In the early 20th century, spectrographs were developed for wavebands outside the optical range. Whether you are measuring visible light or high-energy X-rays, spectrographs decode the information carried in radiation by sorting photons into a spectrum from low to high energy, and measuring the intensity of photons at each wavelength. Early spectrographs captured the spectrum of a source photographically revealing bright and dark lines that could be measured. The bright lines are called emission lines and come from extended hot gas. The dark lines are caused by cooler gas sitting in front of hot gas. In both cases the wavelengths of the lines provide an assured mechanism for identifying the chemical elements producing the lines. Today electronic detectors count photons at specific wavelengths converting the spectrum into more accurate data that can be graphed, allowing astrophysicists to analyze the light of distant galaxies, stars, and even the light reflected from planets and moons, and accurately measure chemical composition and temperature of objects and phenomena across the Universe.
Spectroscopy: Turning Light Into Data - Play the video to see an example of how an optical spectrograph works. Click the triangle to start the video. The double arrows in the lower right corner enlarge the video to full screen. Push the escape key on your keyboard to return to the lesson page. The double bars pauses the video. In the compressed display mode, place your cursor on the bar along the bottom of the video and click to go back to any previous point. If the video is blank, refresh the lesson page.
Spectrographs differ for each region of the electromagnetic spectrum, but they share basic functions: isolate the light, focus the light, disperse the light by wavelength, and measure the light at each wavelength. A slit is often used to isolate the light entering the telescope from a specific source, and reject the light from other sources that the telecope gathers. A collimator straightens out the light into a parallel beam that is directed onto the dispersing element. A dispersing element sorts the beam of light into a spectrum by diverting the path of photons of different wavelengths into beams traveling at separate angles (gratings and prisms are common examples of dispersing elements). A camera focuses the separated beams of photons onto different parts of the detector. The detector records the number of photons collected at each wavelength and converts the spectrum into data. The challenge in designing spectrographs for each region of the electromagnetic spectrum is to select the right tools for isolating, focusing, dispersing, and measuring photons. Spectrograph design varies greatly in how slits, lenses, and mirrors are used to isolate and focus light, and there are different types of dispersing elements and sensors for photons of different wavelengths.
Spectroscopy provides the data astrophysicists need to verify the temperature, chemical composition, velocity, and distance of objects in the Universe. With this information, they can determine the evolution of stars and galaxies and study the conditions in which they form, reveal if the atmosphere of a planet can support life, measure how fast a black hole spins, and track the expansion of the Universe.