It was soon found that there was a practical limit for constructing large lenses. Such telescopes had to be long to get the correct focal length, which is the distance between the lens of a telescope and the image it forma of an object. The longer the focal length, the larger will be the scale of the image formed, but the field of view will be smaller. The ability of the optics in a telescope to focus light is indicated by what is known as the focal ratio, abbreviated as f. The f- ratio is a number obtained by dividing the focal length by the diameter of the mirror or lance, expressed in millimeters.
For example, if the aperture of the lens is 200 mm and the focal length in a telescope is the distance it takes for light to be refracted or reflected to a point of focus. The eye piece magnifies the tiny image at this point.
In order to get a better image, a technique other than that applied in the refractor telescope soon became necessary. The new technique to ensure that reflection does not cause dispersion, emerged, when a concave mirror was used instead of a convex lens to collect star light. It was discovered that different wavelengths of light bend at different angles, as they transit from one medium (e.g. air) to another (e.g. glass). This would result in chromatic aberration, which is reduced by placing achromatic lens that has or more different type of glass.
From 1700 to 1800, metal mirrors were used to reflect light rather than glass lances that could refract light. This enabled astronomers to design shorter and more powerfully telescopes.
Newton built the first practical reflecting telescope in 1668, though in 1616, Galileo’s compatriot, Nicole zucchini, is reported to have developed the first reflecting telescope. John Hadley (1682-1744) is credited with the first working model in 1721 free of spherical and chromatic aberration. Reflecting telescopes extended the field of observation. Eventually special molten glass (instead of pieces cut from glass slabs) was used, as it would not significantly expand or contract with temperature once it cools.
A problem of that persisted since Galileo’s times was the spherical mirror, which caused blurry images as they did not bring light rays to a common point. A parabolic mirror, first envisaged by James Gregory, solved the problem. A Newtonian telescope nowadays has a parabolic (not spherical as Newton had envisaged) concave mirror to reduce burring. Only a parabolic mirror can reflect all the distant light waves to a common focus. A spherical mirror, despite having more curvature than a parabolic one, cannot do so. A primary parabolic mirror focuses light into a mirror that directs the image to the side so as to keep the head of the observer out of its path. The light from the flat mirror is magnified with a secondary mirror. Moreover, it was found that the size of the primary mirror rather than the length of the telescope would indicate its power .
William Herschel (1738-18220, a German musician living in England , became famous for making reflecting telescopes in the 18th century . he was reported to have ground and polished 430 telescope mirror over 20 years until 1795. he observed the planets and their moons as well as the orbital movement of several dozen binary star systems and catalogued more than 800 of them. His largest telescope had a focal length of 12 meters and a mirror with a diameter of 120 cm. It remained the world’s largest telescope for more than 50 years . It was only in 1845 that William parsons built a larger telescope and discovered 14 spiral nebulae.
Basically, two mirrors are used: a larger primary to produce an image and a secondary to magnify it. The resolution can be improved by using shorter wavelengths of light and bigger primary primary mirrors.
In one type of reflecting telescopes, the images are seen through a hole in the main mirror. It is called Cass grain telescope, named after its inventor, Laurent Cassegrain, a French physicist (1629- 16730. Some historians attribute the invention to a French sculptor, Gallium Cassegrain . This is one of the most commonly used telescopes.
In this type of telescope, light is collected as in a normal reflecting telescope by a larger parabolic, concave primary mirror but reflected to a smaller convex secondary mirror facing it. The secondary mirror reflects the light back to a hole in the center of the main mirror. By this technique, the capacity of the telescope could be increased without increasing its length. Another type called Gregorian telescope has a concave secondary mirror.
In 1930 , an optician, Bernhard Schmidt (1879- 1935), introduced a thin piece of a glass (lens corrector in front of the spherical primary mirror, which then produces an image between them and minimized the spherical mirror aberrations.
In an improved version, called Schmidt- Cassegrain telescope, the light is reflected off a convex secondary mirror to produce an image behind the primary. This type telescope is useful for all – sky surveys.
There is, however, a limit to the number of instruments that can be attached to the back of the primary mirror. Hence in what is known as the Nysmyth focus, the image is processed outside the telescope. The image can also be diverted from the telescope mirrors to some distance for processing. Generally, a tertiary mirror is used to redirect light to external device (e.g. a spectrometer). It is called coude, which is French for “bending or elbowing”. In Kavalur, for instance, light from the primary mirror is sent through an optical fiber to a high –resolution coude spectrometer that disperses the light from bright sources for study in greater detail.