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Refracting telescopes

Dear friends, I think it is the high time to discuss and understand the refracting telescopes! So, what types of telescopes there are in our time and how they different? Many people consider that the main attribute of a telescope is in its capacity to magnify the celestial object's image. Indeed this is not true! The main mission of a telescope is to trap as much light as possible. This can be explained in the following way: there is as much light in the human eye as the eye pupil let it pass through! The maximum diameter of the eye pupil (it can change depending on total illumination of the space) is about 7 mm. If we trap the light falling on the bigger square the image will be brighter. This task is done by a telescope. The bigger the diameter of a telescope objective the much light it traps and consequently the more faint celestial objects can be seen through it. It is considered that Galileo Galilei had invented a telescope. In truth terrestrial telescopes had been before him in Holland and Galileo Galilei only had guessed to direct them towards sky. Telescopes producing images with the help of collecting lens had received the name "refractors" (from Latin word - refract rays). And only then Newton had invented another way of image getting - with the help of concave mirror. This type of telescopes was called "reflectors". The telescope of Galileo Galiley was the first refracting telescope. A refracting telescope has a long narrow tube through which light passes from the objective lens to the eyepiece at the lower end of the tube. Refracting telescopes feature many key benefits among which simplicity and reliability due to design; practically it is maintenance free; excellent for planetary, lunar and binary star observing; it is excellent for dilettantes!