4. Light and Telescopes

Overview

In the Modules 1-3, you were introduced to the big picture of astronomical science. Module 1 began your orientation into the world of astronomy. You explored both what it means from a cosmological perspective for astronomers to be situated ‘here’ and ‘now’, and also the implications of our particular vantage point for the way astronomers must view and describe the night sky. In Modules 2 and 3, you were then taken back to the beginnings of astronomy where you explored the emergence of the scientific method as people sought to explain the motions of the planets—the wandering stars that move with respect to the rotating background celestial sphere. You learned that those motions were ultimately explained as a consequence of regular motions within a gravitational field in which planets move freely along elliptical paths through curved space. The apparent wandering motion of the planets was therefore explained as a consequence of viewing all of those regular motions from the perspective of a planet (namely, Earth) that follows one such elliptical path while simultaneously spinning on an axis once every twenty-four hours.

This picture we have of the Solar System and its relation to the apparent motions of other planets is the result of discovering what science is, and then of using science to explore and continually refine our understanding of the world we live in. As you learned in Module 3, the invention of the telescope in the early seventeenth century played a significant part in the transformation from a geocentric to a heliocentric view of the Solar System. However, little else was said about telescopes—e.g. how they work, and what purpose they serve for astronomers. Our concentration was on the big picture and how our understanding transformed as astronomers followed the scientific method. We therefore glossed over the means by which astronomers actually gather their evidence, since the evidence itself was all we needed for the purpose of that discussion.

In this module, our aim is to begin exploring both the nature of the evidence that astronomers collect (i.e. the nature of light) and the means by which we collect and record that evidence (i.e. with telescopes and cameras). We will begin by exploring the nature of light as it is understood by modern physics. Then we will move on to examine how telescopes work, investigating both how they are useful in astronomy and what their limitations are. Finally, we’ll explain how the light from astronomical objects that is gathered by telescopes eventually gets recorded so that it can be used in scientific analyses.

Learning Objectives

When you have finished this module, you should be able to do the following:

1. Explore the nature of light as electromagnetic radiation.
2. Examine how telescopes work and prepare a telescope for observation.
3. Investigate the powers and limitations of telescopes.
4. Explore instrumentation used to record and analyse light gathered by telescopes.
5. Compare ground-based and space telescopes in terms of their strengths and limitations.

Key Terms and Concepts

• electromagnetic radiation
• photon
• photoelectric effect
• electromagnetic spectrum
• light-gathering power
• refraction
• focus
• reflection
• resolving power
• magnifying power
• charge-coupled devices (CCDs)
• spectroscopy
• interferometry