TELESCOPE NOTES

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 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.

A lens of type a (above) is known as double convex and is the type most familiar to medieval spectacle makers. 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, lens type b (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 first telescopes introduced In 1608 by Hans Lipperhey of Holland required a double convex lens (type a) with a long focal length and a double concave lens (type b) with a short focal length. This is the same type of telescope refined and

used by Galileo for his ground - breaking observations. The lens in the front is known as the objective, while the one nearest the eye is the eyepiece.

Actually, the Galilean telescope was quickly abandoned for other types. The only virtue of the Galilean arrangement is that it gives upright images, not important for astronomical purposes. The most common astronomical telescope today is the reflecting telescope that uses a mirror for the objective. This type is outlined below: