- There are two kinds of optical telescopes: Reflecting and Refracting.
Refracting telescopes, or refractors, use a glass lens to bend, or refract,
light and bring it to a focus. The lens is convex becoming thinner toward its
edges. A convex lens bends light at the edge of the lens to a greater angle than
light coming through the center, so all of the rays converge to a focus. The
distance between the lens and the place where the rays converge is called the
focal length of the lens. Such telescopes are usually affected by chromatic
aberration, which causes the image of an object, such as a star of planet, to be
surrounded by circles of different colors, because every color has its own
degree of refraction.
A reflecting telescope uses a precisely curved mirror instead of a lens to
collect light. The mirror is concave, thicker towards the edges. The mirror,
which is usually parabolic in shape, focuses the incoming light instead of a
glass lens. This increases the telescope’s light sensitivity, so dimmer
objects can be seen.