Glossary of Common Astronomical Terms

Achromat Another name for an achromatic lens (q.v.)

Achromatic: Literally, “without colour”

Achromatic lens: A lens designed to produce an image that has true colour and is free of the rainbow colours that border the images produced by simple lenses. An achromatic lens consists of two layers, made of different glasses, fitted together. One lens is convex, while the other is concave so that it fits closely against the first. The lens is designed so that the colour distortion produced by one element of the lens is corrected by opposite distortions produced by the other element.

Air-spaced: Sometimes the elements of an achromatic lens are not cemented together but are separated by a thin film of air, which can be so thin that the two elements are in contact. Large objective lenses for telescopes are almost always air-spaced.

Altazimuth: A mounting for a telescope or camera, which has two mutually perpendicular axes, one horizontal and the other vertical. This permits movement both horizontally and vertically. A camera tripod is an example of an altazimuth mount.

Amici roof prismInvented 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 that produces 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: Canada balsam is a colourless, transparent resin that is used to glue optical components together.

Bloomed: See ‘coating’.

Cemented: The two glass elements of most achromatic lenses are held together by transparent cement. Canada balsam was the traditional material but this has been superseded by modern adhesives. Canada balsam has the advantage that it can be melted over a hot-water bath, so the two components of the lens can be separated later if necessary. Modern adhesives tend to form a permanent bond.

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, transparent and colourless coating applied to their surfaces to minimise reflection of light. This enables more of the light to pass through the lens. Such coatings often appear coloured when viewed by reflected light. The blue or purple tinge of binocular lenses is due to this anti-reflection coating.

Diagonal See ‘Star diagonal’.

Dial-sight eyepiece See ‘Plossl eyepiece’.

Doublet: A lens made from two layers, one of crown glass, the other of flint glass. Doublets are thicker than simple lenses and the join between the two elements is visible at the edge. All achromatic lenses are doublets, but not all doublets are achromatic!

Draw-tube: The sliding tube of a telescope that permits the 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 which has two mutually perpendicular axes, one of which can be pointed to the celestial pole. This type of mount permits the movement of a star across the sky to be more easily followed than with an altazimuth mount.

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 one element, such as the Tolles, two elements, as in the Ramsden, through four elements as in the Plossl and Orthoscopic, to six elements as in the Erfle.

Eye-relief: The distance from the surface of an eyepiece to the position where the observer’s eye sees the greatest field of view. A too-short eye-relief makes the eyepiece uncomfortable to use because the observer’s eye has to be pressed against the eyepiece. A long eye-relief permits an observer to wear spectacles whilst looking through the eyepiece.

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 telescope of low magnification whose eyepiece is fitted with a pointer or cross-hair. This auxiliary telescope is mounted on a larger astronomical telescope and is used to help point the larger telescope to the desired astronomical 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: This is the distance from the edge of a lens to the point at which light from a very distant object is focused. The focal length of a complex system of lenses, such as an eyepiece, is the focal length of the single lens that would produce the same degree of magnification.

Focal plane: The imaginary surface on which a lens projects a focused image. It is separated from the lens by a distance equal to the focal length.

Focal ratio: The diameter of a lens expressed as the focal length divided by a suitable number. For example a lens 4″ in diameter with a focal length of 60″ is said to be a f/15 lens, because 60/15 = 4. Focal ratio is a useful way of describing the ‘speed’ of a lens. Lenses of low focal ratio (say f/2) when used for photography require shorter exposure times, and are thus ‘faster’ than lenses of high focal ratio (say f/15). Higher focal ratios, however, permit greater magnification and usually cause less distortion of the image.

Graticule: A thin disc of glass on which a cross or a scale is engraved. This is fixed inside an eyepiece (as in a rifle sight) and enables the optical system to be lined-up on a target, such as a star. A star finder eyepiece usually contains a graticule.

Guide-scope: A very high magnification telescope fixed to an astronomical telescope. It is used to keep the main telescope fixed on some astronomical object while the main telescope is being used for photography.

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 The large lens at the front of a telescope. It is usually achromatic and is fixed with its most curved surface facing outward from the telescope. It is often called an O. G. (Objective Glass).

Ocular: Another name for eyepiece.

Orthoscopic: A type of 4-element eyepiece which consists of a triplet lens with a singlet eye lens.

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

Plössl Eyepiece A type of 4-element eyepiece in which two achromatic doublets are disposed with their most curved surfaces facing towards one another, and almost touching. This four-element ocular is characterised by a flat field of view and a long eye-relief. It is also called the symmetrical, or dial sight eyepiece. It was designed by Georg Simon Plössl in 1860.

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 A type of eyepiece in which two identical plano-convex lenses are disposed with their curved surfaces towards one another, separated by a distance equal to the focal length of each lens. It is named after Jesse Ramsden.

Refractor: A telescope in which the light from a distant object is gathered and focused by a lens.

Reflector: A telescope (almost always astronomical) in which the light is gathered and focused by a concave mirror.

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 path of the light, from the objective lens of a refractor, through a right-angle. This makes the telescope more comfortable to use when it is pointed upwards because the observer looks downward into the eyepiece, instead of having to crouch under the telescope and look upward into the eyepiece.

Star-finder See ‘Finder’.

Symmetrical eyepiece See ‘Plossl eyepiece’.

Terrestrial telescope A telescope that produces an erect image. This is achieved by the use of extra lenses inside the telescope, which unfortunately also absorb some of the light. To avoid light-loss astronomical telescopes (q.v.) do not have these lenses.

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 7×50 binoculars, for example, should have a true field of view of 71/2 degrees.