What Is A Telescope?

A telescope provides angular magnification of a distant object, and therefore produces an effect as if the object were either large or closer to the eye.

The astronomical telescope consists of an objective of long focal length and an eye-lens of short focal length. The objective forms a real and inverted image of a distant object (a planet or a star) on its focus. This image can be brought within the focus of the eye-lens by adjusting its position. The eye-lens produces an enlarged virtual secondary image. An astronomical telescope produces an inverted image.

In a terrestrial telescope an extra lens is placed between the objective and the eye-lens to produce an erect image of the object.