Optical Board - Reflecting Telescope
To illustrate rays in one type of reflecting telescope
Optical board with convex lens, long focal length concave mirror, and small plane mirror.◙
Use a single positive lens to obtain parallel rays, creating a source (stars) at infinity. The long focal length concave mirror is used as the telescope mirror. Before the focal point of the concave mirror, a small plane mirror is inserted, shifting the focus to a point below the incoming light beam. The small mirror can actually be either plane, concave or convex, producing different types of reflecting telescopes, or it can be eliminated entirely, with the observer or film at the focus of the primary mirror.