In this chapter, we consider the acquisition of spectral line data and the integrity of such data. Several types of measurements can be made on a spectral line: total absorption, characteristic width, detailed shape, asymmetry, wavelength position, and polarization. These measurements are not all independent, but it helps to think of them separately when measurements are to be performed. We shall consider all of these except polarization.

Compared to continuum photometry, studies of spectral lines often require very high spectral resolution. How high depends on the width and structure of the spectral lines to be measured, but λ/Δλ in the range of 100 000 or even higher is what we are talking about. This kind of resolving power is obtained with low-order diffraction gratings, echelle gratings, and interferometers. The emphasis here is on grating spectrographs, an elaboration on the basics discussed in Chapter 3. (For details on interferometers, see Vaughan 1967, Connes 1970, and Ridgway & Brault 1984.) High resolution means a small wavelength interval per detector pixel, perhaps 15 or 20mÅ. Contrast this to ∼1000 Å for a wide-band photometric system, a ratio of ≈60 000 or 12 magnitudes. High-resolution spectroscopy, where we want to get at the physics of spectral lines, is the domain of bright stars. Large-aperture telescopes are just as important for high-resolution spectroscopy as they are for faint galaxy work. In practical terms, one can expect a 30 minute exposure of a sixth magnitude star to yield a signal-to-noise ratio of 100 at a resolving power of 105 using a telescope of 1m aperture.