Reflecting Telescope is a telescope in which a mirror is used to collect and focus light. In 1663 scientist Gregory first invented this telescope. In 1668 Sir Isaac Newton first constructed the most currently used reflecting telescope.
Generally, it can be said that larger the focal length and aperture of the objective of a telescope, larger and bright will be the image of the object. But since color distortion of white light occurs in a large size of the objective lens, hence color defect and size-distortion take place in the image of the object. These defects are removed by the relatively large focal length and aperture of the concave lens of the objective used in a reflecting telescope. For this reason, the astronomical telescopes of the world famous large observatories are reflecting telescopes, not refracting telescopes.
In this telescope, a concave minor C of large focal length and wider aperture is placed at one end of a hollow tube and the other end remains open (Figure). There exists a hollow side-tube at one end of the hollow tube at a distance less than the focal length of the concave lens. In this side tube, a convex lens E is placed as an eve-piece. Besides, at the intersecting point between principal axes of the concave mirror and the convex lens a plane mirror ‘M’ is plated inside the tube at an angle of 450 with the principal axis of the concave mirror. The surface of the plane minor faces towards the concave minor and the lens.