University of California, San Diego
Physics 7 - Introduction to Astronomy
||Physics 7 - Lecture Summary #4 |
This site has a good overview of
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
focus light using lenses and the Principle of
Look here to learn more than you ever wanted to know about
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
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 place is determined by the focal length of the objective;
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 the diameter of the primary lens or mirror.
The Great Refractor
of Lick Observatory in Northern
was completed in 1888 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 human is standing. The objective is actually composed of two lenses
made of Crown glass and Flint glass which have different refracting properties
in order to correct partially for chromatic aberration. 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
Visitors' Program which allows public 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
or this one if you are
The lecture continues with Reflecting Telescopes.
Basic Forces & Light
Physics 7 Lectures
Physics 7 Home
Conducted by Gene Smith, CASS/UCSD.
You may send email to email@example.com
Prof. H. E. (Gene) Smith
CASS 0424 UCSD
9500 Gilman Drive
La Jolla, CA 92093-0424
updated: 13 January 2001