TELESCOPE NOTES

Figure 1

In its simplest form a lens is a clear disk of glass with curved surfaces. When parallel light rays from a distant object pass through the lens they are bent (refracted) and come together at a fixed distance. If the distant object is a point source such as a star, all of the rays come together at a point known as the focal point. The distance between the lens and the focal point is known as the focal length. The curve on the surface of the lens can be either convex (bulging out) or concave (bulging in), as shown in the above illustration. It is also possible for one surface of the lens to be flat, giving rise to the plano-convex and plano-concave designs. The curvature of a lens' surface and its type (concave or convex) determines the location of its focal point. The focusing properties of concave and convex lenses are shown in the following two figures.

Figure 2

Figure 3

There is evidence that there was some knowledge of refraction as early as 2000 BC, as fragments of what archeologists have taken to be crude lenses have been found at ancient sites on the Mediterranean island of Crete. Around 500 BC the Greeks were known to have used lenses made by filling glass globes with water. These were mostly used to concentrate the sun's rays to start fires, but Aristophanes in 424 BC wrote that these lenses could also be used to "read letters, however small and dim". During the medieval period (around 1000 AD), the Persian (today Iranian) scientist Alhazen studied the phenomenon of refraction and the properties of plane, spherical and parabolic mirrors. He made this work known in a treatise on optics which became widely circulated in Europe and was one of the factors which led to the invention of spectacles (eye glasses) in the late thirteenth or early fourteenth century (1200s and 1300s).

By the early 1400s spectacles were common in Europe and lens making was a profitable profession. With this proliferation of lenses, the stage was set for the invention of the telescope. A telescope requires two lenses with different focal lengths, a long focal length for the objective (the lens in front where light from a distant object enters) and an eyepiece (the lens you look through) with a short focal length. This was not known in medieval times, so it had to be discovered accidentally. By the 1500s two types of lenses with the required characteristics were in general circulation: the convex lens for nearsightedness, and the concave lens for far sightedness.

The double convex convergent lens (figure 2) was the type most familiar to medieval spectacle makers. It is known as a convergent lens since the parallel light rays converge in the focal plane. This is a lens for farsightedness, and was needed by all aging scholars as their eye progressively lost their ability to focus close up. By contrast, the lens type shown in figure 3 (double concave) was used to correct nearsightedness, an affliction that prevents a clear focus of distant objects. Note that for the double convex lens the focal point is on the opposite side of the lens from the object, while for the double concave lens, the focal point is on the same side.

The focal lengths of the convex lenses were often longer than their concave counterparts, and by the beginning of the seventeenth century (1600s) the discovery had been made that holding a long focal length convex lens in front of a short focal length concave lens produced a magnified image. The credit for the first practical telescope is usually given to Hans Lipperhey of Holland who began marketing it in 1608. The convex/concave combination is the same type of instrument refined and used by Galileo for his ground - breaking observations and is today known as the Galilean telescope and is shown in the top diagram of figure 4.

Figure 4

Actually, the Galilean telescope was quickly abandoned for other types, in particular the design suggested by Johannes Kepler. This Keplerian telescope (bottom, figure 4) employs a converging lens as the eyepiece and gives a wider field of view (i.e., takes in more of the sky ) than the Galilean arrangement. Its main disadvantage is that the resulting image is upside down, but it was quickly realized that this is has no significance in astronomical observations.

Telescopes which use a lens as the objective are known as refracting telescopes or simply refractors. The second major type of telescope employs a curved mirror as the objective. This is known as a reflecting telescope or reflector. The reflecting telescope was suggested by Leonard Diggs in the late 1500s, but the first practical design was demonstrated by Isaac Newton in 1671.