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A refractor is an optical telescope which, through the use of lenses, exploits the phenomenon of refraction. The largest refractor ever constructed is 40 inches (101.60 cm) Yerkes Observatory, built in the late nineteenth century. The optical component of a refracting telescope consists of a long tube on whose front end is disposed a doublet called Camera, which has the function to collect and focus light. The objective substantially performs the function of the prism: decomposes and recomposes the light radiation at a given point given by instrumental focal length. The optical tube, in addition to fulfilling the objective and eyepiece support function avoids, since it is closed at the two sides, the occurrence of the degradation of the image due to internal air motions. The eyepiece is another set of lenses used to render accessible to the eye all the details contained in the image formed by the objective. The refractors may be of achromatic type, semi-achromatic or apochromatic a function of the ability to focus at the same point the light of different colors. Refractors the higher cost are undoubtedly the so-called apochromatic, tools designed to reduce almost completely chromatic aberration, typical of a slow instruments. To achieve this excellence in the images using low-dispersion optical systems such as lenses to mineral or synthetic fluorite. This aberration is manifested by the appearance of colored halos around the edges of the subjects observed, and it is more evident in refractors from low cost or not perfectly correct. A particular type of refractor is the chronograph, to observe the solar corona. amateur use, the telescope lens finds the best apps in the observation of the planets. In refracting astronomical images possess a great sharpness, for this reason they are generally preferred by amateur astronomers who observe the elusive details of the planets, often lacking contrast.