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A spectrograph splits light into its component wavelengths. First, light travels from a telescope through a small opening in the spectrograph to a collimating mirror that lines up all entering rays of light parallel to one another before they reach a finely scored plate of glass known as a diffraction grating. When light passes through or bounces off this glass grating, its many constituent wavelengths each change speed and direction according to their spectral color. The grating bends red light in a different way from orange light, which bends a little differently from yellow light and so on, spreading the many wavelengths into a rainbow spectrum. Rotating the diffraction grating controls which wavelengths of light reach another mirror, which in turn focuses these wavelengths onto a photodetector, such as a charge-coupled device. The detector converts photons into electrical signals that a computer interprets to measure the strength of different wavelengths.
Image: Illustration by George Retseck