Reflecting Telescope

A reflecting telescope is used to see distant stars and possesses large light-gathering power in order to obtain a bright image of even a faint star deep in space. The objective is made of a concave mirror, having large aperture and large focal length. This concave mirror, being parabolic in shape, is free from spherical aberration.

Before the reflected rays of light meet to form a real, inverted and diminished image of a distant star at the focal plane of concave mirror, they are intercepted and reflected by a plane Mirror M1M2 inclined at an angle of 45° to the principal axis of the concave mirror. This plane mirror deviates the rays and the real image is formed in front of the eye piece, which is at right angle to the principal axis of concave mirror. The function of the eye-piece is to form a magnified, virtual image of the star enabling eye to see it distinctly.

If fo is the focal length of the concave mirror and fe is the focal length of eye piece, the magnifying power of the reflecting telescope is given by

M = fo/fe

If D is the diameter of the objective and d is the diameter of the pupil of the eye, the brightness ratio is given by

B = D2/d2

Advantages of Reflecting Telescope over Refracting Telescope

  1. Since the objective is not a lens, the reflecting telescopes are free from chromatic aberration. Thus, rays of different colors reaching the objective from distant stars are focused at the same point.
  2. Since the spherical mirrors are parabolic mirrors, free from spherical aberration, they produce a very sharp and distinct image.
  3. Even a very faint star can be seen through the reflecting telescope because they have large aperture and have large light-gathering power. The brightness of the image is directly proportional to the area of the objective.
  4. In reflecting type of telescopes, there is negligible absorption of light.
  5. Large apertures of reflecting telescope enable to see minute details of distant stars and explore deeper into space.