University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

Astronomical Telescopes

This site has a good overview of astronomical telescopes. The principal function of an astronomical telescope is light gathering, (magnification is largely incidental; while sometimes useful, the huge magnification of the largest telescopes poses a difficult challenge to astronomical instrument makers). There are two types of optical/infrared telescopes:

Refracting Telescopes

focus light using lenses and the Principle of Refraction. Look here to learn more than you ever wanted to know about refraction.

The Refractive Index of Air is very nearly 1 (nair = 1.0003, depending on Temperature & Pressure, but astronomers need to take this into account when calculating wavelengths in the earth's atmosphere). The refractive index of water is nH2O = 1.33 and refractive indices for various kinds of glass vary from about n = 1.5--1.8. A diamond's luster is partially due to its high refractive index, n = 2.4.
Here's a Java Demonstration of the workings of a converging lens.

In order to look through a telescope you need two lenses, the objective , which is the principal lens of the telescope, and an eyepiece. The image scale in the focal plane is determined by F, the focal length of the objective, the distance between the lens and the focused image. If you look through the telescope, the magnification will be determined by the ratio of the focal lengths of the objective and the eyepiece.

The sensitivity of the telescope is determined by the collecting area of the objective lens (or primary mirror) which is proportional to the square of its diameter.

The Great Refractor of Lick Observatory in Northern California was completed in 1888. It was funded by a bequest from entrepreneur James Lick, who is buried in the telescope mount. The objective lens (at the top of the photograph) has a diameter of 36 inches; the lens has a focal length of 57 feet, bringing light to a focus at the bottom of the telescope where the observer is standing. The objective is actually composed of two lenses, in order to correct partially for chromatic aberration. One is made of Crown glass, the other Flint glass, which have different refracting properties. For a brief period, the Great Refractor was the world's largest optical research telescope. The telescope is no longer used for research, but there is a Summer Visitors' Program that allows visitors to observe through the telescope and hear a popular lecture. Lick Observatory is operated by the University of California for all Faculty, Research Staff and Students in UC, including UCSD astronomers.
The Great Refractor of Lick Observatory

 Check this page if you are farsighted or this one if you are nearsighted.

The tutorial continues with Reflecting Telescopes.

Reflecting Telescopes  Basic Forces & Light  Education & Outreach  CASS Home  

Conducted by Gene Smith, CASS/UCSD.
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Prof. H. E. (Gene) Smith
CASS   0424   UCSD
9500 Gilman Drive
La Jolla, CA    92093-0424

Last updated: 21 April 1999