Glossary of Some Binocular & Scope Terms  Back To Knowledge Bank Index Aberration. An optical error caused by an imperfect optical surface created during manufacture, assembly error, or refraction. Aberrations reduce image quality in different ways, reduction in image sharpness and contrast being the most objectionable. Aberrations that can severely affect image quality in consumer optics include Coma, Astigmatism, Spherical Aberration, Chromatic Aberration, Field Curvature and Distortion. High performance binoculars and spotting scopes have optics that are designed to be free of most, if not all of these aberrations at the centre of the image. With modern high performance instruments, it is usually poor quality control rather than poor design that can lead to the presence of image deteriorating aberrations. Absolute MagnitudeThe apparent brightness a star would have if placed at a distance of 10 parsecs from the earth.Achromatic. Refers to a lens pair or group that is designed to bring two wavelengths of light (red and blue) to the same focal position. White light is made up of many wavelengths (colours), and glass lenses create a dispersion of colour because a lens has a different focal length for each wavelength. Achromatising an objective lens greatly reduces the affect of dispersion on the image. Residual unfocused wavelengths in the on-axis image that are not corrected, are visible as diffuse colour fringing, contribute to the image as aChromatic Aberration and has the term Secondary Spectrum.   Alt-azimuth. A simple mount that allows movement in altitude (up and down) and in azimuth (side to side). Angled scope. A spotting scope where the prisms direct the cone of light out at 45 degrees to the axis of the objective lens. In many cases an angled scope is more comfortable and convenient to use then a straight through type. Antireflection Coating. A thin layer of film applied to an optical surface that reduces the loss of transmission of light. Aperture The diameter of the objective lens or mirror in an optical telescope or binocular. Also see Entrance Pupil. Apochromatic. Refers to a lens pair or group that is designed to bring three wavelengths of light (colours) to the same focal position. Also see Achromatic. The result is an image where no visible chromatic aberration reduces image quality. Apochromatic is a loosely used term in the consumer optics market, sometimes used to describe an achromatic telescope for marketing purposes, as well as telescopes that deserve the term apochromatic. Refracting spotting scopes that are described as apochromatic usually have a denotation of ED(Extra Low Dispersion) , FL (Flourite) or HD (High Definition). Catadioptric spotting scopes are considered to be apochromatic as the main image forming optics are reflecting.  Although we don't usually refer to reflecting optics when we use the term apochromatic, it is nevertheless true to say that a true apochromatic telescope objective contains reflecting optics only, as reflection of light does not disperse individual wavelengths. In reality, catadioptrics and ED spotting scopes all have a tiny amount of chromatic aberration, but it is of such a low order as to be not noticed in the image.  AsterismA group of stars that appear to make an easily recognized shape, such as the "Big Dipper" or the "Coathanger".B. (1) Refers to a particular eyepiece type common on modern binoculars. Typically a B type eyepiece has an eye relief long enough for spectacle wearer's to be able to see the edge of the field of view. (2) Refers to a design style of Porro prism binocular, where each half of the binocular is cast as a single solid chamber. The B is an abbreviation of Bausch & Lomb, the US company that popularised the design. Barlow LensA "negative" lens which, when placed in front of the eyepiece, increases the focal length and magnification and decreases the field.BCF. A binocular with one of the B references above, plus central focusing.BGA. A binocular with one of the B references above, and is rubber armoured. Also see Gummiarmierung.BIF. A binocular with one of the B references above, plus individual eyepiece focusing. 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. An optical error created by white light passing through a lens and dispersing into separate wavelengths (colours). A lens focal length is wavelength dependent. This means different wavelengths come to focus at slightly different positions on the focal plane. The result is unfocused light of certain wavelengths also visible in the image made up focused light from some other wavelengths.  The chromatic aberrations that can be present in consumer optical instruments include Secondary Spectrum, Lateral or Transverse Colour and Spherochromatism.  Coated OpticsIn lenses this is an antireflection coating. In mirrors a coating is applied that preserves the aluminum mirror surface. CollimationThe process of aligning all the elements of an optical system. Collimation is routinely needed in reflectors, often in Catadioptric systems but seldom in refractors.DCF. An abbreviation of Dach with central focusing. Dach is German for roof and is used by some manufacturer's to denote a roof prism binocular with central focusing. DeclinationSimilar 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 SkyA name given by amateur astronomers to objects beyond our Sun and its planets.DefinitionA term used loosely as a reference to the perceived sharpness and contrast of the image. Whereas we can use terms like resolution and contrast to highlight particular areas of image quality, the term definition is more of an umbrella term, covering the overall image quality. A spotting scope or binocular which is described as high definition, generally means the image is sharp with high contrast, free from chromatic aberration and free from any internal light reflections and veiling glare. Depth of FieldThe depth of distance in the field of view of a binocular or spotting scope in which objects and