The telescope is a sighting device usually with a magnification greater than unity used for image enlargement or measurement purposes. It consists of a well corrected objective lens, a reticle at its focal plane and an eyepiece.
The emerging beam is parallel, so that the image observed through the telescope is located at infinity i.e at a long distance. This set up in is known as infinity setting.
 
Similar to collimators, the telescopes can be focused at finite distances. Attaching a draw out tube to the reticle adapter to move the reticle out of the focal of the objective lens, the standard telescope becomes a focusing telescope. Depending on the location of the reticle relatively to the focal plane, it results a real or a virtual image at a finite distance.