Glossary

Basic — the most important terms and interrelations with reference to terrestrial telescopes, i.e. such with an upright image, will be explained in the following. Particular features of astronomical telescopes will not be considered.

Anti-reflection Coatings

Anti-reflection coatings on glass lenses are very thin layers, which have the effect that as little as possible of the light incident on the glass is reflected by it. The aim of the anti-reflection coating is to get as much light as possible to the eye, i.e. achieve a high transmission.

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Aperture

The aperture of an objective is also called entrance pupil diameter. It is an important performance feature of a telescope and is thus often mentioned in connection with the magnification. For example: 8x30 means 8-fold magnification for an objective diameter of 30 mm.

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Aspherics

In the case of a traditional lens the optically effective surfaces have the shape of a sphere. This shape can efficiently be produced, using grinding and polishing methods; however, it is not ideal for imaging.

In an optical system a multitude of radii therefore have to be combined in such a way that the desired effect is achieved with minimized image errors. The aspheric is a lens which has a shape that deviates from the shape of a sphere and is contorted according to the requirements. In this way image errors that are due to physical reasons can be better corrected. A disadvantage of aspherics in comparison to conventional lenses are the clearly increased efforts during manufacture.

It is thanks to the founder of the company, Mr. Bernhard Docter, that a method for the manufacture of aspherics, that is more cost effective, has been transferred to series production. By means of the DOCTER® bright moulding technology optical lenses are produced in standard high quality. DOCTER® was the first company that widely used aspherics in binoculars, spotting scopes, riflescopes and flash lights.

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Exit Pupil

The exit pupil is a bright spot that can be seen in eyepiece lenses, when the telescope is held up in front of a bright background with the arm extended. It appears some way behind the last lens of the eyepiece in free space and should under ideal conditions of observation be identical to the size and position of the eye's pupil. The exit pupil is calculated by dividing the objective diameter by the magnification of the binocular. Large diameters of exit pupils are important for achieving good observation performance in dusk or dawn conditions and in the dark. Binoculars, which are designed for use in daylight, only need a small exit pupil and can be built much smaller for achieving the same magnification.

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Exit Pupil Eye Relief

This is the distance between the exit pupil and the last lens vertex. The larger the distance, the more comfortable the observation. In the case of riflescopes the distance is chosen very large in order to avoid injury of the eye by the recoil of the rifle. A large eye relief requires large lenses in the eyepiece and therefore causes increased weight and cost.

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Eyepiece for Persons Wearing Glasses

Binoculars, which are intended for use by persons wearing glasses, have a large exit pupil eye relief. In this way it is possible for the person wearing glasses to see the whole field of view. These eyepieces are supposed to fulfil certain criteria, and their construction is thus more costly. A special kind of these eyepieces are protective mask eyepieces, which have to have even larger exit pupil eye reliefs.

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Field of View

The sector, which is seen when looking through binoculars and, respectively, a telescopic sight, is called field of view. The criterion for the field of view is the number of yards seen using binoculars on 1,000 yard and, respectively, a telescopic sight on 100 yard.

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Geometrical Light-Gathering Power

This is calculated by taking the square of the exit pupil diameter. For example, an exit pupil diameter of 5 mm is equivalent to a geometrical light-gathering power of 25.

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Magnification

This factor indicates how many times larger the user of a telescope sees an object than with the naked eye. If a binocular with 10-fold magnification is used to view an object at a distance of 200 meters, the object appears as if it was at a distance of 20 meters.

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Resolution

The resolution specifies in how much detail an object under observation is visible. In this respect the distance of the object is important and how it is imaged onto the retina of the human eye.

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Reticles

The aiming mask of a telescopic sight is called reticle. Reticles can have various shapes, but all serve the same purpose: fast and easy aiming at the object. For telescopes with variable magnification the reticle exists in the objective image plane as well as in the eyepiece image plane. The two types differ by the size of the reticle in the objective image plane changing with the magnification, whereas the reticle in the eyepiece image plane remains apparently unchanged. In reality things are the other way around. However, for riflescopes with reticles in the eyepiece image plane the overlapping measures change, whereas they remain unchanged in the case of riflescopes with reticles in the objective image plane, even when the magnification is changing. This is why these aiming marks can also be used for estimating distances. To do this with the other type of device it is necessary to use a defined magnification.

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Transmittance

Specifies how much of the light falling onto the objective is transmitted through the binoculars to the eyepiece.

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Twilight Number

A high twilight number helps in achieving a good performance of the binoculars at low light levels. For its calculation the diameter of the objective is multiplied by the magnification and then the square root is taken. For example: binoculars 10x50; 10 x 50 = 500, twilight number = √ 500 = 22.36. The twilight number is a purely arithmetical value and doesn't have any significance for the optical performance in comparison to another device of the same kind.

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Wide Angle Eyepiece

If the magnified image that is produced by the binoculars appears under an angle of at least 60°, the eyepiece may be called a wide angle eyepiece. Wide angle eyepieces can be discerned by their large lenses that are more expensive to produce.

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