A telescope is an optical instrument that uses lenses or mirrors and other optical devices to observe distant objects. It uses light refracted through a lens or reflected by a concave mirror to enter a small aperture and converge to be seen through a magnifying eyepiece, also known as a "milliscope".

The first role of the telescope is to magnify the opening angle of distant objects, so that the human eye can see the details of the angular distance smaller. The second role of the telescope is to collect the objective lens is much thicker than the diameter of the pupil (maximum 8 mm) of the beam, sent to the human eye, so that the observer can see the original can not see the faint object.

In 1608, a Dutch optician Hans Lieberich accidentally discovered that with two lenses can see the scene in the distance, inspired by this, he manufactured the first telescope in human history. 1609 Italian Florentine Galileo Galilei invented 40 times the double-lens telescope, which is the first practical telescope put into scientific applications. After more than 400 years of development, the telescope has become more and more powerful, and the observation distance has become more and more distant.