With research time in great demand,no single astronomer can sit at the giant telescopes and simply stargaze. Instead,starlight is recorded on photographic plates or film (directly or after passing through electronic imaging systems) for later exhaustive study by many scientists.
Often an instrument called a spectrograph is attached to the telescope. Starlight is not a single color but rather amixture of colors,or wavelengths. Astronomers deduce much information about stars from these separate wavelengths, as you will see in Chapter Three.
A spectrograph separates starlight into its component wavelengths. Starlight enters the spectrograph through a narrow slit and goes through a collimating lens, which produces a beam of parallel rays of light. A prism or grating disperses this light into its separate colors (wavelengths) A camera records this spectrum on photographic plate.
You can produce a spectrum from sunlight (starlight). Place a mirror in a pan of water so that it is under the water and leaning against the side of the pan. Position the pan in bright sunlight so that the sun shines on the mirror. Move the mirror slightly until you see a spectrum on the ceiling or wall.