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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 = fo - feThe advantages of this telescope is
- The final image is erect
- The length of the telescope is small
- The field of view is small
- Cross wires cannot be used






