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Question
Which two aberrations do objectives of refracting telescope suffer from? How are these overcome in reflecting telescope?
Solution
Refracting telescope suffer from chromatic and spherical aberrations.
Chromatic aberration: The image of a white object formed by a lens is coloured and blurred. This inability of lens to form a clear image is known as chromatic aberration.
Spherical aberration: The inability of a lens to form a point image of an axial point object is called spherical aberration.
In the reflecting-type telescope, the objective lens is replaced by the concave parabolic mirror of a large aperture required for observing fainter objects. The use of parabolic mirror makes the resolving power of the telescope high. The parabolic mirrors are free from chromatic and spherical aberrations.
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