Astronomy is an observational science, renewed and even revolutionized by new developments in instrumentation. With the resulting growth of multiwavelength investigation as an engine of discovery, it is increasingly important for astronomers to understand the underlying physical principles and operational characteristics for a broad range of instruments. This comprehensive text is ideal for graduate students, active researchers and instrument developers. It is a thorough review of how astronomers obtain their data, covering current approaches to astronomical measurements from radio to gamma rays. The focus is on current technology rather than the history of the field, allowing each topic to be discussed in depth. Areas covered include telescopes, detectors, photometry, spectroscopy, adaptive optics and high- contrast imaging, millimeter-wave and radio receivers, radio and optical/infrared interferometry, and X-ray and gamma-ray astronomy, all at a level that bridges the gap between the basic principles of optics and the subject's abundant specialist literature. Color versions of figures and solutions to selected problems are available online at www.cambridge.org/9780521762298.