Observations of stellar photospheres require instruments for collecting and analyzing light. Low-resolution spectrographs are used for measuring the continuous spectrum, while high-resolution spectrographs are needed to measure the spectral lines. In this chapter, we delve into several basic aspects of spectrographs and diffraction gratings, then turn our attention briefly to interferometers, and finally to telescopes. Special features and application of spectrophotometric equipment are discussed separately in Chapter 10 (continua) and Chapter 12 (lines). Chapter 4 is about light detectors.

Spectrographs: some general relations

The basic astronomical spectrograph is shown in Fig. 3.1. It consists of an entrance slit placed at the focus of the telescope, a collimator that intercepts the divergent telescope beam, a dispersing element (prism or grating), and a camera that focuses the dispersed light onto a detector. Since the purpose of the collimator is to make the divergent beam parallel, the distance between the slit and the collimator is the focal length of the collimator, Fcoll. Similarly, the distance between the camera and the focused spectrum is the focal length of the camera, Fcam. The distances between the collimator, disperser, and camera affect the detailed design and optimization of the spectrograph, but do not matter for the basics we are considering at the moment. We concentrate on plane diffraction gratings as the dispersing element since these are almost universally used in astronomical spectrographs.