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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30

Lecture 26: Telescopes

Key Ideas:

Types of Telescopes:

Observatory Sites

Special Telescopes:

Extending Human Vision

The basic function of a telescope is:

The simplest telescope is the human eye:

Light Gathering Power

The standard measure of a telescope is its Light Gathering Power:

We usually express the size of a telescope in terms of the diameter of its primary collecting optic:

Refracting Telescopes

Refracting Telescopes use lenses to gather and focus light.

Refracting Telescopes were the most common type of telescope in use before the early 1900s, and are still common for small personal telescopes (binoculars are pairs of matched refracting telescopes).

Schematic of a Refracting Telescope
(Click on the image to view at full scale [Size: 6Kb])

The size of a refracting telescope, and hence its light gathering power, is limited by the size of the largest lens that you can make:

Largest Refractor: 40-inch Yerkes Telescope

Visit the Yerkes Observatory webpage for more info and pictures.

Reflecting Telescopes

The Reflecting Telescopes uses curved mirrors. It was invented by James Gregory & Isaac Newton.

All modern large telescopes are reflecting telescopes.

Schematic of Reflecting Telescopes
(Click on the image to view at full scale [Size: 9Kb])

Advantages of Reflecting Telescopes:

Currently, the largest reflecting telescopes in operating are the two Keck 10-meter telescopes on Mauna Kea in Hawaii:

Plans are afoot to build a new generation of 20- and 30-meter telescopes in the next 10-15 years. In principle, there is no limit to the size of a reflecting telescope except the money to build and operate it.

Mountain-Top Observatories

Telescopes need to be located at special remote sites. The requirements of these sites are:

The best observatory sites are high, dry mountain peaks. Examples of prime observatory sites are:

To see what the Earth looks like at night from composite satellite imagery (the source of the image of the US at night I showed in lecture), visit the Images page of the International Dark-Sky Association.

The Largest Telescopes

The last few years has seen the construction of very large (8-10 meter) telescopes. This sudden burst of activity has been initiated by technological breakthroughs in large mirror design. The links below lead to the webpages for these projects.

10-meter Keck I and II:

8.2-meter Subaru:

8.1-meter GEMINI Telescopes:

8-meter Very Large Telescopes (VLT):

2x8.4-meter Large Binocular Telescope:

Radio Telescopes

Use antennas to detect cosmic radio waves:


Space Telescopes

Only radio waves, visible light, and some infrared light penetrate the atmosphere.

Need to go into space to observe all others:

They are very expensive ($Billions) to build and operate, and only a few are in operation at any given time.

Examples of recent space observatories (not a complete list):

Hubble Space Telescope:
UV, Visible, & near-IR imaging & spectroscopy telescope
Run by NASA and ESA (European Space Agency)

Spitzer Space Infrared Telescope:
Thermal infrared imaging & spectroscopy telescope
Run by NASA (IPAC at Caltech)

Chandra X-Ray Observatory:
X-ray imaging and spectroscopy.
Run by NASA (Chandra Science Center at Harvard)

XMM-Newton X-Ray Observatory:
X-ray imaging and spectroscopy.
Run by ESA

Far Ultraviolet Spectroscopic Explorer (FUSE):
Far-Ultraviolet spectroscopy
Run by NASA (FUSE Science Center at Johns Hopkins)

Return to [ Unit 4 Index | Astronomy 161 Main Page ]
Updated: 2006 October 27
Copyright Richard W. Pogge, All Rights Reserved.