University of California, San Diego
Physics 7 - Introduction to Astronomy

H. E. Smith   Winter 2007

Physics 7 Lecture Summary #4 (cont'd)

Reflecting Telescopes

Reflecting telescopes

use mirrors to focus light following the Principle of Reflection.

Angle of incicence (in) equals angle of reflection (out).

Modern reflecting telescopes use a parabolically shaped primary mirror coated with a thin film of aluminum.

 The 200" (5m) Hale Reflector of Palomar Observatory is shown above. Until recently it was the world's largest optical/infrared telescope. Palomar is located in N. San Diego County and is well worth a visit on a pleasant weekend. The Visitors Gallery of the 200" telescope and a walking tour of the Observatory are open to visitors from 9:00am -- 4:30pm. We are currently using the 200" telescope for a survey of distant infrared galaxies.

 All modern optical/infrared telescopes are reflecting telescopes, because:

  1. Reflecting telescopes do not suffer from chromatic aberration.
  2. Figuring a mirror requires polishing only one precise surface rather than two (or four for a compound lens).
  3. Mirrors are easier to support because they can be supported on the sides and the back; large lenses tend to sag because they can only be supported on the perimeter.
Reflectors are also much more versatile than refractors because they can be used at several different foci.

 The world's largest optical/infrared telescopes are the twin 10-meter Keck Telescopes operated by the University of California and Caltech on the 13,700ft dormant volcano, Mauna Kea, Hawaii. These are the principal research instruments of UCSD optical and infrared astronomers.

The Keck Telescopes employ hexagonal segmented primary mirrors, each made out of 36 hexagonal segments, 1.8m in diameter. In order to maintain a precise optical surface the positions of the segments are monitored by sensors which relay signals to a computer which drives precision actuators, keeping each segment in proper alignment.

Mauna Kea is probably the world's best observatory site because of its stable atmosphere, maintained by the island's marine layer, and its altitude. Over 20 astronomical telescopes from the US, Britain, Canada, France and Japan are in operation or under construction.

The lecture continues with Radio Telescopes

Radio Telescopes  Refracting Telescopes  Physics 7 Lectures  Physics 7 Home 

Conducted by Gene Smith, CASS/UCSD.
Comments? You may send email to

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

Last updated: 12 January 2001