See 'Finder'.

Symmetrical eyepiece See 'Plossl eyepiece'.

Terrestrial telescope A telescope that produces an image that is the right way up and the right way round. This is achieved by the use of a set of relay lenses within the telescope or by a prism. To avoid light loss within the telescope, astronomical telescopes do not have these extra lenses or prisms.

Triplet A lens that is made of three components. Small triplets are always cemented, but the large triplets that are used as the objective lenses for apochromatic refracting telescopes are usually air-spaced. If you are buying a telescope that has such an objective lens, check that the fluorite component is not the front element of the lens. Fluorite is delicate and if your lens has this component as its front element, you could be buying into a lot of trouble.

True field of view This is the angle subtended by the width of the view seen through the instrument. A good pair of 7x50 binoculars, for example, should have a true field of view of 71/2 degrees.