## Refraction

Galileo's instrument, like all of the earliest telescopes, was a refracting telescope, which uses a glass lens to focus the incoming light. For all practical purposes, astronomical objects are so far away from us that we can consider that light rays come to us parallel to one anotherâ€”that is, unfocused. Refraction is the bending of these parallel rays.

The convex (bowed outward) piece of glass we call a o lens bends the incoming rays such that they all con verge at a point called the focus, which is behind the lens directly along its axis. The distance from the cross-sectional center of the lens to the focus is called the focal length of the lens. Positioned behind the focus is the eyepiece lens, which magnifies the focused image for the viewer's eye.

Modern refracting telescopes consist of more than two simple lenses. At both ends of the telescope tube, compound (multiple) lenses are used, consisting of assemblies of individual lenses (called elements) designed to correct for various distortions simple lenses produce. For example, the exact degree to which light bends or refracts in a piece of glass depends on its wavelength. Since light consists of many different wavelengths, a single lens will produce a distortion called "chromatic aberration." The compound eyepiece of many modern telescopes also corrects the image, which a simple eyepiece would see upside down and reversed left to right.

### Star Words

A refracting telescope or refractor creates its image by refracting (bending) light rays in glass lenses. The point at which those incoming parallel bent rays converge is called the focus, and the distance from the cross-sectional center of the lens to the focus is the focal length of the lens.

### Star Words

A refracting telescope or refractor creates its image by refracting (bending) light rays in glass lenses. The point at which those incoming parallel bent rays converge is called the focus, and the distance from the cross-sectional center of the lens to the focus is the focal length of the lens.

Diagram of a refracting telescope. CF represents the objective lens and LL the eyepiece. The observer's eye is identified by E.

(Image from arttoday.com)

Newton gets credit for inventing the reflecting telescope, but another Englishman, John Gregory, actually beat him to it, with a design created in 1663. It was, however, the Newtonian reflector that caught on. The French lens maker Guillaume Cassegrain introduced another variation on the reflector design in 1672. In his design, there is a primary and a secondary mirror, and the focal point of the primary mirror is located behind the primary mirror surface.

## Telescopes Mastery

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