Glossary term: Reflecting Telescope

Description: In a reflecting telescope or mirror telescope, the main optical element is a mirror, the "main mirror", which bundles infalling light. Mirror telescopes are often characterized by main mirror diameter, ranging from a dozen centimeters for smaller amateur telescopes to eight meters for the largest solid mirrors used in professional telescopes. Still larger collecting areas can be obtained by combining several mirror segments, which then act in a way similar to that of a larger solid mirror. There are several types of mirror telescopes. For example, in a Newtonian telescope the light coming from the main mirror is reflected sideways by a smaller mirror into an eyepiece. In a Cassegrain telescope, a smaller secondary mirror reflects the light back, through an opening in the main mirror.

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Related Media

The two Keck telescopes seen from above. The domes are open and the telescopes' mirrors can be seen inside.

Keck Telescopes

Caption: The two Keck telescopes on Mauna Kea on the island of Hawai`i. These two telescopes are reflecting telescopes with primary mirrors 10m across.
Credit: NASA/JPL credit link
License: PD Public Domain icons