ASTR 1230 (O'Connell) Lecture Notes


Summit of Mauna Kea, Hawaii, world's largest
astronomical observing complex

The telescope is the single most important invention for astronomy. Without it, we would have little of the profound understanding we have obtained over the last few hundred years about the physical nature of the universe and its history.

This lecture describes the main features of the design and operating principles of optical-band telescopes.


Maxwell (1865) discovered that electric and magnetic forces can propagate through space at the speed of light. The immediate inference was that light is an electromagnetic disturbance. The propagating disturbance moves through space like a wave through water and is called an electromagnetic ("EM") wave.

A wave disturbance

EM waves are characterized by their wavelengths, or the distance between peaks where the EM forces are strongest. All wavelengths from zero to infinity are possible for EM waves, and this total range is called the EM spectrum. From longest to shortest wavelengths, the EM spectrum includes: Radio, microwaves, infrared, optical light, ultraviolet light, X-rays, and gamma rays.

The human eye is directly sensitive only to a very small range of wavelengths in the EM spectrum. This is called the visible or optical region (see figure below). Within this region, the wavelength of the light determines the sensation of color produced in our eyes. Shorter wavelengths correspond to bluer colors.

Full EM Spectrum with the Optical/Visible band enlarged
(Units marked are microns. 1 micron = 0.001 mm = 10-4 cm = 10,000 Å)

Atmospheric "Windows"

The Earth's atmosphere is opaque to most wavelengths in the EM spectrum. This is a good thing for lifeforms on Earth's surface, because the more energetic types of EM radiation are harmful. But, obviously, it is not convenient for astronomers who want to monitor the universe across the full EM spectrum. (Eliminating atmospheric absorption is the main motivation for space astronomy.) The chart below shows the ability of different wavelengths to penetrate the atmosphere. The two main "atmospheric windows," where cosmic EM radiation can easily reach Earth's surface at sea level, are in the radio and optical bands. (Click for enlargement.)

The rest of this lecture describes telescopes designed to work in the optical band. Telescopes in adjacent regions of the EM spectrum (the infrared and ultraviolet) are quite similar. But "telescopes" for the radio, X-ray, or gamma-ray bands are very different in design.


The telescope is a beautiful example of interplay between technology (fabrication of quality glass, polishing techniques, large mechanical structures, computers) and basic science.



Basic Principle

Objectives: Two Types

  1. Lens: A lens is a piece transparent glass or plastic shaped to refract (or bend) light rays to a focus. The image at the left below shows how a flat glass surface bends light rays (in this case, two flat surfaces at an angle combine to make a prism). The shorter the wavelength (e.g. blue light), the stronger the bending. The image at the right shows how a glass surface can be continuously curved to bring all the light rays passing through it from a distant object to a common focal point. Each element of the lens acts like a small prism.

    Refraction of light by a prism
    (click for descriptive animation)
    Shaped convex lens
    • Note that light rays travel in straight lines through empty space or through any medium (air, glass, water, etc) that has uniform properties. It is only at the boundaries between two uniform media that light rays can be deflected or "bent." The optical elements of a telescope therefore only change the directions of light rays at their surfaces (which represent glass/air boundaries).

  2. Mirror: A mirror is a shaped piece of glass which reflects light rays off its front surface to a common focus. A mirror shaped like a parabola will focus all rays that are parallel to its optical axis to a single point. See picture below.

Reflection of light by a figured mirror

Focal point


"Visual" Use



The telescopes you will use in this class are Celestron 8-in Schmidt-Cassegrain reflectors and use an equatorial fork-mount. This terminology is explained in the rest of the lecture.

A more complete description and full details on operation of the Celestrons are given in Chapter 3 of the ASTR 1230 Laboratory Manual.


Focal ratio or "f/ number"

Magnification or "power"

Field of View

Light Gathering Power

Resolution: Optical Figuring Tolerance

Resolution: Diffraction of Light Waves

Resolution: Atmospheric "Seeing"


Three basic types of telescope optics

Telescope Designs

A great variety! Here are four common types of reflector designs:

Mounting designs

Again, a great variety. Two primary types:

Why are all large telescopes reflectors?

Glass mirror blank for one of the two 8.4-m diameter
mirrors of the Large Binocular Telescope.



A binocular is simply a pair of two small, co-aligned refracting telescopes mounted together in such a way that each eye can look through one of telescopes.

Sunset over the William Herschel Telescope
(La Palma, Spain; N. Szymanek)


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Last modified December 2020 by rwo

Text copyright © 1998-2020 Robert W. O'Connell. All rights reserved. Some images copyright © by Prentice-Hall and by the University of Tennessee at Knoxville. WHT image copyright © by N. Szymanek. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.