While I disagree with Proust about the thrill of seeing utterly new things (I’m sorry, that is an adventure), if I wonder about seeing things “with new eyes,” telescopes immediately come to mind. No instrument has so revolutionized a science, nor so long and thoroughly dominated its practice, as has the telescope astronomy. No instrument so simple (amateurs still make their own) has produced such a sustained transformation in humanity’s understanding of the universe.

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.

Upon foundations of evidence, astronomers erect splendid narratives about the lives of stars, the prevalence of habitable planets, or the fate of the universe. Inaccurate or imprecise evidence weakens the foundation and imperils the astronomical story it supports. Incorrect ideas and theories are vital to science, which normally works by proving many, many ideas to be wrong until only one remains. Wrong data, on the other hand, are deadly.

Beginning in 1862, Huggins used a spectroscope to probe the chemical nature of stars and nebulae. Since then, spectrometry has been the tool for the observational investigation of almost every important astrophysical question, through direct or indirect measurement of temperature, chemical abundance, gas pressure, wavelength shift, and magnetic field strength. The book by Hearnshaw (1986), from which the above quotes were taken, provides a history of astronomical spectroscopy prior to 1965. Since 1965, the importance of spectroscopy has only increased. This chapter introduces some basic ideas about spectrometer design and use. Kitchin (1995, 2008) and Schroeder (1987) give a more complete and advanced treatment, and Hearnshaw (2009) provides a history of the actual instruments.

Astronomy is not for the faint of heart. Almost everything it cares for is forbiddingly 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 collect the flimsy evidence that trickles in from the universe outside our atmosphere and hope it may hold a message.

Astronomers have measured apparent brightness since ancient times, and, as is usual in science, technology has acutely influenced their success. Prior to the 1860s, observers estimated brightness using only their eyes, expressing the results in the uncannily persistent magnitude system that Ptolemy introduced in the second century.1 As Arago notes, the results were not satisfactory.