Astronomy is not for the faint of heart. Almost everything it cares for is indescribably remote, tantalizingly untouchable, and invisible in the daytime, when most sensible people do their work. Nevertheless, many – including you, brave reader – have enough curiosity and courage to go about collecting the flimsy evidence that reaches us from the universe outside our atmosphere, and to hope it may hold a message.

This chapter introduces you to astronomical evidence. Some evidence is in the form of material (like meteorites), but most is in the form of light from faraway objects. Accordingly, after a brief consideration of the material evidence, we will examine three theories for describing the behavior of light: light as a wave, light as a quantum entity called a photon, and light as a geometrical ray. The ray picture is simplest, and we use it to introduce some basic ideas like the apparent brightness of a source and how that varies with distance. Most information in astronomy, however, comes from the analysis of how brightness changes with wavelength, so we will next introduce the important idea of spectroscopy. We end with a discussion of the astronomical magnitude system. We begin, however, with a few thoughts on the nature of astronomy as an intellectual enterprise.

Astronomers normally present the output of a sensor array in the form of a digital image, a picture, but a mathematical picture. One appealing characteristic of a digital image is that the astronomer can readily subject it to mathematical manipulation, both for purposes of improving the image itself, as well as for purposes of extracting information.

Accordingly, the chapter will proceed by first presenting some general thoughts about array data, and some general algorithms for image manipulation. Because they are so useful in astronomy, we next examine some procedures for removing image flaws introduced by the observing system, as well as some operations that can combine multiple images into a single image. Finally, we look at one important method for extracting information: digital photometry, and derive the CCD equation, an expression that describes the quality you can expect from a digital photometric measurement.

Arrays

Astronomers usually use panoramic detectors to record two-dimensional images and, at optical wavelengths, they most often use a charge-coupled device (CCD). Unlike a photographic plate (until the 1980s, the panoramic detector of choice), a CCD is an array – a grid of spatially discrete but identical light-detecting elements. Although this chapter discusses the CCD specifically, most of its ideas are relevant to images from other kinds of arrays.

Where is Mars? The center of our Galaxy? The brightest X-ray source? Where, indeed, are we? Astronomers have always needed to locate objects and events in space. As our science evolves, it demands ever more exact locations. Suppose, for example, an astronomer observes with an X-ray telescope and discovers a source that flashes on and off with a curious rhythm. Is this source a planet, a star, or the core of a galaxy? It is possible that the X-ray source will appear to be quite unremarkable at other wavelengths. The exact position for the X-ray source might be the only way to identify its optical or radio counterpart. Astronomers need to know where things are.

Likewise, knowing when something happens is often as important as where it happens. The rhythms of the spinning and orbiting Earth gave astronomy an early and intimate connection to timekeeping. Because our Universe is always changing, astronomers need to know what time it is.

Astronomical detection, even more than the work of Sherlock Holmes, is an exact science. Watson, though, has an equally important point: no astronomer, not even the coldest and most unemotional, is immune to that pleasant, even romantic, thrill that comes when the detector does work, and the Universe does seem to be speaking.

An astronomical detector receives photons from a source and produces a corresponding signal. The signal characterizes the incoming photons: it may measure their rate of arrival, their energy distribution, or perhaps their wave phase or polarization. Although detecting the signal may be an exact science, its characterization of the source is rarely exact. Photons never pass directly from source to detector without some mediation. They traverse both space and the Earth's atmosphere, and in both places emissions and absorptions may modify the photon stream. A telescope and other elements of the observing system, like correcting lenses, mirrors, filters, optical fibers, and spectrograph gratings, collect and direct the photons, but also alter them. Only in the end does the detector do its work.

While I disagree with Proust about the thrill of seeing utterly new things (I'm sorry, that is an adventure), astronomers immediately come to mind if I wonder who might be obsessively concerned with the acquisition of new “eyes.” No instrument has so revolutionized a science, nor so long and thoroughly dominated its practice, as has the telescope astronomy. With the possible exception of the printing press, no instrument so simple (amateurs still make their own) has produced such a sustained transformation in humanity's understanding of the Universe.

In this chapter, we examine the basic one- and two-mirror optical layouts of the preferred modern designs, as well as the layouts of a few telescopes that use both transmitting and reflecting elements. Schroeder (1987) provides a more advanced treatment.

Space-based telescopes have some pronounced advantages, disadvantages, and special requirements compared with their ground-based cousins, and we will consider these in some detail, along with recent advances in the construction of very large telescopes. Because it is such an important technology, we will take some trouble to understand the principles of adaptive optics, and its potential for removing at least some of the natural but nasty (for astronomy) consequences of living on a planet with an atmosphere.

Certainly Bacon's judgment that optics is the gateway to other sciences is particularly true of astronomy, since virtually all astronomical information arrives in the form of light. We devote the next two chapters to how astronomers utilize the sweetness and beauty of optical science. This chapter introduces the fundamentals.

We first examine the simple laws of reflection and refraction as basic consequences of Fermat's principle, then review the behavior of optical materials and the operation of fundamental optical elements: films, mirrors, lenses, fibers, and prisms.

Telescopes, of course, are a central concern, and we introduce the simple concept of a telescope as camera. We will see that the clarity of the image produced by a telescopic camera depends on many things: the diameter of the light-gathering element, the turbulence and refraction of the air, and, if the telescope uses lenses, the phenomenon of chromatic aberration. Concern with image quality, finally, will lead us to an extended discussion of monochromatic aberrations and the difference between the first-order and higher-order ray theories of light.