detail are in focus. The depth of field applies to both close focus images and far distance, in that an instrument with greater depth of field over another instrument when both are focused to distance, will also have the greater depth of field at close focus. With photographic lenses we are used to the relationship between the focal ratio (photographic aperture) of the lens and the consequential depth of field. With binoculars and telescopes the depth of field is determined by magnification and physical aperture (objective or entrance pupil diameter). Similarly, because of the relationship between entrance pupil and exit pupil, we can also consider that depth of field is determined by magnification and exit pupil diameter. Entrance pupil diameter and magnification, or exit pupil and magnification, it amounts to the same thing. This means that two binoculars that share the same aperture and magnification (and hence the same size exit pupil), will have the same depth of field. If we look at three binoculars all with 7X magnification, one with an exit pupil of 4mm, another with 5mm and the other with 6mm. This means they all have different apertures. The largest aperture has the largest exit pupil, and its images will have the greater depth of field. Alternatively, we can look at this the other way and choose three binoculars with the same aperture, one having an exit pupil of 4mm, one with 5mm and one with 6mm. This means they all have different magnification. The lowest magnification binocular has the largest exit pupil, and its images will have the greatest depth of field.  However, the above is true for the instruments only. Once the pupil of the eye is introduced into the equation we have another contributor into the optical train. If the pupil of the eye is smaller than the exit pupil, it becomes the restricting aperture that determines depth of field. E.g. A 7X50 binocular used in light conditions where the pupil of the eye is 5mm, has the depth of field reduced to the equivalent of a 7X35 binocular. An instrument with good depth of field is not necessary for astronomy, as all objects are at infinity. For terrestrial uses a good depth of field is useful, particularly for subjects like bird watching, general nature observation and hunting. Dew CapA 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. DiagonalA mirror or prism system which changes the angle and orientation of the light rays coming from the telescope to the eyepiece. Double StarTwo 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. Dioptre adjustment or Dioptre SettingA design feature of all central focus binoculars. It is the facility, usually on the right eyepiece, that enables separate focusing between the right and left eyepiece. This is necessary as many binocular users have subtly different focus position requirements for individual eyes. When using an unfamiliar binocular, the observer uses the central focusing wheel to focus the image for the left eye first, then the dioptre setting on the right eyepiece, so that both eyes see a sharp image. From that point on, only central focusing is needed for viewing objects at different distances. There is usually a dioptre scale on the right eyepiece so that a personal setting can be noted and set more permanently. Some modern roof prism binoculars make a feature out of a separate section on the central focus wheel as the dioptre adjustment, instead of turning the right eyepiece. EclipseThe blocking of one astronomical body by another as seen from the earth. The most common of these events are Solar and Lunar eclipses. Equatorial MountA 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. EyepieceAlso 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. Entrance pupilRefers to either the objective lens or mirror aperture, or any aperture-restricting stop. E.g. A spotting scope objective lens has a clear aperture of 60mm. The manufacture adds a light baffle ring at the rear of the objective cell, that happens to cut into the converging cone of light by 1mm, restricting the effective aperture to 58mm. This ring becomes the entrance pupil as it now defines the true aperture instead of the objective lens aperture. The exit pupil in the eyepiece is an image of the entrance pupil.Exit Pupil. A small bright disc of light that can be seen floating in the eyepiece of a binocular or spotting scope, when the eyepiece is looked at rather than through. The exit pupil is an image of the Entrance Pupil. The exit pupil is actually a pencil of light exiting the eyepiece. The diameter of the exit pupil is found by dividing the magnification into the aperture in millimetres. E.g. A 10X50 binocular has a 5mm exit pupil (50 divided by 10). EyepieceAlso 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 ReliefThe property of an eyepiece that dictates at what distance from the eyepiece the eye can be positioned in order to be able to see the edge of the field of view. At too great a distance the edge of the field of view is hidden by the physical constraints of the eyepiece. Too close, and the edge of the field is swamped by image tunnelling. Spectacle wearers require greater eye relief. Manufacturers of binoculars and spotting scope eyepieces normally cater for the varied requirements of different users, by designing eyepieces with either folding soft rubber eyecups or retractable stiffer eyecups. Some eyepieces or denoted as LER meaning Long Eye Relief.Field Of View. The expression in either linear measurement increments, or angular measurement increments of the visible field of an instrument. TheTrue or Real field refers to the width of the field of view at a given distance from the observer. The Apparent field is the diameter of an eyepiece field stop expressed in angular increments. FilterThis 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.) FinderscopeA low power telescope attached parallel to the main instrument which provides easy object locating and telescope aiming. Focal LengthThe 