Reflecting Telescope

Refraction Telescope

The objective lens is replaced by a concave parabolic mirror of large aperture. The images in such telescopes are brighter and have a high resolving power compared to astronomical telescope.

Cassegrainan Reflecting Telescope

where O-objective, B-convex mirror, E- eye piece

Newtonian Reflecting Telescope

Galilean Telescope

This consists of an objective lens O, a convex lens of large focal length and aperture and an eyepiece (E), a concave lens of a short focal length and small aperture separated by a distance equal to the difference in their focal lengths. The parallel beam passing through the objective would have converged at a distance f₀ to form the image A¹B¹. But the eyepiece intercepts these rays and converts them into a parallel beam.

The magnifying power is given by

Telescope is given by

L = f_o - f_e

The advantages of this telescope is

• The final image is erect

• The length of the telescope is small

The disadvantages are:

• The field of view is small

• Cross wires cannot be used