A simple refracting telescope may be constructed from a pair of convex lenses, where the larger of the two lenses is the objective. This lens produces a real image at the focal plane that is viewed with an eyepiece. The eyepiece is placed so that its focal plane coincides with the focal plane of the objective, resulting in a virtual image at 'infinity'.

Parallel light rays from a distant source cross at the focal point of the objective lens.

The eyepiece is placed such that its focal plane coincides with the focal plane of the objective lens.

A simple refracting telecsope using a convex objective lens with a longer focal length than the convex eyepiece.

The aperture of the objective, D_o, is used to calculate the field of view, the resolving power and the magnification of the telescope.