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 RatioThis is found by dividing an optical system's Focal Length by its Aperture. The resulting value is sometimes called the system's "speed". FocuserA 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 CoordinatesA 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 ClusterA 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. Gummiarmierung (GA)German word for rubber armouring. Originally a term used on German binoculars, but now sometimes used on Asian manufactured binoculars. Inter-Pupillary Distance (IPD)The measurement of the distance between the pupils of an observer's eyes (centre to centre). With binoculars this corresponds to a term Inter-Ocular Distance, which is the distance the eyepieces must be separated to coincide with the IPD. If this is not correct, the circular shape of the visible field of view can be lost, and vignetting and/or off-axis aberrations are more easily seen, severely degrading image quality. Lanyard. The modern term for a binocular strap, for carrying the binocular around the neck. LensA transparent optical element consisting of one or more pieces of glass. A lens has curved surfaces that bring distant light to a focus. M MagnificationIn simple terms, the size of the image of an object formed by a telescope or binocular, compared to the size of the object seen with the naked eye. Magnification is angular so it is normally expressed as  Magnification = Angular size of an object in the image ÷Angular size of the object viewed with the naked eye. The magnification of a binocular is usually stated on the body of the instrument. With a spotting scope the magnification of the combination of scope and eyepiece  is usually stated on the particular eyepiece used on the scope. If the magnification is unknown it can be found by dividing the focal length of the objective by the focal length of an eyepiece. If no focal lengths are known, magnification can be found by Entrance pupil diameter ÷ Exit pupil diameter.  Magnifying PowerThe 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. MeteorThe 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. MirrorIn 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. NebulaA 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. ObjectiveUsually refers to the objective lens in a binocular or spotting scope, but can also be used to describe the primary mirror in a catadioptric. It refers to the main image forming optic. The area of the aperture of the objective determines the light grasp of the instrument. This means it generally determines how useful the instrument is in poor light, and the image brightness at higher magnifications. The diameter of the objective aperture determines the theoretical resolving limit of the telescope or binocular. However, with spotting scopes and binoculars this resolving limit is rarely if ever realised, as very high magnifications are required for the eye to be able to detect this limit. With low magnification instruments such as binoculars and spotting scopes the term resolution is preferred as this refers to the general sharpness of the image. The optical quality of the surface(s) of the objective (free from optical aberrations) is also important in the instrument achieving high resolution images. The aperture of the objective plays no part in the field of view of the instrument, only the focal length contributes to this. The aperture of the objective is sometimes denoted as D. E.g. D = 80mm. The focal length of the objective as f. E.g. f = 480mm, and the ratio between the two is the focal ratio (f divided by D), and is often written as f/. E.g. f/6.   Ocular. A term occasionally seen used in place of the more common eyepiece. Open ClusterA 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 AssemblyThe housing and optical train of a telescope; not including the mount, diagonal, eyepiece or accessories. Optical coatings. (1) Transmission coatings. The coating of lens and prism surfaces by the evaporation of certain metals onto the optical surfaces in a vacuum chamber. Single layer coatings to lenses usually involve the metal Magnesium Fluoride (MgF2) and has the general application term of Bloomingor Anti-reflection coating (AR coating). Multiple layer coatings are known asMulti-coatings and can involve various metals that serve both as high transmission and protective coatings. At every air to glass and glass to air surface of a lens or group of lenses, there is a reflection of approximately 4% of the incident light. In an optical instrument such as a binocular or spotting scope, the number of surfaces can add up to a major loss of light. A single layer coating to each surface halves the light loss and can transform the image brightness and contrast. Broadband (greater visible spectral range) multi-coatings applied to the same surfaces can reduce the light loss to less than 1% per surface. The optimum coatings aim to provide the user with images that are as close to the naked eye experience as possible. Great advances in the optical coating industry in the last few decades have transformed the image quality possible with modern telescopes and binoculars. High quality carefully considered optical coatings applied to the lenses and prisms by the various manufacturers, is now an essential ingredient of any product in the consumer optics market. (2) Reflection coatings. Catadioptric spotting scopes use mirror surfaces to form the image. The metal most commonly used as a high reflective coating is evaporated aluminium. Aluminising a mirror takes place in a vacuum chamber. Metals such as Silicon Oxide or Silicon Di-oxide are then used as over-coatings to protect the aluminium's reflectivity over time. Modern aluminium coatings can be over 90% reflective, approaching the reflectivity of the much more expensive silver coating. A more recent reflecting coating process is Di-electric coating. This involves multiple layers, each layer reflecting a specific spectral