A telescope that uses a concave mirror to focus light. The main advantage of reflection (by a mirror) over refraction (by a lens) is that all colours of light are reflected equally, so the images do not suffer from chromatic aberration. Additionally, there is no practical upper limit to the size of a mirror and only one surface needs to be optically worked, thus making large mirrors cheaper than large lenses. It is also comparatively easy to make reflecting telescopes with focal ratios smaller than f/6. All telescopes with apertures greater than about 1 m are reflectors.

The principal drawback of reflectors is that some obstruction of the light path is inevitable, resulting in a loss of light and image contrast. Furthermore, a mirror’s coating loses its reflectivity over time and must periodically be renewed; and normally the tube of a reflector is open at the top end, so that air currents (tube currents) can circulate within it and disturb the quality of the image. The largest reflectors are now made with segmented mirrors. The first design for a reflecting telescope was published by J. Gregory in 1663, but the first person to build a reflector was I. Newton in 1668. The main designs of reflector are the Cassegrain, Newtonian, and Ritchey–Chrétien telescopes.