A telescope that produces images by refraction using a lens. The advantage of this over reflection by a mirror is that the aperture remains completely unobstructed, thus yielding images of the highest contrast and brightness. The light transmission of a coated lens is better than 90%, and remains better than that of a set of mirrors for apertures up to about 0.4 m. Refractors are considered ideal for observing fine, low-contrast details, as on planets. Their drawbacks are that in order to overcome chromatic aberration each lens must have several optical elements, and that the weight of a large lens is difficult to support. In addition, it is difficult to make lenses of focal ratios smaller than about f/8, because of the steepness of the curves required, so tubes become very long in large-aperture instruments.

Refractors were the first type of telescope to be invented, in the early 17th century; the invention is usually attributed to the Dutch optician Hans Lipperhey (c.1570–c.1619), although Galileo was the first to use the telescope for serious astronomy. The practical upper size limit of refractors is represented by the 40-inch (1-m) f/19 refractor at Yerkes Observatory. Virtually all large refractors were made before the end of the 19th century. However, small refractors of aperture 50–100 mm continue to be popular for amateur use.