Observational Astronomy

It is safe to say that the spectrograph, a relatively simple instrument, brought about a virtual revolution in astronomy. Although Newton had examined the spectrum of sunlight and Fraunhofer had seen the spectra of a few stars, the spectroscope was not extensively used on telescopes until the latter half of the nineteenth century. Beginning in about 1860, Sir William Huggins in England and Fr. Angelo Secchi in Rome performed their first experiments on the light from the Moon, the planets and the brighter stars. The spectrograph was slowly refined and improved, and eventually it made possible a series of new understandings of the nature of the Sun and stars. First came the identification of a few absorption lines in solar and stellar spectra; then came recognition of several distinct classes of spectra. By the end of the century the construction of spectrographs had been refined to the point that radial velocities could be measured with confidence. Today we have a remarkable understanding of the physical processes that occur in nebulae and the atmospheres of the Sun and stars.

Since the nineteenth century several technological developments have increased the efficiency of the spectrograph, and there have been changes in the means by which the light is dispersed. The general principles of the spectrograph are not complicated, however, and we will outline them below. We shall discuss first the prism and then the grating as dispersive elements. Then we will describe the practical considerations in the design and use of a spectrograph.