Every recorded quasar spectrum is a blemished version of an otherwise pure light beam. It is blurred by the atmosphere and suffers interference and scattering when reflected off optical elements. It is imperfectly collimated, impurely dispersed, iteratively refocused, and inefficiently discretized when recorded. It is then converted to analog and re-digitized, which introduces “read” errors to an already noise-ridden Poissonian process of photon counting. To understand spectra, one needs to understand its recording device, the spectrograph. In this chapter, a range of long-slit low-resolution spectrographs and high-resolution echelle spectrographs are described. Grating equations, blaze functions, and cross dispersers are examined in detail. The equations for resolving power and instrumental resolution are derived from first principles, followed by illustrations showing the impact of CCD pixelization and line broadening on recorded absorption lines. We present quantitative models for the recorded counts in observed spectra. Flux calibration is also derived from first principles of telescope characteristics and spectrograph design. Finally, integrated field units are described.

The main tool for studying stars is the spectrograph.Here we look at the astronomical aspects of spectrographs, how they work, and how to optimize them for stellar work.The characteristics of diffraction gratings are a central theme.The roles of the collimator and camera are then discussed, as is the resolving power of the final unit.Spectrographs using gratings in low orders are contrasted to echelle spectrographs used in high orders.Interferometers and Fourier spectroscopy isdiscussed briefly, as are some aspects of telescopes.