Types of Telescope Lenses

Refractors vs. Reflectors

• There are two basic types of telescopes: refractors and reflectors. Reflectors collect light and form images through the use of a series of mirrors that catch and reflect the light back into the eyepiece. The lens (or lenses) in the eyepiece are the only lenses in a reflector. Refractors, on the other hand, have what is called the objective lens at the larger end of the telescope, as well as lenses in the eyepiece like a reflector telescope.

Different Refractor Lenses

• The objective lens works to focus the light being collected by a refractor telescope. Light rays passing through the objective lens are bent inward to a point, where an image forms. This image is viewed through the eyepiece. While the objective lens serves to collect the light and create a picture with it, the lenses in the eyepiece magnify that picture and make it clearer.

Concave and Convex Lenses

• While lenses can vary greatly by number and configuration, there are only two basic types--convex and concave. Convex lenses are rounded outward, like the curve of a sphere. Concave lenses are rounded inward, like the impression of a sphere. A third type of lens has one side which is convex and one side which is concave, and this is called convex-concave or meniscus. Combinations of these three types form the more specific configurations in telescope lenses.

Doublets and Triplets

• Some telescopic lenses are made of several basic lenses put together. The simplest of these is a convex lens placed against a concave one. These may be set directly against one another or placed with space between them. Combinations of two lenses are called doublets, and those with three are called triplets. The combining of lenses lets the telescope focus different wavelengths of light at the same time.

Eyepiece Lenses

• The most compound lens layouts are found within the eyepiece of a telescope. The Huygenian, Barlow and Ramsden eyepieces all use two lenses, while the Kellner and RKE configurations use three. The Orthoscopic and Plössl use four lenses, the Erfle eyepiece uses five, and the complex Nagler uses seven. The eyepiece variations in lens quantity, type and spacing make it possible for one telescope to view objects of differing distance and brightness.