The Physics
Factbook
An encyclopedia of scientific essays

Focal Length of an Astronomical Telescope

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Arny, Thomas T., and Nicholas A. Pananides. Introductory Astronomy. New York: Addison-Wesley, 1973: 53. "Thus a 10-inch, f-9 telescope has an objective with a diameter of 10 inches and a focal length of 90 inches." 2.2860 m
Muirden, James. How to Use an Astronomical Telescope. New York: Simon & Shuster, 1985: 14. "Astronomical eyepieces are available in a considerable range of focal lengths. The longest are up to 50 mm or so, the shortest may be as little as 4 mm." 0.004โ€“0.050 m
Chaisson, Eric J. The Hubble Wars. New York: HarperCollins, 1994: 150. "Hubble's focal length โ€” the distance between the main mirror and the point at which it focuses light โ€” is 189 feet (nearly 58 meters), or about three times as long as the shuttle, but because the light is additionally reflected by a secondary mirror back through a hole in the main mirror, a long focus can be attained within the confines of telescope." 57.6072 m
Worvill, Roy. Stars and Telescopes for the Beginner. New York: Taplinger, 1979: 50. "A Newtonian reflector … … made by J.H. Hindle. The focal length is 7½ ft." 2.2860 m
Hsu, Daniel. NASA. Electronic mail. 29 May 2002. "The primary mirror is 2.4 meters and the telescope has a focal ratio of f/24. The focal length is then 2.4 x 24 = 57.6 m." 57.6 m

As evidenced by the wide array of books about astronomy aimed at beginners and amateurs it is clear to see that this a field of science in which nonprofessionals can easily partake. All that is needed is an astronomical telescope and the sky is the limit. An astronomical telescope is a device used by astronomers to examine planets, stars, and other heavenly bodies. The telescope can magnify these distant objects or produce images that are otherwise too faint for the naked eye to see. There is no standard telescope used by all astronomers but rather an assortment of designs to choose from.

The astronomical telescope is an instrument that works based on a relatively simple premise. A lens or mirror, known as the objective, collects light from an object and focuses it at a location called the focal point. The focal length is the distance from the focal point to the surface of the mirror or the center of the lens.

The Hubble Space Telescope, arguably the most famous telescope, has a focal length of roughly 58 M. This is longer than the space shuttle in which it is delivered into orbit but not a problem because a secondary mirror is used to reflect the light back through a hole in the main mirror. For comparison, a Newtonian reflector made by J.H. Hindle has a focal length of 2.860 m and a 10-inch (25.4 cm), f-9 telescope also has a focal length of 2.2860 m.

An eyepiece (essentially a magnifying glass) is also needed in an astronomical telescope. It is used to bring the human eye closer to the image produced at the focal point and examine it. Astronomical eyepieces also have focal lengths, which vary from 0.004 m to 0.050 m depending on design.

Jennifer Hu -- 2002