band. The result is near total reflection for the entire visible spectrum. Parabolic MirrorA 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 NebulaA 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 AxisA 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. PowerSee Magnifying Power. Prime FocusThe focal point of the objective mirror or lens. Rainguard. A modern single piece rubber or plastic cover for the eyepieces of a binocular, instead of the traditional separate caps for the eyepieces which can be easily dropped and lost in the field.Resolution. Refers to the fine detail that is visible with a spotting scope or binocular at a given magnification. Whereas the resolving power of a telescope is determined by aperture, and is only realised at very high magnification, the resolution of fine detail is the product of image sharpness due to freedom from optical aberrations. Resolution is then a term that is relevant for terrestrial telescopes and binoculars and for the medium or lower magnifications the instruments are used at. Resolving PowerThe ability of a telescope to separate closely positioned points. Right AscensionSimilar 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 CirclesCircular 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 AberrationA 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 ClusterA group of stars that travel together through space. See Globular Cluster and Open Cluster. Setting CirclesCircular 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 AberrationA 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 ClusterA group of stars that travel together through space. See Globular Cluster and Open Cluster. Setting CirclesCircular 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 AberrationA 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 ClusterA group of stars that travel together through space. See Globular Cluster and Open Cluster. Setting CirclesCircular 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 AberrationA 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 ClusterA group of stars that travel together through space. See Globular Cluster and Open Cluster. True FieldHow much sky, in angular measure, is available at the eyepiece. It is contrasted with Apparent Field, which measures the field of the eyepiece alone. Twilight Factor. A term used as a marketing tool by some binocular manufacturers to quantify the performance of an instrument in low light. The higher the Twilight factor number, the better the low light performance. By performance we mean what detail we can see when the light drops to a low level. The Twilight factor is found by multiplying the magnification by the aperture in millimetres, and then finding the square root of the result. E.g. A 7X50 binocular has a Twilight factor of 18.71  An 8X40 binocular has a Twilight factor of 17.9. This means that even though the 8X40 binocular has a higher magnification, the Twilight factor suggests that the 7X50 binocular will reveal more detail in low light. The error in using the Twilight factor as a deciding factor when choosing a binocular for low light use, is that if taken to logical conclusion, the square root of the product of aperture and magnification means that, for example, a binocular of 100X50 (if such a binocular existed) would have a Twilight factor of 70.71. This binocular should be excellent in low light! It doesn't take much thought to realise that in low light, the image created by a 50mm aperture instrument with 100 magnification would be very dark, and we would be very lucky to see any useful detail at all. Low light performance of a binocular or spotting scope depends on the relationship between aperture, magnification and the diameter of the pupil of the eye. The Twilight factor (as stated and used by modern manufacturers), should therefore be consigned to the Twilight Zone.Vignetting.  The unavoidable failure of a lens, group of lenses or prisms in a binocular or telescope, to provide equal illumination for all points of the image. This can be seen either as lower brightness at the field edge, or, with severe vignetting, a total loss at the field edge. With a total loss, the edge of the field of view is greyed out producing an image with a narrower field. The loss of light with vignetting is caused by light from objects that are off-axis failing to illuminate all image points. Light from objects that are on-axis illuminate all points of the image.W and WW. Modern terminology that denotes a wide field, whether true or apparent field of view of a binocular (e.g. 10 X 50W), or the apparent field of a spotting scope eyepiece (e.g. 20XWW). Sometimes WW is replaced by XW(extra-wide).  Widefield EyepieceAn eyepiece with an Apparent field of more than 50 degrees. ZenithThe point in the sky directly overhead. Zoom LensAn optical system which provides a variable focal length. ZCF. Refers to a style of porro prism binocular, where each half of the binocular is made up of two units joined together; a prism housing and an objective housing. The Z is an abbreviation of Zeiss, the German company that popularised the design. The CF refers to central focusing.Zoom eyepiece. A spotting scope eyepiece with variable magnification. Common magnification ranges on modern spotting scope zoom eyepieces include 15 - 45X, 16 - 48X, 20 - 50X, 20 - 60X, 22 - 67X. Zoom eyepieces usually have a helical mechanism that moves lens groups to and from each other. This separation changes the magnification. A spotting scope with a zoom eyepiece can be very useful for plane spotting and bird watching. Traditionally, the accepted wisdom was that fixed focal length eyepieces gave superior image quality over zoom eyepieces at the same magnification setting. Today, this is still largely true, although the difference is sometimes marginal. Zoom eyepieces, particularly from the top German, Austrian and Japanese manufacturers have seen a marked improvement in optical quality over the past ten years.