ABSTRACT
In this and the immediately succeeding sections, the emphasis is upon the detection of the radiation or other information carrier and upon the instruments and techniques used to facilitate and optimise that detection. There is inevitably some overlap with other sections and chapters, and some material might arguably be more logically discussed in a different order from the one chosen. In particular in this section, telescopes are included as a necessary adjunct to the detectors themselves. The theory of the formation of an image of a point source by a telescope, which is all that is required for simple detection, takes us most of the way through the theory of imaging extended sources. Both theories are, therefore, discussed together, even though the latter should perhaps be in Chapter 2. There are many other examples such as X-ray spectroscopy and polarimetry that appear in Section 1.3 instead of Sections 4.2 and 5.2. In general, the author has tried to follow the route detection→imaging→ancillary techniques throughout the book but has dealt with items out of this order when it seemed more natural to do so.
