Mirrors are used in reflecting telescopes, which allow astronomers to see distant objects in space more clearly.
A mirror forms an image by gathering light from spatial objects. A second mirror receives the picture that was reflected by the first mirror, which may be rather wide. The light is reflected by this tiny mirror onto an eyepiece lens, which magnifies or enlarges the image of the object.
Hence, the secondary mirror is used to move the eyepiece outside the telescopic tube.