Choosing Your Terrestrial Telescope
A Guide to choosing the correct Telescope by Opticron

The majority of terrestrial telescopes are prismatic and as such follow the basic design of a large monocular (half a binocular) usually between 6omm and 80mm in diameter, designed for use with a range of different magnification eyepieces.

Telescope specifications
Using the example 20x60, the first number '20' represents the magnifying power and brings objects 20X nearer when compared to the naked eye. The second number '6o' denotes the diameter in mm of the objective (OG) lens through which light enters the telescope. A 20-6ox6o has a 6o mm OG with a zoom eyepiece that enables the magnification to be varied between 20X and 6ox. As stated, many now come in 'body only' format e.g. HR66. This has a 66mm OG lens and no built-in eyepiece allowing the choice of lens to be made separately.

What magnification?
Terrestrial telescopes are most commonly used to provide high magnification viewing over long distances. Image quality at different magnifications will largely depend on the design of the optical system, the quality of glass used and the coatings applied to the surfaces of each lens. There are, however, a few general rules that can be applied in determining the right specification for your needs. Firstly, the relationship between the magnification, OG lens and the size of the eye pupil, and secondly the quality of the optics inside the scope. In normal daylight, when the pupil is dilated to approximately a 6omm telescope will deliver optimum performance at 24X magnification i.e. when the exit pupil diameter equals that of the iris. In twilight when the pupil opens to objectives.

Light Transmission
The relative light transmitting capacity of a telescope (Twilight Factor) is sometimes said to be found by calculating the square root of the multiplication of the eyepiece magnification with the objective lens diameter. Using this calculation to assess image brightness can be confusing as under these rules a 20X70 combination will give a tower light transmission than a 30x70 combination. The best way to assess the actual brightness of any telescope and eyepiece combination, assuming equal optical systems is to calculate the exit pupil diameter in the same way as with a binocular.

Field of view
As with light transmission is directly related to the magnification. The greater the magnification, the smaller the field of view. Normally expressed as the width in metres of the image when viewing at a distance of boom, there are exceptions, namely wide-angle eyepieces designed specifically to offer greater fields of vision. It is important to note that the objective lens, irrespective of diameter has no influence over the field of view.

Resolution
As a general rule a good telescope should be able to resolve two black dots 1.5mm distance apart on a white surface, in bright daylight from a distance of 5om.

Eyerelief
Is the distance between the eye lens and the point where the pupil is positioned for full field of view and varies from eyepiece to eyepiece. In most cases the eye-relief is shorter than that required by spectacle wearers to obtain the full field of view - especially at higher magnifications. If it is important that you obtain the full field of view with spectacles please refer to either Opticron HDF eyepieces or those eyepieces with the suffix LE. Remember, rubber eyecups although fitted to many telescope eyepieces are no guarantee that the full field of view is obtainable when folded down and used with glasses.

Straight-through or 45º angled
Common advantages of an angled telescope are that:
i) the upper body is in a more relaxed approximately 7mm the same telescope will deliver optimum performance at just 9x. As with binoculars, the higher the magnification, the shorter the depth of field and the narrower the field of view. See 'Choosing your Binocular.' Higher magnification also increases image and colour distortion. This effect can be reduced by using ED or Fluorite combination objective lenses that minimise chromatic aberration, enhancing overall contrast and clarity across the magnification range. At magnifications of iox or lower, the benefits of these expensive lenses are hardly noticeable when compared to conventional glass position when looking through the scope, ii) the tripod can be set at a lower position making it easier for people of different heights to use, (and the equipment more stable in outdoor conditions). Straight-through telescopes are easier to use when following fast moving objects, using the instrument from the confined spaces of a hide or vehicle, or when hand-held.

Tele-photography
Most telescopes can be converted into long focal length telephoto lenses for wildlife and general photography. It should be noted however, that due to the small 'f' numbers and comparatively long exposure times needed, high-rated ASA films and a solid support should be used when using telescopes in this way.