Early telescopes, like Galileo's, were designed on the principles of refraction, or redirecting light by bending its rays as it passes from one medium, like air, into and out of another medium, like glass. A convex objective lens, located at one end of the telescope's tubular body, gathered as much light as possible from a distant object. The larger the lens, the more light it could gather. As light rays passed through the objective lens, its curvature directed the rays to converge and form an image of the viewed object near the other end of the tube, at a place called the focal plane. A concave eyepiece then magnified the tiny image for the viewer to see.

The lenses in Galileo's telescope had a greenish hue from iron contained in the glass and were filled with tiny bubbles that distorted the image. In addition, as explained in more detail below, a colored halo surrounded the images seen in the small field of view. Yet another shortcoming of Galileo's telescope was that magnification could only be improved by increasing the focal length, which meant focusing the light farther behind the objective lens. Tubes were made longer and lenses larger, but there were practical limits to this design: a glass lens large enough to capture the sparse rays from more distant stellar objects would sag under its own weight.

Isaac Newton introduced a new concept in telescope design in which mirrors replaced glass lenses. In a reflecting telescope, a curved concave mirror at the base of the tube gathered light and reflected it to a point of focus situated about halfway back along the tube. There, a second mirror -- flat and angled -- reflected light to an eyepiece located on the side of the tube.

Newton's design held some distinct advantages over Galileo's. For example, lenses naturally cause different wavelengths of light to separate as they pass through them. Light of different colors focuses at different points, causing distortions in the color of the objects under observation, especially around their edges. Mirrors, by contrast, do not separate the colors in this way.