The optical telescope

There are essentially two types of telescope - refractors and reflectors. In a refracting system, a lens is used to collect the light and bend it to a point of focus, while a reflector makes use of a specially curved mirror to collect and reflect the light back to the focal point. In each system, an eyepiece is placed at the focal point to magnify the image produced. There are advantages and disadvantages with both types, and before either system is selected these must be weighed in relation to the needs of the observer.

Reflectors are less expensive, size for size, than refractors, and they tend also to be more compact, especially with larger instruments. In a reflector, however, the mirror needs periodic recoating and realigning.

The design of a reflector means that air currents can be created inside the tube, resulting in slightly imperfect images. A refractor has a closed tube and so air currents cannot form, but light refracted by the lens can produce images displaying false colour fringes. This occurs because the lens cannot bring all the different wavelengths of light to a common focus.

The focal length of a telescope is the distance from the primary (lens or mirror) to the actual point of focus. Eyepieces, too, have different focal lengths, and magnification is altered simply by changing the eyepiece. To work out magnification, the focal length of the primary is divided by that of the eyepiece. In the case of a telescope of 60 cm (23.5 in), an eyepiece with a focal length of 2.5 cm (0.98 in) will give a magnification of 24 times.

The larger the focal length of the telescope, the higher the possible magnification will be. Refractors, with their long focal lengths, tend to be useful for lunar and planetary work, but the wide field of view obtained with reflectors makes them more suitable for the observation of nebulae and other objects in deep space.
 
The more light that reaches the eyepiece in a telescope, the brighter the image of the heavens will be. Astronomers made their lenses and mirror bigger, they changed the focal length of the telescopes, and combined honeycombs of smaller mirrors to make a single, large reflective surface in order to capture the greatest amount of light and focus it on to a single point. During the 19th century refracting telescopes were preferred and opticians devoted themselves to perfecting large lenses free of blemishes. In 20th century there were advances in technology for grinding a polishing mirrors. A large mirror will intercept more light than a small one, but mirrors larger than 4 m (13ft) in diameter will sag under their own weight and cause distortion. The development of multiple-mirror instruments in the late 20th century has introduced bigger and better optical telescopes.(below) Smaller mirrors mounted side by side make up a mirror than any single one could be.

MULTI-MIRROR TECHNOLOGY
The limitations of size imposed by the difficulties of casting a single large mirror have been overcome by the invention of multi-mirror telescope technology (MMT).
GRINDING MIRRORS
The 5-m (16 ft) mirror of the famous Hale telescope on Mount Palomar in California, US was cast in 1934 from 35 tonnes of molten Pyrex. The Second World War interrupted the grinding of the mirror to achieve the correct curved shape. It was not completed until 1947. Mount Palomar was one of the first high altitude observatories, built where the atmosphere is thinner and the effects of pollution is reduced.
MEASURING ACROSS VAST DISTANCES
The bigger the telescope, the larger its scale will be. This means that measurements become increasingly crude. A micrometer can be set to provide extremely fine gradations, a necessary element when measuring the distances between two stars in the sky that are a very long way away. William Herschel made this micrometer (above). To pinpoint the location of a star, a fine hair or spider's web was threaded between two holders that were adjusted by means of the finely turned screw on the side.
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