In observational astronomy we study the processes by which Earth-bound astronomers obtain and interpret information about distant parts of the universe. Theoretical descriptions of the natural world and observational/experimental data are complementary, and their interplay is a fundamental feature of scientific inquiry. For progress in astronomy we need extensive, sensitive, and accurate observations. But such data do not come for free. They are not just lying around for anybody to pick up. Work is required. An observer who simply accepts data at face value is likely to encounter problems.

In studying the observational process it will be helpful to adopt the following point of view. There is something we will call information which is present in an astronomical source. This information leaves the source, perhaps in the form of electromagnetic radiation. As it travels from the source to the observer it passes through intervening regions, often being modified in the process. The information then reaches the detection system. This final stage inevitably involves significant modification of the information. Noise is added, much information is lost, and other changes occur. From this final state the astronomer attempts to infer characteristics of the original source.

Electromagnetic radiation

The most important carrier of astronomical information is electromagnetic radiation. The electromagnetic spectrum is commonly broken down into various wavelength bands, as indicated in Table 1.1. Each band may be roughly described by both a characteristic photon energy hν and a characteristic temperature hν/k.