Image illustrating light beams passing through a simple Cassegrain Reflecting Telescope. This simulation shows parallel beams of monochromatic light striking a concave converging mirror (the objective or primary mirror) at right. The rays are reflected backwards to strike the small secondary mirror (slightly convex or diverging) which reflects them back down the optical axis of the telescope towards the primary mirror. A gap in the primary mirror allows these rays to exit the telescope, where they are inspected by a simple eyepiece in this graphic.