Telescopes

A Telescope is an instrument that is used to study distant objects. An optical telescope uses a convex lens or a concave mirror, or in some cases a combination of both, to collect light from, and form an image of, a distant object. The image produced by the collector can be recorded photographically or electronically. When used for direct visual observation, a telescope also incorporates an eyepiece, a short-focus lens that enables the eye to view a magnified version of the image produced by the collector. An eyepiece may also be used to project an enlarged image onto a photographic emulsion or electronic detector.  If the collector is a lens, the telescope is called a refractor, whereas if the collector is a mirror, the telescope is a reflector. A catadioptric telescope uses a combination of lens (es) and mirror(s) to collect light and form an image.

The invention of the telescope is usually credited to the Dutch spectacle-maker, Hans Lippershey, who constructed a simple refractor in 1608. However, some historians contend that a form of telescope may have been constructed and used by the Englishman, Leonard Digges, in or around 1550. Although the Scot, James Gregory had produced a design for a reflecting telescope in 1663, the first working reflector is generally believed to have been designed and constructed in 1668 by the English natural philosopher Isaac Newton. However, it was Galileo Galeilei, then Professor of Mathematics at the University of Padua in Italy, who was the first to construct a telescope for systematic astronomical observations.

A telescope is categorized by its aperture (the clear diameter of the collector) and focal length or focal ratio, the former being the distance between the collector and the image that it produces, and the latter being the ratio of the focal length to the aperture. Because a telescope collects more light than the unaided human eye, it can reveal fainter (and more distant) objects than the eye alone; the larger the aperture, the greater the light gathering power. The ability of a telescope to reveal fine detail (resolving power) also depends on the aperture. The size of the image produced by a telescope is directly proportional to the focal length of the collector. Additional lenses or mirrors may be used to increase the effective focal length of the telescope and thereby produce larger images.

The objective lens of a refractor usually consists of at least two components, each of which has front and rear surfaces. Consequently there are at least four surfaces that must be figured (ground to a precise shape). By contrast, because light is reflected from the front surface of an astronomical mirror, only one surface needs to be figured.

Furthermore, lenses can be supported only around the perimeter; while mirrors can be supported across the back as well as round the edge. For these, and other, reasons, large reflectors are easier and cheaper to construct and, in most respects, are more efficient. All large modern optical telescopes are of the reflecting type, the largest being the two Keck telescopes, each with an aperture of 10 m, that are located on Mauna Kea, Hawaii.

Questions to Ponder:

• What are observational capabilities of the Hubble Space Telescope (HST)?
• What are the advantages and disadvantages of using a telescope in space?